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1 | =head1 NAME |
2 | ||
3 | perlfunc - Perl builtin functions | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The functions in this section can serve as terms in an expression. | |
8 | They fall into two major categories: list operators and named unary | |
9 | operators. These differ in their precedence relationship with a | |
10 | following comma. (See the precedence table in L<perlop>.) List | |
11 | operators take more than one argument, while unary operators can never | |
12 | take more than one argument. Thus, a comma terminates the argument of | |
13 | a unary operator, but merely separates the arguments of a list | |
14 | operator. A unary operator generally provides a scalar context to its | |
15 | argument, while a list operator may provide either scalar or list | |
16 | contexts for its arguments. If it does both, the scalar arguments will | |
17 | be first, and the list argument will follow. (Note that there can ever | |
18 | be only one such list argument.) For instance, splice() has three scalar | |
19 | arguments followed by a list, whereas gethostbyname() has four scalar | |
20 | arguments. | |
21 | ||
22 | In the syntax descriptions that follow, list operators that expect a | |
23 | list (and provide list context for the elements of the list) are shown | |
24 | with LIST as an argument. Such a list may consist of any combination | |
25 | of scalar arguments or list values; the list values will be included | |
26 | in the list as if each individual element were interpolated at that | |
27 | point in the list, forming a longer single-dimensional list value. | |
28 | Elements of the LIST should be separated by commas. | |
29 | ||
30 | Any function in the list below may be used either with or without | |
31 | parentheses around its arguments. (The syntax descriptions omit the | |
32 | parentheses.) If you use the parentheses, the simple (but occasionally | |
33 | surprising) rule is this: It I<looks> like a function, therefore it I<is> a | |
34 | function, and precedence doesn't matter. Otherwise it's a list | |
35 | operator or unary operator, and precedence does matter. And whitespace | |
36 | between the function and left parenthesis doesn't count--so you need to | |
37 | be careful sometimes: | |
38 | ||
39 | print 1+2+4; # Prints 7. | |
40 | print(1+2) + 4; # Prints 3. | |
41 | print (1+2)+4; # Also prints 3! | |
42 | print +(1+2)+4; # Prints 7. | |
43 | print ((1+2)+4); # Prints 7. | |
44 | ||
45 | If you run Perl with the B<-w> switch it can warn you about this. For | |
46 | example, the third line above produces: | |
47 | ||
48 | print (...) interpreted as function at - line 1. | |
49 | Useless use of integer addition in void context at - line 1. | |
50 | ||
51 | A few functions take no arguments at all, and therefore work as neither | |
52 | unary nor list operators. These include such functions as C<time> | |
53 | and C<endpwent>. For example, C<time+86_400> always means | |
54 | C<time() + 86_400>. | |
55 | ||
56 | For functions that can be used in either a scalar or list context, | |
57 | nonabortive failure is generally indicated in a scalar context by | |
58 | returning the undefined value, and in a list context by returning the | |
59 | null list. | |
60 | ||
61 | Remember the following important rule: There is B<no rule> that relates | |
62 | the behavior of an expression in list context to its behavior in scalar | |
63 | context, or vice versa. It might do two totally different things. | |
64 | Each operator and function decides which sort of value it would be most | |
65 | appropriate to return in scalar context. Some operators return the | |
66 | length of the list that would have been returned in list context. Some | |
67 | operators return the first value in the list. Some operators return the | |
68 | last value in the list. Some operators return a count of successful | |
69 | operations. In general, they do what you want, unless you want | |
70 | consistency. | |
71 | ||
72 | A named array in scalar context is quite different from what would at | |
73 | first glance appear to be a list in scalar context. You can't get a list | |
74 | like C<(1,2,3)> into being in scalar context, because the compiler knows | |
75 | the context at compile time. It would generate the scalar comma operator | |
76 | there, not the list construction version of the comma. That means it | |
77 | was never a list to start with. | |
78 | ||
79 | In general, functions in Perl that serve as wrappers for system calls | |
80 | of the same name (like chown(2), fork(2), closedir(2), etc.) all return | |
81 | true when they succeed and C<undef> otherwise, as is usually mentioned | |
82 | in the descriptions below. This is different from the C interfaces, | |
83 | which return C<-1> on failure. Exceptions to this rule are C<wait>, | |
84 | C<waitpid>, and C<syscall>. System calls also set the special C<$!> | |
85 | variable on failure. Other functions do not, except accidentally. | |
86 | ||
87 | =head2 Perl Functions by Category | |
88 | ||
89 | Here are Perl's functions (including things that look like | |
90 | functions, like some keywords and named operators) | |
91 | arranged by category. Some functions appear in more | |
92 | than one place. | |
93 | ||
94 | =over 4 | |
95 | ||
96 | =item Functions for SCALARs or strings | |
97 | ||
98 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, | |
99 | C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, | |
100 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> | |
101 | ||
102 | =item Regular expressions and pattern matching | |
103 | ||
104 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> | |
105 | ||
106 | =item Numeric functions | |
107 | ||
108 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, | |
109 | C<sin>, C<sqrt>, C<srand> | |
110 | ||
111 | =item Functions for real @ARRAYs | |
112 | ||
113 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> | |
114 | ||
115 | =item Functions for list data | |
116 | ||
117 | C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> | |
118 | ||
119 | =item Functions for real %HASHes | |
120 | ||
121 | C<delete>, C<each>, C<exists>, C<keys>, C<values> | |
122 | ||
123 | =item Input and output functions | |
124 | ||
125 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, | |
126 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, | |
127 | C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, | |
128 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, | |
129 | C<warn>, C<write> | |
130 | ||
131 | =item Functions for fixed length data or records | |
132 | ||
133 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> | |
134 | ||
135 | =item Functions for filehandles, files, or directories | |
136 | ||
137 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, | |
138 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, | |
139 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>, | |
140 | C<umask>, C<unlink>, C<utime> | |
141 | ||
142 | =item Keywords related to the control flow of your perl program | |
143 | ||
144 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, | |
145 | C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> | |
146 | ||
147 | =item Keywords related to scoping | |
148 | ||
149 | C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use> | |
150 | ||
151 | =item Miscellaneous functions | |
152 | ||
153 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>, | |
154 | C<scalar>, C<undef>, C<wantarray> | |
155 | ||
156 | =item Functions for processes and process groups | |
157 | ||
158 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, | |
159 | C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, | |
160 | C<times>, C<wait>, C<waitpid> | |
161 | ||
162 | =item Keywords related to perl modules | |
163 | ||
164 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> | |
165 | ||
166 | =item Keywords related to classes and object-orientedness | |
167 | ||
168 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, | |
169 | C<untie>, C<use> | |
170 | ||
171 | =item Low-level socket functions | |
172 | ||
173 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, | |
174 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
175 | C<socket>, C<socketpair> | |
176 | ||
177 | =item System V interprocess communication functions | |
178 | ||
179 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, | |
180 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
181 | ||
182 | =item Fetching user and group info | |
183 | ||
184 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, | |
185 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
186 | C<getpwuid>, C<setgrent>, C<setpwent> | |
187 | ||
188 | =item Fetching network info | |
189 | ||
190 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, | |
191 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
192 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
193 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
194 | C<setnetent>, C<setprotoent>, C<setservent> | |
195 | ||
196 | =item Time-related functions | |
197 | ||
198 | C<gmtime>, C<localtime>, C<time>, C<times> | |
199 | ||
200 | =item Functions new in perl5 | |
201 | ||
202 | C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, | |
203 | C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>, | |
204 | C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, | |
205 | C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> | |
206 | ||
207 | * - C<sub> was a keyword in perl4, but in perl5 it is an | |
208 | operator, which can be used in expressions. | |
209 | ||
210 | =item Functions obsoleted in perl5 | |
211 | ||
212 | C<dbmclose>, C<dbmopen> | |
213 | ||
214 | =back | |
215 | ||
216 | =head2 Portability | |
217 | ||
218 | Perl was born in Unix and can therefore access all common Unix | |
219 | system calls. In non-Unix environments, the functionality of some | |
220 | Unix system calls may not be available, or details of the available | |
221 | functionality may differ slightly. The Perl functions affected | |
222 | by this are: | |
223 | ||
224 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, | |
225 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, | |
226 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, | |
227 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, | |
228 | C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
229 | C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, | |
230 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, | |
231 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, | |
232 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, | |
233 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, | |
234 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, | |
235 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, | |
236 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, | |
237 | C<shmwrite>, C<socket>, C<socketpair>, | |
238 | C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, | |
239 | C<times>, C<truncate>, C<umask>, C<unlink>, | |
240 | C<utime>, C<wait>, C<waitpid> | |
241 | ||
242 | For more information about the portability of these functions, see | |
243 | L<perlport> and other available platform-specific documentation. | |
244 | ||
245 | =head2 Alphabetical Listing of Perl Functions | |
246 | ||
247 | =over 8 | |
248 | ||
249 | =item I<-X> FILEHANDLE | |
250 | ||
251 | =item I<-X> EXPR | |
252 | ||
253 | =item I<-X> | |
254 | ||
255 | A file test, where X is one of the letters listed below. This unary | |
256 | operator takes one argument, either a filename or a filehandle, and | |
257 | tests the associated file to see if something is true about it. If the | |
258 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. | |
259 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or | |
260 | the undefined value if the file doesn't exist. Despite the funny | |
261 | names, precedence is the same as any other named unary operator, and | |
262 | the argument may be parenthesized like any other unary operator. The | |
263 | operator may be any of: | |
264 | X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> | |
265 | X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> | |
266 | ||
267 | -r File is readable by effective uid/gid. | |
268 | -w File is writable by effective uid/gid. | |
269 | -x File is executable by effective uid/gid. | |
270 | -o File is owned by effective uid. | |
271 | ||
272 | -R File is readable by real uid/gid. | |
273 | -W File is writable by real uid/gid. | |
274 | -X File is executable by real uid/gid. | |
275 | -O File is owned by real uid. | |
276 | ||
277 | -e File exists. | |
278 | -z File has zero size (is empty). | |
279 | -s File has nonzero size (returns size in bytes). | |
280 | ||
281 | -f File is a plain file. | |
282 | -d File is a directory. | |
283 | -l File is a symbolic link. | |
284 | -p File is a named pipe (FIFO), or Filehandle is a pipe. | |
285 | -S File is a socket. | |
286 | -b File is a block special file. | |
287 | -c File is a character special file. | |
288 | -t Filehandle is opened to a tty. | |
289 | ||
290 | -u File has setuid bit set. | |
291 | -g File has setgid bit set. | |
292 | -k File has sticky bit set. | |
293 | ||
294 | -T File is an ASCII text file (heuristic guess). | |
295 | -B File is a "binary" file (opposite of -T). | |
296 | ||
297 | -M Script start time minus file modification time, in days. | |
298 | -A Same for access time. | |
299 | -C Same for inode change time (Unix, may differ for other platforms) | |
300 | ||
301 | Example: | |
302 | ||
303 | while (<>) { | |
304 | chomp; | |
305 | next unless -f $_; # ignore specials | |
306 | #... | |
307 | } | |
308 | ||
309 | The interpretation of the file permission operators C<-r>, C<-R>, | |
310 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode | |
311 | of the file and the uids and gids of the user. There may be other | |
312 | reasons you can't actually read, write, or execute the file. Such | |
313 | reasons may be for example network filesystem access controls, ACLs | |
314 | (access control lists), read-only filesystems, and unrecognized | |
315 | executable formats. | |
316 | ||
317 | Also note that, for the superuser on the local filesystems, the C<-r>, | |
318 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 | |
319 | if any execute bit is set in the mode. Scripts run by the superuser | |
320 | may thus need to do a stat() to determine the actual mode of the file, | |
321 | or temporarily set their effective uid to something else. | |
322 | ||
323 | If you are using ACLs, there is a pragma called C<filetest> that may | |
324 | produce more accurate results than the bare stat() mode bits. | |
325 | When under the C<use filetest 'access'> the above-mentioned filetests | |
326 | will test whether the permission can (not) be granted using the | |
327 | access() family of system calls. Also note that the C<-x> and C<-X> may | |
328 | under this pragma return true even if there are no execute permission | |
329 | bits set (nor any extra execute permission ACLs). This strangeness is | |
330 | due to the underlying system calls' definitions. Read the | |
331 | documentation for the C<filetest> pragma for more information. | |
332 | ||
333 | Note that C<-s/a/b/> does not do a negated substitution. Saying | |
334 | C<-exp($foo)> still works as expected, however--only single letters | |
335 | following a minus are interpreted as file tests. | |
336 | ||
337 | The C<-T> and C<-B> switches work as follows. The first block or so of the | |
338 | file is examined for odd characters such as strange control codes or | |
339 | characters with the high bit set. If too many strange characters (>30%) | |
340 | are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file | |
341 | containing null in the first block is considered a binary file. If C<-T> | |
342 | or C<-B> is used on a filehandle, the current IO buffer is examined | |
343 | rather than the first block. Both C<-T> and C<-B> return true on a null | |
344 | file, or a file at EOF when testing a filehandle. Because you have to | |
345 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> | |
346 | against the file first, as in C<next unless -f $file && -T $file>. | |
347 | ||
348 | If any of the file tests (or either the C<stat> or C<lstat> operators) are given | |
349 | the special filehandle consisting of a solitary underline, then the stat | |
350 | structure of the previous file test (or stat operator) is used, saving | |
351 | a system call. (This doesn't work with C<-t>, and you need to remember | |
352 | that lstat() and C<-l> will leave values in the stat structure for the | |
353 | symbolic link, not the real file.) (Also, if the stat buffer was filled by | |
354 | a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>). | |
355 | Example: | |
356 | ||
357 | print "Can do.\n" if -r $a || -w _ || -x _; | |
358 | ||
359 | stat($filename); | |
360 | print "Readable\n" if -r _; | |
361 | print "Writable\n" if -w _; | |
362 | print "Executable\n" if -x _; | |
363 | print "Setuid\n" if -u _; | |
364 | print "Setgid\n" if -g _; | |
365 | print "Sticky\n" if -k _; | |
366 | print "Text\n" if -T _; | |
367 | print "Binary\n" if -B _; | |
368 | ||
369 | =item abs VALUE | |
370 | ||
371 | =item abs | |
372 | ||
373 | Returns the absolute value of its argument. | |
374 | If VALUE is omitted, uses C<$_>. | |
375 | ||
376 | =item accept NEWSOCKET,GENERICSOCKET | |
377 | ||
378 | Accepts an incoming socket connect, just as the accept(2) system call | |
379 | does. Returns the packed address if it succeeded, false otherwise. | |
380 | See the example in L<perlipc/"Sockets: Client/Server Communication">. | |
381 | ||
382 | On systems that support a close-on-exec flag on files, the flag will | |
383 | be set for the newly opened file descriptor, as determined by the | |
384 | value of $^F. See L<perlvar/$^F>. | |
385 | ||
386 | =item alarm SECONDS | |
387 | ||
388 | =item alarm | |
389 | ||
390 | Arranges to have a SIGALRM delivered to this process after the | |
391 | specified number of wallclock seconds have elapsed. If SECONDS is not | |
392 | specified, the value stored in C<$_> is used. (On some machines, | |
393 | unfortunately, the elapsed time may be up to one second less or more | |
394 | than you specified because of how seconds are counted, and process | |
395 | scheduling may delay the delivery of the signal even further.) | |
396 | ||
397 | Only one timer may be counting at once. Each call disables the | |
398 | previous timer, and an argument of C<0> may be supplied to cancel the | |
399 | previous timer without starting a new one. The returned value is the | |
400 | amount of time remaining on the previous timer. | |
401 | ||
402 | For delays of finer granularity than one second, you may use Perl's | |
403 | four-argument version of select() leaving the first three arguments | |
404 | undefined, or you might be able to use the C<syscall> interface to | |
405 | access setitimer(2) if your system supports it. The Time::HiRes | |
406 | module (from CPAN, and starting from Perl 5.8 part of the standard | |
407 | distribution) may also prove useful. | |
408 | ||
409 | It is usually a mistake to intermix C<alarm> and C<sleep> calls. | |
410 | (C<sleep> may be internally implemented in your system with C<alarm>) | |
411 | ||
412 | If you want to use C<alarm> to time out a system call you need to use an | |
413 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to | |
414 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to | |
415 | restart system calls on some systems. Using C<eval>/C<die> always works, | |
416 | modulo the caveats given in L<perlipc/"Signals">. | |
417 | ||
418 | eval { | |
419 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required | |
420 | alarm $timeout; | |
421 | $nread = sysread SOCKET, $buffer, $size; | |
422 | alarm 0; | |
423 | }; | |
424 | if ($@) { | |
425 | die unless $@ eq "alarm\n"; # propagate unexpected errors | |
426 | # timed out | |
427 | } | |
428 | else { | |
429 | # didn't | |
430 | } | |
431 | ||
432 | =item atan2 Y,X | |
433 | ||
434 | Returns the arctangent of Y/X in the range -PI to PI. | |
435 | ||
436 | For the tangent operation, you may use the C<Math::Trig::tan> | |
437 | function, or use the familiar relation: | |
438 | ||
439 | sub tan { sin($_[0]) / cos($_[0]) } | |
440 | ||
441 | =item bind SOCKET,NAME | |
442 | ||
443 | Binds a network address to a socket, just as the bind system call | |
444 | does. Returns true if it succeeded, false otherwise. NAME should be a | |
445 | packed address of the appropriate type for the socket. See the examples in | |
446 | L<perlipc/"Sockets: Client/Server Communication">. | |
447 | ||
448 | =item binmode FILEHANDLE, LAYER | |
449 | ||
450 | =item binmode FILEHANDLE | |
451 | ||
452 | Arranges for FILEHANDLE to be read or written in "binary" or "text" | |
453 | mode on systems where the run-time libraries distinguish between | |
454 | binary and text files. If FILEHANDLE is an expression, the value is | |
455 | taken as the name of the filehandle. Returns true on success, | |
456 | C<undef> on failure. | |
457 | ||
458 | If LAYER is omitted or specified as C<:raw> the filehandle is made | |
459 | suitable for passing binary data. This includes turning off possible CRLF | |
460 | translation and marking it as bytes (as opposed to Unicode characters). | |
461 | Note that as desipite what may be implied in I<"Programming Perl"> | |
462 | (the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf> | |
463 | -- other layers which would affect binary nature of the stream are | |
464 | I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the | |
465 | PERLIO environment variable. | |
466 | ||
467 | I<The LAYER parameter of the binmode() function is described as "DISCIPLINE" | |
468 | in "Programming Perl, 3rd Edition". However, since the publishing of this | |
469 | book, by many known as "Camel III", the consensus of the naming of this | |
470 | functionality has moved from "discipline" to "layer". All documentation | |
471 | of this version of Perl therefore refers to "layers" rather than to | |
472 | "disciplines". Now back to the regularly scheduled documentation...> | |
473 | ||
474 | On some systems (in general, DOS and Windows-based systems) binmode() | |
475 | is necessary when you're not working with a text file. For the sake | |
476 | of portability it is a good idea to always use it when appropriate, | |
477 | and to never use it when it isn't appropriate. | |
478 | ||
479 | In other words: regardless of platform, use binmode() on binary files | |
480 | (like for example images). | |
481 | ||
482 | If LAYER is present it is a single string, but may contain | |
483 | multiple directives. The directives alter the behaviour of the | |
484 | file handle. When LAYER is present using binmode on text | |
485 | file makes sense. | |
486 | ||
487 | To mark FILEHANDLE as UTF-8, use C<:utf8>. | |
488 | ||
489 | The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the | |
490 | form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to | |
491 | establish default I/O layers. See L<open>. | |
492 | ||
493 | In general, binmode() should be called after open() but before any I/O | |
494 | is done on the filehandle. Calling binmode() will normally flush any | |
495 | pending buffered output data (and perhaps pending input data) on the | |
496 | handle. An exception to this is the C<:encoding> layer that | |
497 | changes the default character encoding of the handle, see L<open>. | |
498 | The C<:encoding> layer sometimes needs to be called in | |
499 | mid-stream, and it doesn't flush the stream. | |
500 | ||
501 | The operating system, device drivers, C libraries, and Perl run-time | |
502 | system all work together to let the programmer treat a single | |
503 | character (C<\n>) as the line terminator, irrespective of the external | |
504 | representation. On many operating systems, the native text file | |
505 | representation matches the internal representation, but on some | |
506 | platforms the external representation of C<\n> is made up of more than | |
507 | one character. | |
508 | ||
509 | Mac OS, all variants of Unix, and Stream_LF files on VMS use a single | |
510 | character to end each line in the external representation of text (even | |
511 | though that single character is CARRIAGE RETURN on Mac OS and LINE FEED | |
512 | on Unix and most VMS files). In other systems like OS/2, DOS and the | |
513 | various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, | |
514 | but what's stored in text files are the two characters C<\cM\cJ>. That | |
515 | means that, if you don't use binmode() on these systems, C<\cM\cJ> | |
516 | sequences on disk will be converted to C<\n> on input, and any C<\n> in | |
517 | your program will be converted back to C<\cM\cJ> on output. This is what | |
518 | you want for text files, but it can be disastrous for binary files. | |
519 | ||
520 | Another consequence of using binmode() (on some systems) is that | |
521 | special end-of-file markers will be seen as part of the data stream. | |
522 | For systems from the Microsoft family this means that if your binary | |
523 | data contains C<\cZ>, the I/O subsystem will regard it as the end of | |
524 | the file, unless you use binmode(). | |
525 | ||
526 | binmode() is not only important for readline() and print() operations, | |
527 | but also when using read(), seek(), sysread(), syswrite() and tell() | |
528 | (see L<perlport> for more details). See the C<$/> and C<$\> variables | |
529 | in L<perlvar> for how to manually set your input and output | |
530 | line-termination sequences. | |
531 | ||
532 | =item bless REF,CLASSNAME | |
533 | ||
534 | =item bless REF | |
535 | ||
536 | This function tells the thingy referenced by REF that it is now an object | |
537 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
538 | is used. Because a C<bless> is often the last thing in a constructor, | |
539 | it returns the reference for convenience. Always use the two-argument | |
540 | version if the function doing the blessing might be inherited by a | |
541 | derived class. See L<perltoot> and L<perlobj> for more about the blessing | |
542 | (and blessings) of objects. | |
543 | ||
544 | Consider always blessing objects in CLASSNAMEs that are mixed case. | |
545 | Namespaces with all lowercase names are considered reserved for | |
546 | Perl pragmata. Builtin types have all uppercase names, so to prevent | |
547 | confusion, you may wish to avoid such package names as well. Make sure | |
548 | that CLASSNAME is a true value. | |
549 | ||
550 | See L<perlmod/"Perl Modules">. | |
551 | ||
552 | =item caller EXPR | |
553 | ||
554 | =item caller | |
555 | ||
556 | Returns the context of the current subroutine call. In scalar context, | |
557 | returns the caller's package name if there is a caller, that is, if | |
558 | we're in a subroutine or C<eval> or C<require>, and the undefined value | |
559 | otherwise. In list context, returns | |
560 | ||
561 | ($package, $filename, $line) = caller; | |
562 | ||
563 | With EXPR, it returns some extra information that the debugger uses to | |
564 | print a stack trace. The value of EXPR indicates how many call frames | |
565 | to go back before the current one. | |
566 | ||
567 | ($package, $filename, $line, $subroutine, $hasargs, | |
568 | $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); | |
569 | ||
570 | Here $subroutine may be C<(eval)> if the frame is not a subroutine | |
571 | call, but an C<eval>. In such a case additional elements $evaltext and | |
572 | C<$is_require> are set: C<$is_require> is true if the frame is created by a | |
573 | C<require> or C<use> statement, $evaltext contains the text of the | |
574 | C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, | |
575 | $filename is C<(eval)>, but $evaltext is undefined. (Note also that | |
576 | each C<use> statement creates a C<require> frame inside an C<eval EXPR> | |
577 | frame.) $subroutine may also be C<(unknown)> if this particular | |
578 | subroutine happens to have been deleted from the symbol table. | |
579 | C<$hasargs> is true if a new instance of C<@_> was set up for the frame. | |
580 | C<$hints> and C<$bitmask> contain pragmatic hints that the caller was | |
581 | compiled with. The C<$hints> and C<$bitmask> values are subject to change | |
582 | between versions of Perl, and are not meant for external use. | |
583 | ||
584 | Furthermore, when called from within the DB package, caller returns more | |
585 | detailed information: it sets the list variable C<@DB::args> to be the | |
586 | arguments with which the subroutine was invoked. | |
587 | ||
588 | Be aware that the optimizer might have optimized call frames away before | |
589 | C<caller> had a chance to get the information. That means that C<caller(N)> | |
590 | might not return information about the call frame you expect it do, for | |
591 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the | |
592 | previous time C<caller> was called. | |
593 | ||
594 | =item chdir EXPR | |
595 | ||
596 | Changes the working directory to EXPR, if possible. If EXPR is omitted, | |
597 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, | |
598 | changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the | |
599 | variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If | |
600 | neither is set, C<chdir> does nothing. It returns true upon success, | |
601 | false otherwise. See the example under C<die>. | |
602 | ||
603 | =item chmod LIST | |
604 | ||
605 | Changes the permissions of a list of files. The first element of the | |
606 | list must be the numerical mode, which should probably be an octal | |
607 | number, and which definitely should I<not> a string of octal digits: | |
608 | C<0644> is okay, C<'0644'> is not. Returns the number of files | |
609 | successfully changed. See also L</oct>, if all you have is a string. | |
610 | ||
611 | $cnt = chmod 0755, 'foo', 'bar'; | |
612 | chmod 0755, @executables; | |
613 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to | |
614 | # --w----r-T | |
615 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better | |
616 | $mode = 0644; chmod $mode, 'foo'; # this is best | |
617 | ||
618 | You can also import the symbolic C<S_I*> constants from the Fcntl | |
619 | module: | |
620 | ||
621 | use Fcntl ':mode'; | |
622 | ||
623 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; | |
624 | # This is identical to the chmod 0755 of the above example. | |
625 | ||
626 | =item chomp VARIABLE | |
627 | ||
628 | =item chomp( LIST ) | |
629 | ||
630 | =item chomp | |
631 | ||
632 | This safer version of L</chop> removes any trailing string | |
633 | that corresponds to the current value of C<$/> (also known as | |
634 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total | |
635 | number of characters removed from all its arguments. It's often used to | |
636 | remove the newline from the end of an input record when you're worried | |
637 | that the final record may be missing its newline. When in paragraph | |
638 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
639 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is | |
640 | a reference to an integer or the like, see L<perlvar>) chomp() won't | |
641 | remove anything. | |
642 | If VARIABLE is omitted, it chomps C<$_>. Example: | |
643 | ||
644 | while (<>) { | |
645 | chomp; # avoid \n on last field | |
646 | @array = split(/:/); | |
647 | # ... | |
648 | } | |
649 | ||
650 | If VARIABLE is a hash, it chomps the hash's values, but not its keys. | |
651 | ||
652 | You can actually chomp anything that's an lvalue, including an assignment: | |
653 | ||
654 | chomp($cwd = `pwd`); | |
655 | chomp($answer = <STDIN>); | |
656 | ||
657 | If you chomp a list, each element is chomped, and the total number of | |
658 | characters removed is returned. | |
659 | ||
660 | Note that parentheses are necessary when you're chomping anything | |
661 | that is not a simple variable. This is because C<chomp $cwd = `pwd`;> | |
662 | is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as | |
663 | C<chomp( $cwd = `pwd` )> which you might expect. Similarly, | |
664 | C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than | |
665 | as C<chomp($a, $b)>. | |
666 | ||
667 | =item chop VARIABLE | |
668 | ||
669 | =item chop( LIST ) | |
670 | ||
671 | =item chop | |
672 | ||
673 | Chops off the last character of a string and returns the character | |
674 | chopped. It is much more efficient than C<s/.$//s> because it neither | |
675 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. | |
676 | If VARIABLE is a hash, it chops the hash's values, but not its keys. | |
677 | ||
678 | You can actually chop anything that's an lvalue, including an assignment. | |
679 | ||
680 | If you chop a list, each element is chopped. Only the value of the | |
681 | last C<chop> is returned. | |
682 | ||
683 | Note that C<chop> returns the last character. To return all but the last | |
684 | character, use C<substr($string, 0, -1)>. | |
685 | ||
686 | See also L</chomp>. | |
687 | ||
688 | =item chown LIST | |
689 | ||
690 | Changes the owner (and group) of a list of files. The first two | |
691 | elements of the list must be the I<numeric> uid and gid, in that | |
692 | order. A value of -1 in either position is interpreted by most | |
693 | systems to leave that value unchanged. Returns the number of files | |
694 | successfully changed. | |
695 | ||
696 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
697 | chown $uid, $gid, @filenames; | |
698 | ||
699 | Here's an example that looks up nonnumeric uids in the passwd file: | |
700 | ||
701 | print "User: "; | |
702 | chomp($user = <STDIN>); | |
703 | print "Files: "; | |
704 | chomp($pattern = <STDIN>); | |
705 | ||
706 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
707 | or die "$user not in passwd file"; | |
708 | ||
709 | @ary = glob($pattern); # expand filenames | |
710 | chown $uid, $gid, @ary; | |
711 | ||
712 | On most systems, you are not allowed to change the ownership of the | |
713 | file unless you're the superuser, although you should be able to change | |
714 | the group to any of your secondary groups. On insecure systems, these | |
715 | restrictions may be relaxed, but this is not a portable assumption. | |
716 | On POSIX systems, you can detect this condition this way: | |
717 | ||
718 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
719 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
720 | ||
721 | =item chr NUMBER | |
722 | ||
723 | =item chr | |
724 | ||
725 | Returns the character represented by that NUMBER in the character set. | |
726 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and | |
727 | chr(0x263a) is a Unicode smiley face. Note that characters from 127 | |
728 | to 255 (inclusive) are by default not encoded in Unicode for backward | |
729 | compatibility reasons (but see L<encoding>). | |
730 | ||
731 | For the reverse, use L</ord>. | |
732 | See L<perlunicode> and L<encoding> for more about Unicode. | |
733 | ||
734 | If NUMBER is omitted, uses C<$_>. | |
735 | ||
736 | =item chroot FILENAME | |
737 | ||
738 | =item chroot | |
739 | ||
740 | This function works like the system call by the same name: it makes the | |
741 | named directory the new root directory for all further pathnames that | |
742 | begin with a C</> by your process and all its children. (It doesn't | |
743 | change your current working directory, which is unaffected.) For security | |
744 | reasons, this call is restricted to the superuser. If FILENAME is | |
745 | omitted, does a C<chroot> to C<$_>. | |
746 | ||
747 | =item close FILEHANDLE | |
748 | ||
749 | =item close | |
750 | ||
751 | Closes the file or pipe associated with the file handle, returning | |
752 | true only if IO buffers are successfully flushed and closes the system | |
753 | file descriptor. Closes the currently selected filehandle if the | |
754 | argument is omitted. | |
755 | ||
756 | You don't have to close FILEHANDLE if you are immediately going to do | |
757 | another C<open> on it, because C<open> will close it for you. (See | |
758 | C<open>.) However, an explicit C<close> on an input file resets the line | |
759 | counter (C<$.>), while the implicit close done by C<open> does not. | |
760 | ||
761 | If the file handle came from a piped open C<close> will additionally | |
762 | return false if one of the other system calls involved fails or if the | |
763 | program exits with non-zero status. (If the only problem was that the | |
764 | program exited non-zero C<$!> will be set to C<0>.) Closing a pipe | |
765 | also waits for the process executing on the pipe to complete, in case you | |
766 | want to look at the output of the pipe afterwards, and | |
767 | implicitly puts the exit status value of that command into C<$?>. | |
768 | ||
769 | Prematurely closing the read end of a pipe (i.e. before the process | |
770 | writing to it at the other end has closed it) will result in a | |
771 | SIGPIPE being delivered to the writer. If the other end can't | |
772 | handle that, be sure to read all the data before closing the pipe. | |
773 | ||
774 | Example: | |
775 | ||
776 | open(OUTPUT, '|sort >foo') # pipe to sort | |
777 | or die "Can't start sort: $!"; | |
778 | #... # print stuff to output | |
779 | close OUTPUT # wait for sort to finish | |
780 | or warn $! ? "Error closing sort pipe: $!" | |
781 | : "Exit status $? from sort"; | |
782 | open(INPUT, 'foo') # get sort's results | |
783 | or die "Can't open 'foo' for input: $!"; | |
784 | ||
785 | FILEHANDLE may be an expression whose value can be used as an indirect | |
786 | filehandle, usually the real filehandle name. | |
787 | ||
788 | =item closedir DIRHANDLE | |
789 | ||
790 | Closes a directory opened by C<opendir> and returns the success of that | |
791 | system call. | |
792 | ||
793 | DIRHANDLE may be an expression whose value can be used as an indirect | |
794 | dirhandle, usually the real dirhandle name. | |
795 | ||
796 | =item connect SOCKET,NAME | |
797 | ||
798 | Attempts to connect to a remote socket, just as the connect system call | |
799 | does. Returns true if it succeeded, false otherwise. NAME should be a | |
800 | packed address of the appropriate type for the socket. See the examples in | |
801 | L<perlipc/"Sockets: Client/Server Communication">. | |
802 | ||
803 | =item continue BLOCK | |
804 | ||
805 | Actually a flow control statement rather than a function. If there is a | |
806 | C<continue> BLOCK attached to a BLOCK (typically in a C<while> or | |
807 | C<foreach>), it is always executed just before the conditional is about to | |
808 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
809 | it can be used to increment a loop variable, even when the loop has been | |
810 | continued via the C<next> statement (which is similar to the C C<continue> | |
811 | statement). | |
812 | ||
813 | C<last>, C<next>, or C<redo> may appear within a C<continue> | |
814 | block. C<last> and C<redo> will behave as if they had been executed within | |
815 | the main block. So will C<next>, but since it will execute a C<continue> | |
816 | block, it may be more entertaining. | |
817 | ||
818 | while (EXPR) { | |
819 | ### redo always comes here | |
820 | do_something; | |
821 | } continue { | |
822 | ### next always comes here | |
823 | do_something_else; | |
824 | # then back the top to re-check EXPR | |
825 | } | |
826 | ### last always comes here | |
827 | ||
828 | Omitting the C<continue> section is semantically equivalent to using an | |
829 | empty one, logically enough. In that case, C<next> goes directly back | |
830 | to check the condition at the top of the loop. | |
831 | ||
832 | =item cos EXPR | |
833 | ||
834 | =item cos | |
835 | ||
836 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, | |
837 | takes cosine of C<$_>. | |
838 | ||
839 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> | |
840 | function, or use this relation: | |
841 | ||
842 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
843 | ||
844 | =item crypt PLAINTEXT,SALT | |
845 | ||
846 | Encrypts a string exactly like the crypt(3) function in the C library | |
847 | (assuming that you actually have a version there that has not been | |
848 | extirpated as a potential munition). This can prove useful for checking | |
849 | the password file for lousy passwords, amongst other things. Only the | |
850 | guys wearing white hats should do this. | |
851 | ||
852 | Note that C<crypt> is intended to be a one-way function, much like | |
853 | breaking eggs to make an omelette. There is no (known) corresponding | |
854 | decrypt function (in other words, the crypt() is a one-way hash | |
855 | function). As a result, this function isn't all that useful for | |
856 | cryptography. (For that, see your nearby CPAN mirror.) | |
857 | ||
858 | When verifying an existing encrypted string you should use the | |
859 | encrypted text as the salt (like C<crypt($plain, $crypted) eq | |
860 | $crypted>). This allows your code to work with the standard C<crypt> | |
861 | and with more exotic implementations. In other words, do not assume | |
862 | anything about the returned string itself, or how many bytes in | |
863 | the encrypted string matter. | |
864 | ||
865 | Traditionally the result is a string of 13 bytes: two first bytes of | |
866 | the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only | |
867 | the first eight bytes of the encrypted string mattered, but | |
868 | alternative hashing schemes (like MD5), higher level security schemes | |
869 | (like C2), and implementations on non-UNIX platforms may produce | |
870 | different strings. | |
871 | ||
872 | When choosing a new salt create a random two character string whose | |
873 | characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', | |
874 | '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). | |
875 | ||
876 | Here's an example that makes sure that whoever runs this program knows | |
877 | their own password: | |
878 | ||
879 | $pwd = (getpwuid($<))[1]; | |
880 | ||
881 | system "stty -echo"; | |
882 | print "Password: "; | |
883 | chomp($word = <STDIN>); | |
884 | print "\n"; | |
885 | system "stty echo"; | |
886 | ||
887 | if (crypt($word, $pwd) ne $pwd) { | |
888 | die "Sorry...\n"; | |
889 | } else { | |
890 | print "ok\n"; | |
891 | } | |
892 | ||
893 | Of course, typing in your own password to whoever asks you | |
894 | for it is unwise. | |
895 | ||
896 | The L<crypt> function is unsuitable for encrypting large quantities | |
897 | of data, not least of all because you can't get the information | |
898 | back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories | |
899 | on your favorite CPAN mirror for a slew of potentially useful | |
900 | modules. | |
901 | ||
902 | If using crypt() on a Unicode string (which I<potentially> has | |
903 | characters with codepoints above 255), Perl tries to make sense | |
904 | of the situation by trying to downgrade (a copy of the string) | |
905 | the string back to an eight-bit byte string before calling crypt() | |
906 | (on that copy). If that works, good. If not, crypt() dies with | |
907 | C<Wide character in crypt>. | |
908 | ||
909 | =item dbmclose HASH | |
910 | ||
911 | [This function has been largely superseded by the C<untie> function.] | |
912 | ||
913 | Breaks the binding between a DBM file and a hash. | |
914 | ||
915 | =item dbmopen HASH,DBNAME,MASK | |
916 | ||
917 | [This function has been largely superseded by the C<tie> function.] | |
918 | ||
919 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a | |
920 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first | |
921 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
922 | is the name of the database (without the F<.dir> or F<.pag> extension if | |
923 | any). If the database does not exist, it is created with protection | |
924 | specified by MASK (as modified by the C<umask>). If your system supports | |
925 | only the older DBM functions, you may perform only one C<dbmopen> in your | |
926 | program. In older versions of Perl, if your system had neither DBM nor | |
927 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to | |
928 | sdbm(3). | |
929 | ||
930 | If you don't have write access to the DBM file, you can only read hash | |
931 | variables, not set them. If you want to test whether you can write, | |
932 | either use file tests or try setting a dummy hash entry inside an C<eval>, | |
933 | which will trap the error. | |
934 | ||
935 | Note that functions such as C<keys> and C<values> may return huge lists | |
936 | when used on large DBM files. You may prefer to use the C<each> | |
937 | function to iterate over large DBM files. Example: | |
938 | ||
939 | # print out history file offsets | |
940 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
941 | while (($key,$val) = each %HIST) { | |
942 | print $key, ' = ', unpack('L',$val), "\n"; | |
943 | } | |
944 | dbmclose(%HIST); | |
945 | ||
946 | See also L<AnyDBM_File> for a more general description of the pros and | |
947 | cons of the various dbm approaches, as well as L<DB_File> for a particularly | |
948 | rich implementation. | |
949 | ||
950 | You can control which DBM library you use by loading that library | |
951 | before you call dbmopen(): | |
952 | ||
953 | use DB_File; | |
954 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
955 | or die "Can't open netscape history file: $!"; | |
956 | ||
957 | =item defined EXPR | |
958 | ||
959 | =item defined | |
960 | ||
961 | Returns a Boolean value telling whether EXPR has a value other than | |
962 | the undefined value C<undef>. If EXPR is not present, C<$_> will be | |
963 | checked. | |
964 | ||
965 | Many operations return C<undef> to indicate failure, end of file, | |
966 | system error, uninitialized variable, and other exceptional | |
967 | conditions. This function allows you to distinguish C<undef> from | |
968 | other values. (A simple Boolean test will not distinguish among | |
969 | C<undef>, zero, the empty string, and C<"0">, which are all equally | |
970 | false.) Note that since C<undef> is a valid scalar, its presence | |
971 | doesn't I<necessarily> indicate an exceptional condition: C<pop> | |
972 | returns C<undef> when its argument is an empty array, I<or> when the | |
973 | element to return happens to be C<undef>. | |
974 | ||
975 | You may also use C<defined(&func)> to check whether subroutine C<&func> | |
976 | has ever been defined. The return value is unaffected by any forward | |
977 | declarations of C<&foo>. Note that a subroutine which is not defined | |
978 | may still be callable: its package may have an C<AUTOLOAD> method that | |
979 | makes it spring into existence the first time that it is called -- see | |
980 | L<perlsub>. | |
981 | ||
982 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
983 | used to report whether memory for that aggregate has ever been | |
984 | allocated. This behavior may disappear in future versions of Perl. | |
985 | You should instead use a simple test for size: | |
986 | ||
987 | if (@an_array) { print "has array elements\n" } | |
988 | if (%a_hash) { print "has hash members\n" } | |
989 | ||
990 | When used on a hash element, it tells you whether the value is defined, | |
991 | not whether the key exists in the hash. Use L</exists> for the latter | |
992 | purpose. | |
993 | ||
994 | Examples: | |
995 | ||
996 | print if defined $switch{'D'}; | |
997 | print "$val\n" while defined($val = pop(@ary)); | |
998 | die "Can't readlink $sym: $!" | |
999 | unless defined($value = readlink $sym); | |
1000 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } | |
1001 | $debugging = 0 unless defined $debugging; | |
1002 | ||
1003 | Note: Many folks tend to overuse C<defined>, and then are surprised to | |
1004 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, | |
1005 | defined values. For example, if you say | |
1006 | ||
1007 | "ab" =~ /a(.*)b/; | |
1008 | ||
1009 | The pattern match succeeds, and C<$1> is defined, despite the fact that it | |
1010 | matched "nothing". But it didn't really match nothing--rather, it | |
1011 | matched something that happened to be zero characters long. This is all | |
1012 | very above-board and honest. When a function returns an undefined value, | |
1013 | it's an admission that it couldn't give you an honest answer. So you | |
1014 | should use C<defined> only when you're questioning the integrity of what | |
1015 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is | |
1016 | what you want. | |
1017 | ||
1018 | See also L</undef>, L</exists>, L</ref>. | |
1019 | ||
1020 | =item delete EXPR | |
1021 | ||
1022 | Given an expression that specifies a hash element, array element, hash slice, | |
1023 | or array slice, deletes the specified element(s) from the hash or array. | |
1024 | In the case of an array, if the array elements happen to be at the end, | |
1025 | the size of the array will shrink to the highest element that tests | |
1026 | true for exists() (or 0 if no such element exists). | |
1027 | ||
1028 | Returns each element so deleted or the undefined value if there was no such | |
1029 | element. Deleting from C<$ENV{}> modifies the environment. Deleting from | |
1030 | a hash tied to a DBM file deletes the entry from the DBM file. Deleting | |
1031 | from a C<tie>d hash or array may not necessarily return anything. | |
1032 | ||
1033 | Deleting an array element effectively returns that position of the array | |
1034 | to its initial, uninitialized state. Subsequently testing for the same | |
1035 | element with exists() will return false. Note that deleting array | |
1036 | elements in the middle of an array will not shift the index of the ones | |
1037 | after them down--use splice() for that. See L</exists>. | |
1038 | ||
1039 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: | |
1040 | ||
1041 | foreach $key (keys %HASH) { | |
1042 | delete $HASH{$key}; | |
1043 | } | |
1044 | ||
1045 | foreach $index (0 .. $#ARRAY) { | |
1046 | delete $ARRAY[$index]; | |
1047 | } | |
1048 | ||
1049 | And so do these: | |
1050 | ||
1051 | delete @HASH{keys %HASH}; | |
1052 | ||
1053 | delete @ARRAY[0 .. $#ARRAY]; | |
1054 | ||
1055 | But both of these are slower than just assigning the empty list | |
1056 | or undefining %HASH or @ARRAY: | |
1057 | ||
1058 | %HASH = (); # completely empty %HASH | |
1059 | undef %HASH; # forget %HASH ever existed | |
1060 | ||
1061 | @ARRAY = (); # completely empty @ARRAY | |
1062 | undef @ARRAY; # forget @ARRAY ever existed | |
1063 | ||
1064 | Note that the EXPR can be arbitrarily complicated as long as the final | |
1065 | operation is a hash element, array element, hash slice, or array slice | |
1066 | lookup: | |
1067 | ||
1068 | delete $ref->[$x][$y]{$key}; | |
1069 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; | |
1070 | ||
1071 | delete $ref->[$x][$y][$index]; | |
1072 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
1073 | ||
1074 | =item die LIST | |
1075 | ||
1076 | Outside an C<eval>, prints the value of LIST to C<STDERR> and | |
1077 | exits with the current value of C<$!> (errno). If C<$!> is C<0>, | |
1078 | exits with the value of C<<< ($? >> 8) >>> (backtick `command` | |
1079 | status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside | |
1080 | an C<eval(),> the error message is stuffed into C<$@> and the | |
1081 | C<eval> is terminated with the undefined value. This makes | |
1082 | C<die> the way to raise an exception. | |
1083 | ||
1084 | Equivalent examples: | |
1085 | ||
1086 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
1087 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" | |
1088 | ||
1089 | If the last element of LIST does not end in a newline, the current | |
1090 | script line number and input line number (if any) are also printed, | |
1091 | and a newline is supplied. Note that the "input line number" (also | |
1092 | known as "chunk") is subject to whatever notion of "line" happens to | |
1093 | be currently in effect, and is also available as the special variable | |
1094 | C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1095 | ||
1096 | Hint: sometimes appending C<", stopped"> to your message will cause it | |
1097 | to make better sense when the string C<"at foo line 123"> is appended. | |
1098 | Suppose you are running script "canasta". | |
1099 | ||
1100 | die "/etc/games is no good"; | |
1101 | die "/etc/games is no good, stopped"; | |
1102 | ||
1103 | produce, respectively | |
1104 | ||
1105 | /etc/games is no good at canasta line 123. | |
1106 | /etc/games is no good, stopped at canasta line 123. | |
1107 | ||
1108 | See also exit(), warn(), and the Carp module. | |
1109 | ||
1110 | If LIST is empty and C<$@> already contains a value (typically from a | |
1111 | previous eval) that value is reused after appending C<"\t...propagated">. | |
1112 | This is useful for propagating exceptions: | |
1113 | ||
1114 | eval { ... }; | |
1115 | die unless $@ =~ /Expected exception/; | |
1116 | ||
1117 | If LIST is empty and C<$@> contains an object reference that has a | |
1118 | C<PROPAGATE> method, that method will be called with additional file | |
1119 | and line number parameters. The return value replaces the value in | |
1120 | C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>> | |
1121 | were called. | |
1122 | ||
1123 | If C<$@> is empty then the string C<"Died"> is used. | |
1124 | ||
1125 | die() can also be called with a reference argument. If this happens to be | |
1126 | trapped within an eval(), $@ contains the reference. This behavior permits | |
1127 | a more elaborate exception handling implementation using objects that | |
1128 | maintain arbitrary state about the nature of the exception. Such a scheme | |
1129 | is sometimes preferable to matching particular string values of $@ using | |
1130 | regular expressions. Here's an example: | |
1131 | ||
1132 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; | |
1133 | if ($@) { | |
1134 | if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { | |
1135 | # handle Some::Module::Exception | |
1136 | } | |
1137 | else { | |
1138 | # handle all other possible exceptions | |
1139 | } | |
1140 | } | |
1141 | ||
1142 | Because perl will stringify uncaught exception messages before displaying | |
1143 | them, you may want to overload stringification operations on such custom | |
1144 | exception objects. See L<overload> for details about that. | |
1145 | ||
1146 | You can arrange for a callback to be run just before the C<die> | |
1147 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
1148 | handler will be called with the error text and can change the error | |
1149 | message, if it sees fit, by calling C<die> again. See | |
1150 | L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and | |
1151 | L<"eval BLOCK"> for some examples. Although this feature was meant | |
1152 | to be run only right before your program was to exit, this is not | |
1153 | currently the case--the C<$SIG{__DIE__}> hook is currently called | |
1154 | even inside eval()ed blocks/strings! If one wants the hook to do | |
1155 | nothing in such situations, put | |
1156 | ||
1157 | die @_ if $^S; | |
1158 | ||
1159 | as the first line of the handler (see L<perlvar/$^S>). Because | |
1160 | this promotes strange action at a distance, this counterintuitive | |
1161 | behavior may be fixed in a future release. | |
1162 | ||
1163 | =item do BLOCK | |
1164 | ||
1165 | Not really a function. Returns the value of the last command in the | |
1166 | sequence of commands indicated by BLOCK. When modified by a loop | |
1167 | modifier, executes the BLOCK once before testing the loop condition. | |
1168 | (On other statements the loop modifiers test the conditional first.) | |
1169 | ||
1170 | C<do BLOCK> does I<not> count as a loop, so the loop control statements | |
1171 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. | |
1172 | See L<perlsyn> for alternative strategies. | |
1173 | ||
1174 | =item do SUBROUTINE(LIST) | |
1175 | ||
1176 | A deprecated form of subroutine call. See L<perlsub>. | |
1177 | ||
1178 | =item do EXPR | |
1179 | ||
1180 | Uses the value of EXPR as a filename and executes the contents of the | |
1181 | file as a Perl script. Its primary use is to include subroutines | |
1182 | from a Perl subroutine library. | |
1183 | ||
1184 | do 'stat.pl'; | |
1185 | ||
1186 | is just like | |
1187 | ||
1188 | eval `cat stat.pl`; | |
1189 | ||
1190 | except that it's more efficient and concise, keeps track of the current | |
1191 | filename for error messages, searches the @INC libraries, and updates | |
1192 | C<%INC> if the file is found. See L<perlvar/Predefined Names> for these | |
1193 | variables. It also differs in that code evaluated with C<do FILENAME> | |
1194 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the | |
1195 | same, however, in that it does reparse the file every time you call it, | |
1196 | so you probably don't want to do this inside a loop. | |
1197 | ||
1198 | If C<do> cannot read the file, it returns undef and sets C<$!> to the | |
1199 | error. If C<do> can read the file but cannot compile it, it | |
1200 | returns undef and sets an error message in C<$@>. If the file is | |
1201 | successfully compiled, C<do> returns the value of the last expression | |
1202 | evaluated. | |
1203 | ||
1204 | Note that inclusion of library modules is better done with the | |
1205 | C<use> and C<require> operators, which also do automatic error checking | |
1206 | and raise an exception if there's a problem. | |
1207 | ||
1208 | You might like to use C<do> to read in a program configuration | |
1209 | file. Manual error checking can be done this way: | |
1210 | ||
1211 | # read in config files: system first, then user | |
1212 | for $file ("/share/prog/defaults.rc", | |
1213 | "$ENV{HOME}/.someprogrc") | |
1214 | { | |
1215 | unless ($return = do $file) { | |
1216 | warn "couldn't parse $file: $@" if $@; | |
1217 | warn "couldn't do $file: $!" unless defined $return; | |
1218 | warn "couldn't run $file" unless $return; | |
1219 | } | |
1220 | } | |
1221 | ||
1222 | =item dump LABEL | |
1223 | ||
1224 | =item dump | |
1225 | ||
1226 | This function causes an immediate core dump. See also the B<-u> | |
1227 | command-line switch in L<perlrun>, which does the same thing. | |
1228 | Primarily this is so that you can use the B<undump> program (not | |
1229 | supplied) to turn your core dump into an executable binary after | |
1230 | having initialized all your variables at the beginning of the | |
1231 | program. When the new binary is executed it will begin by executing | |
1232 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1233 | Think of it as a goto with an intervening core dump and reincarnation. | |
1234 | If C<LABEL> is omitted, restarts the program from the top. | |
1235 | ||
1236 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1237 | be open any more when the program is reincarnated, with possible | |
1238 | resulting confusion on the part of Perl. | |
1239 | ||
1240 | This function is now largely obsolete, partly because it's very | |
1241 | hard to convert a core file into an executable, and because the | |
1242 | real compiler backends for generating portable bytecode and compilable | |
1243 | C code have superseded it. That's why you should now invoke it as | |
1244 | C<CORE::dump()>, if you don't want to be warned against a possible | |
1245 | typo. | |
1246 | ||
1247 | If you're looking to use L<dump> to speed up your program, consider | |
1248 | generating bytecode or native C code as described in L<perlcc>. If | |
1249 | you're just trying to accelerate a CGI script, consider using the | |
1250 | C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast. | |
1251 | You might also consider autoloading or selfloading, which at least | |
1252 | make your program I<appear> to run faster. | |
1253 | ||
1254 | =item each HASH | |
1255 | ||
1256 | When called in list context, returns a 2-element list consisting of the | |
1257 | key and value for the next element of a hash, so that you can iterate over | |
1258 | it. When called in scalar context, returns only the key for the next | |
1259 | element in the hash. | |
1260 | ||
1261 | Entries are returned in an apparently random order. The actual random | |
1262 | order is subject to change in future versions of perl, but it is guaranteed | |
1263 | to be in the same order as either the C<keys> or C<values> function | |
1264 | would produce on the same (unmodified) hash. | |
1265 | ||
1266 | When the hash is entirely read, a null array is returned in list context | |
1267 | (which when assigned produces a false (C<0>) value), and C<undef> in | |
1268 | scalar context. The next call to C<each> after that will start iterating | |
1269 | again. There is a single iterator for each hash, shared by all C<each>, | |
1270 | C<keys>, and C<values> function calls in the program; it can be reset by | |
1271 | reading all the elements from the hash, or by evaluating C<keys HASH> or | |
1272 | C<values HASH>. If you add or delete elements of a hash while you're | |
1273 | iterating over it, you may get entries skipped or duplicated, so | |
1274 | don't. Exception: It is always safe to delete the item most recently | |
1275 | returned by C<each()>, which means that the following code will work: | |
1276 | ||
1277 | while (($key, $value) = each %hash) { | |
1278 | print $key, "\n"; | |
1279 | delete $hash{$key}; # This is safe | |
1280 | } | |
1281 | ||
1282 | The following prints out your environment like the printenv(1) program, | |
1283 | only in a different order: | |
1284 | ||
1285 | while (($key,$value) = each %ENV) { | |
1286 | print "$key=$value\n"; | |
1287 | } | |
1288 | ||
1289 | See also C<keys>, C<values> and C<sort>. | |
1290 | ||
1291 | =item eof FILEHANDLE | |
1292 | ||
1293 | =item eof () | |
1294 | ||
1295 | =item eof | |
1296 | ||
1297 | Returns 1 if the next read on FILEHANDLE will return end of file, or if | |
1298 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
1299 | gives the real filehandle. (Note that this function actually | |
1300 | reads a character and then C<ungetc>s it, so isn't very useful in an | |
1301 | interactive context.) Do not read from a terminal file (or call | |
1302 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such | |
1303 | as terminals may lose the end-of-file condition if you do. | |
1304 | ||
1305 | An C<eof> without an argument uses the last file read. Using C<eof()> | |
1306 | with empty parentheses is very different. It refers to the pseudo file | |
1307 | formed from the files listed on the command line and accessed via the | |
1308 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, | |
1309 | as a normal filehandle is, an C<eof()> before C<< <> >> has been | |
1310 | used will cause C<@ARGV> to be examined to determine if input is | |
1311 | available. Similarly, an C<eof()> after C<< <> >> has returned | |
1312 | end-of-file will assume you are processing another C<@ARGV> list, | |
1313 | and if you haven't set C<@ARGV>, will read input from C<STDIN>; | |
1314 | see L<perlop/"I/O Operators">. | |
1315 | ||
1316 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to | |
1317 | detect the end of each file, C<eof()> will only detect the end of the | |
1318 | last file. Examples: | |
1319 | ||
1320 | # reset line numbering on each input file | |
1321 | while (<>) { | |
1322 | next if /^\s*#/; # skip comments | |
1323 | print "$.\t$_"; | |
1324 | } continue { | |
1325 | close ARGV if eof; # Not eof()! | |
1326 | } | |
1327 | ||
1328 | # insert dashes just before last line of last file | |
1329 | while (<>) { | |
1330 | if (eof()) { # check for end of current file | |
1331 | print "--------------\n"; | |
1332 | close(ARGV); # close or last; is needed if we | |
1333 | # are reading from the terminal | |
1334 | } | |
1335 | print; | |
1336 | } | |
1337 | ||
1338 | Practical hint: you almost never need to use C<eof> in Perl, because the | |
1339 | input operators typically return C<undef> when they run out of data, or if | |
1340 | there was an error. | |
1341 | ||
1342 | =item eval EXPR | |
1343 | ||
1344 | =item eval BLOCK | |
1345 | ||
1346 | In the first form, the return value of EXPR is parsed and executed as if it | |
1347 | were a little Perl program. The value of the expression (which is itself | |
1348 | determined within scalar context) is first parsed, and if there weren't any | |
1349 | errors, executed in the lexical context of the current Perl program, so | |
1350 | that any variable settings or subroutine and format definitions remain | |
1351 | afterwards. Note that the value is parsed every time the eval executes. | |
1352 | If EXPR is omitted, evaluates C<$_>. This form is typically used to | |
1353 | delay parsing and subsequent execution of the text of EXPR until run time. | |
1354 | ||
1355 | In the second form, the code within the BLOCK is parsed only once--at the | |
1356 | same time the code surrounding the eval itself was parsed--and executed | |
1357 | within the context of the current Perl program. This form is typically | |
1358 | used to trap exceptions more efficiently than the first (see below), while | |
1359 | also providing the benefit of checking the code within BLOCK at compile | |
1360 | time. | |
1361 | ||
1362 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1363 | the BLOCK. | |
1364 | ||
1365 | In both forms, the value returned is the value of the last expression | |
1366 | evaluated inside the mini-program; a return statement may be also used, just | |
1367 | as with subroutines. The expression providing the return value is evaluated | |
1368 | in void, scalar, or list context, depending on the context of the eval itself. | |
1369 | See L</wantarray> for more on how the evaluation context can be determined. | |
1370 | ||
1371 | If there is a syntax error or runtime error, or a C<die> statement is | |
1372 | executed, an undefined value is returned by C<eval>, and C<$@> is set to the | |
1373 | error message. If there was no error, C<$@> is guaranteed to be a null | |
1374 | string. Beware that using C<eval> neither silences perl from printing | |
1375 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. | |
1376 | To do either of those, you have to use the C<$SIG{__WARN__}> facility, or | |
1377 | turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>. | |
1378 | See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>. | |
1379 | ||
1380 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for | |
1381 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
1382 | is implemented. It is also Perl's exception trapping mechanism, where | |
1383 | the die operator is used to raise exceptions. | |
1384 | ||
1385 | If the code to be executed doesn't vary, you may use the eval-BLOCK | |
1386 | form to trap run-time errors without incurring the penalty of | |
1387 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1388 | Examples: | |
1389 | ||
1390 | # make divide-by-zero nonfatal | |
1391 | eval { $answer = $a / $b; }; warn $@ if $@; | |
1392 | ||
1393 | # same thing, but less efficient | |
1394 | eval '$answer = $a / $b'; warn $@ if $@; | |
1395 | ||
1396 | # a compile-time error | |
1397 | eval { $answer = }; # WRONG | |
1398 | ||
1399 | # a run-time error | |
1400 | eval '$answer ='; # sets $@ | |
1401 | ||
1402 | Due to the current arguably broken state of C<__DIE__> hooks, when using | |
1403 | the C<eval{}> form as an exception trap in libraries, you may wish not | |
1404 | to trigger any C<__DIE__> hooks that user code may have installed. | |
1405 | You can use the C<local $SIG{__DIE__}> construct for this purpose, | |
1406 | as shown in this example: | |
1407 | ||
1408 | # a very private exception trap for divide-by-zero | |
1409 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; | |
1410 | warn $@ if $@; | |
1411 | ||
1412 | This is especially significant, given that C<__DIE__> hooks can call | |
1413 | C<die> again, which has the effect of changing their error messages: | |
1414 | ||
1415 | # __DIE__ hooks may modify error messages | |
1416 | { | |
1417 | local $SIG{'__DIE__'} = | |
1418 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
1419 | eval { die "foo lives here" }; | |
1420 | print $@ if $@; # prints "bar lives here" | |
1421 | } | |
1422 | ||
1423 | Because this promotes action at a distance, this counterintuitive behavior | |
1424 | may be fixed in a future release. | |
1425 | ||
1426 | With an C<eval>, you should be especially careful to remember what's | |
1427 | being looked at when: | |
1428 | ||
1429 | eval $x; # CASE 1 | |
1430 | eval "$x"; # CASE 2 | |
1431 | ||
1432 | eval '$x'; # CASE 3 | |
1433 | eval { $x }; # CASE 4 | |
1434 | ||
1435 | eval "\$$x++"; # CASE 5 | |
1436 | $$x++; # CASE 6 | |
1437 | ||
1438 | Cases 1 and 2 above behave identically: they run the code contained in | |
1439 | the variable $x. (Although case 2 has misleading double quotes making | |
1440 | the reader wonder what else might be happening (nothing is).) Cases 3 | |
1441 | and 4 likewise behave in the same way: they run the code C<'$x'>, which | |
1442 | does nothing but return the value of $x. (Case 4 is preferred for | |
1443 | purely visual reasons, but it also has the advantage of compiling at | |
1444 | compile-time instead of at run-time.) Case 5 is a place where | |
1445 | normally you I<would> like to use double quotes, except that in this | |
1446 | particular situation, you can just use symbolic references instead, as | |
1447 | in case 6. | |
1448 | ||
1449 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements | |
1450 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. | |
1451 | ||
1452 | =item exec LIST | |
1453 | ||
1454 | =item exec PROGRAM LIST | |
1455 | ||
1456 | The C<exec> function executes a system command I<and never returns>-- | |
1457 | use C<system> instead of C<exec> if you want it to return. It fails and | |
1458 | returns false only if the command does not exist I<and> it is executed | |
1459 | directly instead of via your system's command shell (see below). | |
1460 | ||
1461 | Since it's a common mistake to use C<exec> instead of C<system>, Perl | |
1462 | warns you if there is a following statement which isn't C<die>, C<warn>, | |
1463 | or C<exit> (if C<-w> is set - but you always do that). If you | |
1464 | I<really> want to follow an C<exec> with some other statement, you | |
1465 | can use one of these styles to avoid the warning: | |
1466 | ||
1467 | exec ('foo') or print STDERR "couldn't exec foo: $!"; | |
1468 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
1469 | ||
1470 | If there is more than one argument in LIST, or if LIST is an array | |
1471 | with more than one value, calls execvp(3) with the arguments in LIST. | |
1472 | If there is only one scalar argument or an array with one element in it, | |
1473 | the argument is checked for shell metacharacters, and if there are any, | |
1474 | the entire argument is passed to the system's command shell for parsing | |
1475 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1476 | If there are no shell metacharacters in the argument, it is split into | |
1477 | words and passed directly to C<execvp>, which is more efficient. | |
1478 | Examples: | |
1479 | ||
1480 | exec '/bin/echo', 'Your arguments are: ', @ARGV; | |
1481 | exec "sort $outfile | uniq"; | |
1482 | ||
1483 | If you don't really want to execute the first argument, but want to lie | |
1484 | to the program you are executing about its own name, you can specify | |
1485 | the program you actually want to run as an "indirect object" (without a | |
1486 | comma) in front of the LIST. (This always forces interpretation of the | |
1487 | LIST as a multivalued list, even if there is only a single scalar in | |
1488 | the list.) Example: | |
1489 | ||
1490 | $shell = '/bin/csh'; | |
1491 | exec $shell '-sh'; # pretend it's a login shell | |
1492 | ||
1493 | or, more directly, | |
1494 | ||
1495 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell | |
1496 | ||
1497 | When the arguments get executed via the system shell, results will | |
1498 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
1499 | for details. | |
1500 | ||
1501 | Using an indirect object with C<exec> or C<system> is also more | |
1502 | secure. This usage (which also works fine with system()) forces | |
1503 | interpretation of the arguments as a multivalued list, even if the | |
1504 | list had just one argument. That way you're safe from the shell | |
1505 | expanding wildcards or splitting up words with whitespace in them. | |
1506 | ||
1507 | @args = ( "echo surprise" ); | |
1508 | ||
1509 | exec @args; # subject to shell escapes | |
1510 | # if @args == 1 | |
1511 | exec { $args[0] } @args; # safe even with one-arg list | |
1512 | ||
1513 | The first version, the one without the indirect object, ran the I<echo> | |
1514 | program, passing it C<"surprise"> an argument. The second version | |
1515 | didn't--it tried to run a program literally called I<"echo surprise">, | |
1516 | didn't find it, and set C<$?> to a non-zero value indicating failure. | |
1517 | ||
1518 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
1519 | output before the exec, but this may not be supported on some platforms | |
1520 | (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH | |
1521 | in English) or call the C<autoflush()> method of C<IO::Handle> on any | |
1522 | open handles in order to avoid lost output. | |
1523 | ||
1524 | Note that C<exec> will not call your C<END> blocks, nor will it call | |
1525 | any C<DESTROY> methods in your objects. | |
1526 | ||
1527 | =item exists EXPR | |
1528 | ||
1529 | Given an expression that specifies a hash element or array element, | |
1530 | returns true if the specified element in the hash or array has ever | |
1531 | been initialized, even if the corresponding value is undefined. The | |
1532 | element is not autovivified if it doesn't exist. | |
1533 | ||
1534 | print "Exists\n" if exists $hash{$key}; | |
1535 | print "Defined\n" if defined $hash{$key}; | |
1536 | print "True\n" if $hash{$key}; | |
1537 | ||
1538 | print "Exists\n" if exists $array[$index]; | |
1539 | print "Defined\n" if defined $array[$index]; | |
1540 | print "True\n" if $array[$index]; | |
1541 | ||
1542 | A hash or array element can be true only if it's defined, and defined if | |
1543 | it exists, but the reverse doesn't necessarily hold true. | |
1544 | ||
1545 | Given an expression that specifies the name of a subroutine, | |
1546 | returns true if the specified subroutine has ever been declared, even | |
1547 | if it is undefined. Mentioning a subroutine name for exists or defined | |
1548 | does not count as declaring it. Note that a subroutine which does not | |
1549 | exist may still be callable: its package may have an C<AUTOLOAD> | |
1550 | method that makes it spring into existence the first time that it is | |
1551 | called -- see L<perlsub>. | |
1552 | ||
1553 | print "Exists\n" if exists &subroutine; | |
1554 | print "Defined\n" if defined &subroutine; | |
1555 | ||
1556 | Note that the EXPR can be arbitrarily complicated as long as the final | |
1557 | operation is a hash or array key lookup or subroutine name: | |
1558 | ||
1559 | if (exists $ref->{A}->{B}->{$key}) { } | |
1560 | if (exists $hash{A}{B}{$key}) { } | |
1561 | ||
1562 | if (exists $ref->{A}->{B}->[$ix]) { } | |
1563 | if (exists $hash{A}{B}[$ix]) { } | |
1564 | ||
1565 | if (exists &{$ref->{A}{B}{$key}}) { } | |
1566 | ||
1567 | Although the deepest nested array or hash will not spring into existence | |
1568 | just because its existence was tested, any intervening ones will. | |
1569 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring | |
1570 | into existence due to the existence test for the $key element above. | |
1571 | This happens anywhere the arrow operator is used, including even: | |
1572 | ||
1573 | undef $ref; | |
1574 | if (exists $ref->{"Some key"}) { } | |
1575 | print $ref; # prints HASH(0x80d3d5c) | |
1576 | ||
1577 | This surprising autovivification in what does not at first--or even | |
1578 | second--glance appear to be an lvalue context may be fixed in a future | |
1579 | release. | |
1580 | ||
1581 | See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics | |
1582 | on how exists() acts when used on a pseudo-hash. | |
1583 | ||
1584 | Use of a subroutine call, rather than a subroutine name, as an argument | |
1585 | to exists() is an error. | |
1586 | ||
1587 | exists ⊂ # OK | |
1588 | exists &sub(); # Error | |
1589 | ||
1590 | =item exit EXPR | |
1591 | ||
1592 | Evaluates EXPR and exits immediately with that value. Example: | |
1593 | ||
1594 | $ans = <STDIN>; | |
1595 | exit 0 if $ans =~ /^[Xx]/; | |
1596 | ||
1597 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only | |
1598 | universally recognized values for EXPR are C<0> for success and C<1> | |
1599 | for error; other values are subject to interpretation depending on the | |
1600 | environment in which the Perl program is running. For example, exiting | |
1601 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1602 | the mailer to return the item undelivered, but that's not true everywhere. | |
1603 | ||
1604 | Don't use C<exit> to abort a subroutine if there's any chance that | |
1605 | someone might want to trap whatever error happened. Use C<die> instead, | |
1606 | which can be trapped by an C<eval>. | |
1607 | ||
1608 | The exit() function does not always exit immediately. It calls any | |
1609 | defined C<END> routines first, but these C<END> routines may not | |
1610 | themselves abort the exit. Likewise any object destructors that need to | |
1611 | be called are called before the real exit. If this is a problem, you | |
1612 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. | |
1613 | See L<perlmod> for details. | |
1614 | ||
1615 | =item exp EXPR | |
1616 | ||
1617 | =item exp | |
1618 | ||
1619 | Returns I<e> (the natural logarithm base) to the power of EXPR. | |
1620 | If EXPR is omitted, gives C<exp($_)>. | |
1621 | ||
1622 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
1623 | ||
1624 | Implements the fcntl(2) function. You'll probably have to say | |
1625 | ||
1626 | use Fcntl; | |
1627 | ||
1628 | first to get the correct constant definitions. Argument processing and | |
1629 | value return works just like C<ioctl> below. | |
1630 | For example: | |
1631 | ||
1632 | use Fcntl; | |
1633 | fcntl($filehandle, F_GETFL, $packed_return_buffer) | |
1634 | or die "can't fcntl F_GETFL: $!"; | |
1635 | ||
1636 | You don't have to check for C<defined> on the return from C<fnctl>. | |
1637 | Like C<ioctl>, it maps a C<0> return from the system call into | |
1638 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
1639 | in numeric context. It is also exempt from the normal B<-w> warnings | |
1640 | on improper numeric conversions. | |
1641 | ||
1642 | Note that C<fcntl> will produce a fatal error if used on a machine that | |
1643 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) | |
1644 | manpage to learn what functions are available on your system. | |
1645 | ||
1646 | =item fileno FILEHANDLE | |
1647 | ||
1648 | Returns the file descriptor for a filehandle, or undefined if the | |
1649 | filehandle is not open. This is mainly useful for constructing | |
1650 | bitmaps for C<select> and low-level POSIX tty-handling operations. | |
1651 | If FILEHANDLE is an expression, the value is taken as an indirect | |
1652 | filehandle, generally its name. | |
1653 | ||
1654 | You can use this to find out whether two handles refer to the | |
1655 | same underlying descriptor: | |
1656 | ||
1657 | if (fileno(THIS) == fileno(THAT)) { | |
1658 | print "THIS and THAT are dups\n"; | |
1659 | } | |
1660 | ||
1661 | (Filehandles connected to memory objects via new features of C<open> may | |
1662 | return undefined even though they are open.) | |
1663 | ||
1664 | ||
1665 | =item flock FILEHANDLE,OPERATION | |
1666 | ||
1667 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true | |
1668 | for success, false on failure. Produces a fatal error if used on a | |
1669 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). | |
1670 | C<flock> is Perl's portable file locking interface, although it locks | |
1671 | only entire files, not records. | |
1672 | ||
1673 | Two potentially non-obvious but traditional C<flock> semantics are | |
1674 | that it waits indefinitely until the lock is granted, and that its locks | |
1675 | B<merely advisory>. Such discretionary locks are more flexible, but offer | |
1676 | fewer guarantees. This means that files locked with C<flock> may be | |
1677 | modified by programs that do not also use C<flock>. See L<perlport>, | |
1678 | your port's specific documentation, or your system-specific local manpages | |
1679 | for details. It's best to assume traditional behavior if you're writing | |
1680 | portable programs. (But if you're not, you should as always feel perfectly | |
1681 | free to write for your own system's idiosyncrasies (sometimes called | |
1682 | "features"). Slavish adherence to portability concerns shouldn't get | |
1683 | in the way of your getting your job done.) | |
1684 | ||
1685 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with | |
1686 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
1687 | you can use the symbolic names if you import them from the Fcntl module, | |
1688 | either individually, or as a group using the ':flock' tag. LOCK_SH | |
1689 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
1690 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with | |
1691 | LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking | |
1692 | waiting for the lock (check the return status to see if you got it). | |
1693 | ||
1694 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE | |
1695 | before locking or unlocking it. | |
1696 | ||
1697 | Note that the emulation built with lockf(3) doesn't provide shared | |
1698 | locks, and it requires that FILEHANDLE be open with write intent. These | |
1699 | are the semantics that lockf(3) implements. Most if not all systems | |
1700 | implement lockf(3) in terms of fcntl(2) locking, though, so the | |
1701 | differing semantics shouldn't bite too many people. | |
1702 | ||
1703 | Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE | |
1704 | be open with read intent to use LOCK_SH and requires that it be open | |
1705 | with write intent to use LOCK_EX. | |
1706 | ||
1707 | Note also that some versions of C<flock> cannot lock things over the | |
1708 | network; you would need to use the more system-specific C<fcntl> for | |
1709 | that. If you like you can force Perl to ignore your system's flock(2) | |
1710 | function, and so provide its own fcntl(2)-based emulation, by passing | |
1711 | the switch C<-Ud_flock> to the F<Configure> program when you configure | |
1712 | perl. | |
1713 | ||
1714 | Here's a mailbox appender for BSD systems. | |
1715 | ||
1716 | use Fcntl ':flock'; # import LOCK_* constants | |
1717 | ||
1718 | sub lock { | |
1719 | flock(MBOX,LOCK_EX); | |
1720 | # and, in case someone appended | |
1721 | # while we were waiting... | |
1722 | seek(MBOX, 0, 2); | |
1723 | } | |
1724 | ||
1725 | sub unlock { | |
1726 | flock(MBOX,LOCK_UN); | |
1727 | } | |
1728 | ||
1729 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") | |
1730 | or die "Can't open mailbox: $!"; | |
1731 | ||
1732 | lock(); | |
1733 | print MBOX $msg,"\n\n"; | |
1734 | unlock(); | |
1735 | ||
1736 | On systems that support a real flock(), locks are inherited across fork() | |
1737 | calls, whereas those that must resort to the more capricious fcntl() | |
1738 | function lose the locks, making it harder to write servers. | |
1739 | ||
1740 | See also L<DB_File> for other flock() examples. | |
1741 | ||
1742 | =item fork | |
1743 | ||
1744 | Does a fork(2) system call to create a new process running the | |
1745 | same program at the same point. It returns the child pid to the | |
1746 | parent process, C<0> to the child process, or C<undef> if the fork is | |
1747 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
1748 | are shared, while everything else is copied. On most systems supporting | |
1749 | fork(), great care has gone into making it extremely efficient (for | |
1750 | example, using copy-on-write technology on data pages), making it the | |
1751 | dominant paradigm for multitasking over the last few decades. | |
1752 | ||
1753 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
1754 | output before forking the child process, but this may not be supported | |
1755 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1756 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1757 | C<IO::Handle> on any open handles in order to avoid duplicate output. | |
1758 | ||
1759 | If you C<fork> without ever waiting on your children, you will | |
1760 | accumulate zombies. On some systems, you can avoid this by setting | |
1761 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
1762 | forking and reaping moribund children. | |
1763 | ||
1764 | Note that if your forked child inherits system file descriptors like | |
1765 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
1766 | if you exit, then the remote server (such as, say, a CGI script or a | |
1767 | backgrounded job launched from a remote shell) won't think you're done. | |
1768 | You should reopen those to F</dev/null> if it's any issue. | |
1769 | ||
1770 | =item format | |
1771 | ||
1772 | Declare a picture format for use by the C<write> function. For | |
1773 | example: | |
1774 | ||
1775 | format Something = | |
1776 | Test: @<<<<<<<< @||||| @>>>>> | |
1777 | $str, $%, '$' . int($num) | |
1778 | . | |
1779 | ||
1780 | $str = "widget"; | |
1781 | $num = $cost/$quantity; | |
1782 | $~ = 'Something'; | |
1783 | write; | |
1784 | ||
1785 | See L<perlform> for many details and examples. | |
1786 | ||
1787 | =item formline PICTURE,LIST | |
1788 | ||
1789 | This is an internal function used by C<format>s, though you may call it, | |
1790 | too. It formats (see L<perlform>) a list of values according to the | |
1791 | contents of PICTURE, placing the output into the format output | |
1792 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). | |
1793 | Eventually, when a C<write> is done, the contents of | |
1794 | C<$^A> are written to some filehandle, but you could also read C<$^A> | |
1795 | yourself and then set C<$^A> back to C<"">. Note that a format typically | |
1796 | does one C<formline> per line of form, but the C<formline> function itself | |
1797 | doesn't care how many newlines are embedded in the PICTURE. This means | |
1798 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. | |
1799 | You may therefore need to use multiple formlines to implement a single | |
1800 | record format, just like the format compiler. | |
1801 | ||
1802 | Be careful if you put double quotes around the picture, because an C<@> | |
1803 | character may be taken to mean the beginning of an array name. | |
1804 | C<formline> always returns true. See L<perlform> for other examples. | |
1805 | ||
1806 | =item getc FILEHANDLE | |
1807 | ||
1808 | =item getc | |
1809 | ||
1810 | Returns the next character from the input file attached to FILEHANDLE, | |
1811 | or the undefined value at end of file, or if there was an error. | |
1812 | If FILEHANDLE is omitted, reads from STDIN. This is not particularly | |
1813 | efficient. However, it cannot be used by itself to fetch single | |
1814 | characters without waiting for the user to hit enter. For that, try | |
1815 | something more like: | |
1816 | ||
1817 | if ($BSD_STYLE) { | |
1818 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; | |
1819 | } | |
1820 | else { | |
1821 | system "stty", '-icanon', 'eol', "\001"; | |
1822 | } | |
1823 | ||
1824 | $key = getc(STDIN); | |
1825 | ||
1826 | if ($BSD_STYLE) { | |
1827 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; | |
1828 | } | |
1829 | else { | |
1830 | system "stty", 'icanon', 'eol', '^@'; # ASCII null | |
1831 | } | |
1832 | print "\n"; | |
1833 | ||
1834 | Determination of whether $BSD_STYLE should be set | |
1835 | is left as an exercise to the reader. | |
1836 | ||
1837 | The C<POSIX::getattr> function can do this more portably on | |
1838 | systems purporting POSIX compliance. See also the C<Term::ReadKey> | |
1839 | module from your nearest CPAN site; details on CPAN can be found on | |
1840 | L<perlmodlib/CPAN>. | |
1841 | ||
1842 | =item getlogin | |
1843 | ||
1844 | Implements the C library function of the same name, which on most | |
1845 | systems returns the current login from F</etc/utmp>, if any. If null, | |
1846 | use C<getpwuid>. | |
1847 | ||
1848 | $login = getlogin || getpwuid($<) || "Kilroy"; | |
1849 | ||
1850 | Do not consider C<getlogin> for authentication: it is not as | |
1851 | secure as C<getpwuid>. | |
1852 | ||
1853 | =item getpeername SOCKET | |
1854 | ||
1855 | Returns the packed sockaddr address of other end of the SOCKET connection. | |
1856 | ||
1857 | use Socket; | |
1858 | $hersockaddr = getpeername(SOCK); | |
1859 | ($port, $iaddr) = sockaddr_in($hersockaddr); | |
1860 | $herhostname = gethostbyaddr($iaddr, AF_INET); | |
1861 | $herstraddr = inet_ntoa($iaddr); | |
1862 | ||
1863 | =item getpgrp PID | |
1864 | ||
1865 | Returns the current process group for the specified PID. Use | |
1866 | a PID of C<0> to get the current process group for the | |
1867 | current process. Will raise an exception if used on a machine that | |
1868 | doesn't implement getpgrp(2). If PID is omitted, returns process | |
1869 | group of current process. Note that the POSIX version of C<getpgrp> | |
1870 | does not accept a PID argument, so only C<PID==0> is truly portable. | |
1871 | ||
1872 | =item getppid | |
1873 | ||
1874 | Returns the process id of the parent process. | |
1875 | ||
1876 | =item getpriority WHICH,WHO | |
1877 | ||
1878 | Returns the current priority for a process, a process group, or a user. | |
1879 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a | |
1880 | machine that doesn't implement getpriority(2). | |
1881 | ||
1882 | =item getpwnam NAME | |
1883 | ||
1884 | =item getgrnam NAME | |
1885 | ||
1886 | =item gethostbyname NAME | |
1887 | ||
1888 | =item getnetbyname NAME | |
1889 | ||
1890 | =item getprotobyname NAME | |
1891 | ||
1892 | =item getpwuid UID | |
1893 | ||
1894 | =item getgrgid GID | |
1895 | ||
1896 | =item getservbyname NAME,PROTO | |
1897 | ||
1898 | =item gethostbyaddr ADDR,ADDRTYPE | |
1899 | ||
1900 | =item getnetbyaddr ADDR,ADDRTYPE | |
1901 | ||
1902 | =item getprotobynumber NUMBER | |
1903 | ||
1904 | =item getservbyport PORT,PROTO | |
1905 | ||
1906 | =item getpwent | |
1907 | ||
1908 | =item getgrent | |
1909 | ||
1910 | =item gethostent | |
1911 | ||
1912 | =item getnetent | |
1913 | ||
1914 | =item getprotoent | |
1915 | ||
1916 | =item getservent | |
1917 | ||
1918 | =item setpwent | |
1919 | ||
1920 | =item setgrent | |
1921 | ||
1922 | =item sethostent STAYOPEN | |
1923 | ||
1924 | =item setnetent STAYOPEN | |
1925 | ||
1926 | =item setprotoent STAYOPEN | |
1927 | ||
1928 | =item setservent STAYOPEN | |
1929 | ||
1930 | =item endpwent | |
1931 | ||
1932 | =item endgrent | |
1933 | ||
1934 | =item endhostent | |
1935 | ||
1936 | =item endnetent | |
1937 | ||
1938 | =item endprotoent | |
1939 | ||
1940 | =item endservent | |
1941 | ||
1942 | These routines perform the same functions as their counterparts in the | |
1943 | system library. In list context, the return values from the | |
1944 | various get routines are as follows: | |
1945 | ||
1946 | ($name,$passwd,$uid,$gid, | |
1947 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* | |
1948 | ($name,$passwd,$gid,$members) = getgr* | |
1949 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
1950 | ($name,$aliases,$addrtype,$net) = getnet* | |
1951 | ($name,$aliases,$proto) = getproto* | |
1952 | ($name,$aliases,$port,$proto) = getserv* | |
1953 | ||
1954 | (If the entry doesn't exist you get a null list.) | |
1955 | ||
1956 | The exact meaning of the $gcos field varies but it usually contains | |
1957 | the real name of the user (as opposed to the login name) and other | |
1958 | information pertaining to the user. Beware, however, that in many | |
1959 | system users are able to change this information and therefore it | |
1960 | cannot be trusted and therefore the $gcos is tainted (see | |
1961 | L<perlsec>). The $passwd and $shell, user's encrypted password and | |
1962 | login shell, are also tainted, because of the same reason. | |
1963 | ||
1964 | In scalar context, you get the name, unless the function was a | |
1965 | lookup by name, in which case you get the other thing, whatever it is. | |
1966 | (If the entry doesn't exist you get the undefined value.) For example: | |
1967 | ||
1968 | $uid = getpwnam($name); | |
1969 | $name = getpwuid($num); | |
1970 | $name = getpwent(); | |
1971 | $gid = getgrnam($name); | |
1972 | $name = getgrgid($num; | |
1973 | $name = getgrent(); | |
1974 | #etc. | |
1975 | ||
1976 | In I<getpw*()> the fields $quota, $comment, and $expire are special | |
1977 | cases in the sense that in many systems they are unsupported. If the | |
1978 | $quota is unsupported, it is an empty scalar. If it is supported, it | |
1979 | usually encodes the disk quota. If the $comment field is unsupported, | |
1980 | it is an empty scalar. If it is supported it usually encodes some | |
1981 | administrative comment about the user. In some systems the $quota | |
1982 | field may be $change or $age, fields that have to do with password | |
1983 | aging. In some systems the $comment field may be $class. The $expire | |
1984 | field, if present, encodes the expiration period of the account or the | |
1985 | password. For the availability and the exact meaning of these fields | |
1986 | in your system, please consult your getpwnam(3) documentation and your | |
1987 | F<pwd.h> file. You can also find out from within Perl what your | |
1988 | $quota and $comment fields mean and whether you have the $expire field | |
1989 | by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, | |
1990 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password | |
1991 | files are only supported if your vendor has implemented them in the | |
1992 | intuitive fashion that calling the regular C library routines gets the | |
1993 | shadow versions if you're running under privilege or if there exists | |
1994 | the shadow(3) functions as found in System V ( this includes Solaris | |
1995 | and Linux.) Those systems which implement a proprietary shadow password | |
1996 | facility are unlikely to be supported. | |
1997 | ||
1998 | The $members value returned by I<getgr*()> is a space separated list of | |
1999 | the login names of the members of the group. | |
2000 | ||
2001 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
2002 | C, it will be returned to you via C<$?> if the function call fails. The | |
2003 | C<@addrs> value returned by a successful call is a list of the raw | |
2004 | addresses returned by the corresponding system library call. In the | |
2005 | Internet domain, each address is four bytes long and you can unpack it | |
2006 | by saying something like: | |
2007 | ||
2008 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); | |
2009 | ||
2010 | The Socket library makes this slightly easier: | |
2011 | ||
2012 | use Socket; | |
2013 | $iaddr = inet_aton("127.1"); # or whatever address | |
2014 | $name = gethostbyaddr($iaddr, AF_INET); | |
2015 | ||
2016 | # or going the other way | |
2017 | $straddr = inet_ntoa($iaddr); | |
2018 | ||
2019 | If you get tired of remembering which element of the return list | |
2020 | contains which return value, by-name interfaces are provided | |
2021 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
2022 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
2023 | and C<User::grent>. These override the normal built-ins, supplying | |
2024 | versions that return objects with the appropriate names | |
2025 | for each field. For example: | |
2026 | ||
2027 | use File::stat; | |
2028 | use User::pwent; | |
2029 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
2030 | ||
2031 | Even though it looks like they're the same method calls (uid), | |
2032 | they aren't, because a C<File::stat> object is different from | |
2033 | a C<User::pwent> object. | |
2034 | ||
2035 | =item getsockname SOCKET | |
2036 | ||
2037 | Returns the packed sockaddr address of this end of the SOCKET connection, | |
2038 | in case you don't know the address because you have several different | |
2039 | IPs that the connection might have come in on. | |
2040 | ||
2041 | use Socket; | |
2042 | $mysockaddr = getsockname(SOCK); | |
2043 | ($port, $myaddr) = sockaddr_in($mysockaddr); | |
2044 | printf "Connect to %s [%s]\n", | |
2045 | scalar gethostbyaddr($myaddr, AF_INET), | |
2046 | inet_ntoa($myaddr); | |
2047 | ||
2048 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
2049 | ||
2050 | Returns the socket option requested, or undef if there is an error. | |
2051 | ||
2052 | =item glob EXPR | |
2053 | ||
2054 | =item glob | |
2055 | ||
2056 | In list context, returns a (possibly empty) list of filename expansions on | |
2057 | the value of EXPR such as the standard Unix shell F</bin/csh> would do. In | |
2058 | scalar context, glob iterates through such filename expansions, returning | |
2059 | undef when the list is exhausted. This is the internal function | |
2060 | implementing the C<< <*.c> >> operator, but you can use it directly. If | |
2061 | EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in | |
2062 | more detail in L<perlop/"I/O Operators">. | |
2063 | ||
2064 | Beginning with v5.6.0, this operator is implemented using the standard | |
2065 | C<File::Glob> extension. See L<File::Glob> for details. | |
2066 | ||
2067 | =item gmtime EXPR | |
2068 | ||
2069 | Converts a time as returned by the time function to an 8-element list | |
2070 | with the time localized for the standard Greenwich time zone. | |
2071 | Typically used as follows: | |
2072 | ||
2073 | # 0 1 2 3 4 5 6 7 | |
2074 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = | |
2075 | gmtime(time); | |
2076 | ||
2077 | All list elements are numeric, and come straight out of the C `struct | |
2078 | tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the | |
2079 | specified time. $mday is the day of the month, and $mon is the month | |
2080 | itself, in the range C<0..11> with 0 indicating January and 11 | |
2081 | indicating December. $year is the number of years since 1900. That | |
2082 | is, $year is C<123> in year 2023. $wday is the day of the week, with | |
2083 | 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of | |
2084 | the year, in the range C<0..364> (or C<0..365> in leap years.) | |
2085 | ||
2086 | Note that the $year element is I<not> simply the last two digits of | |
2087 | the year. If you assume it is, then you create non-Y2K-compliant | |
2088 | programs--and you wouldn't want to do that, would you? | |
2089 | ||
2090 | The proper way to get a complete 4-digit year is simply: | |
2091 | ||
2092 | $year += 1900; | |
2093 | ||
2094 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
2095 | ||
2096 | $year = sprintf("%02d", $year % 100); | |
2097 | ||
2098 | If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>). | |
2099 | ||
2100 | In scalar context, C<gmtime()> returns the ctime(3) value: | |
2101 | ||
2102 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
2103 | ||
2104 | Also see the C<timegm> function provided by the C<Time::Local> module, | |
2105 | and the strftime(3) function available via the POSIX module. | |
2106 | ||
2107 | This scalar value is B<not> locale dependent (see L<perllocale>), but | |
2108 | is instead a Perl builtin. Also see the C<Time::Local> module, and the | |
2109 | strftime(3) and mktime(3) functions available via the POSIX module. To | |
2110 | get somewhat similar but locale dependent date strings, set up your | |
2111 | locale environment variables appropriately (please see L<perllocale>) | |
2112 | and try for example: | |
2113 | ||
2114 | use POSIX qw(strftime); | |
2115 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; | |
2116 | ||
2117 | Note that the C<%a> and C<%b> escapes, which represent the short forms | |
2118 | of the day of the week and the month of the year, may not necessarily | |
2119 | be three characters wide in all locales. | |
2120 | ||
2121 | =item goto LABEL | |
2122 | ||
2123 | =item goto EXPR | |
2124 | ||
2125 | =item goto &NAME | |
2126 | ||
2127 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes | |
2128 | execution there. It may not be used to go into any construct that | |
2129 | requires initialization, such as a subroutine or a C<foreach> loop. It | |
2130 | also can't be used to go into a construct that is optimized away, | |
2131 | or to get out of a block or subroutine given to C<sort>. | |
2132 | It can be used to go almost anywhere else within the dynamic scope, | |
2133 | including out of subroutines, but it's usually better to use some other | |
2134 | construct such as C<last> or C<die>. The author of Perl has never felt the | |
2135 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
2136 | (The difference being that C does not offer named loops combined with | |
2137 | loop control. Perl does, and this replaces most structured uses of C<goto> | |
2138 | in other languages.) | |
2139 | ||
2140 | The C<goto-EXPR> form expects a label name, whose scope will be resolved | |
2141 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
2142 | necessarily recommended if you're optimizing for maintainability: | |
2143 | ||
2144 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2145 | ||
2146 | The C<goto-&NAME> form is quite different from the other forms of | |
2147 | C<goto>. In fact, it isn't a goto in the normal sense at all, and | |
2148 | doesn't have the stigma associated with other gotos. Instead, it | |
2149 | exits the current subroutine (losing any changes set by local()) and | |
2150 | immediately calls in its place the named subroutine using the current | |
2151 | value of @_. This is used by C<AUTOLOAD> subroutines that wish to | |
2152 | load another subroutine and then pretend that the other subroutine had | |
2153 | been called in the first place (except that any modifications to C<@_> | |
2154 | in the current subroutine are propagated to the other subroutine.) | |
2155 | After the C<goto>, not even C<caller> will be able to tell that this | |
2156 | routine was called first. | |
2157 | ||
2158 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
2159 | containing a code reference, or a block which evaluates to a code | |
2160 | reference. | |
2161 | ||
2162 | =item grep BLOCK LIST | |
2163 | ||
2164 | =item grep EXPR,LIST | |
2165 | ||
2166 | This is similar in spirit to, but not the same as, grep(1) and its | |
2167 | relatives. In particular, it is not limited to using regular expressions. | |
2168 | ||
2169 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting | |
2170 | C<$_> to each element) and returns the list value consisting of those | |
2171 | elements for which the expression evaluated to true. In scalar | |
2172 | context, returns the number of times the expression was true. | |
2173 | ||
2174 | @foo = grep(!/^#/, @bar); # weed out comments | |
2175 | ||
2176 | or equivalently, | |
2177 | ||
2178 | @foo = grep {!/^#/} @bar; # weed out comments | |
2179 | ||
2180 | Note that C<$_> is an alias to the list value, so it can be used to | |
2181 | modify the elements of the LIST. While this is useful and supported, | |
2182 | it can cause bizarre results if the elements of LIST are not variables. | |
2183 | Similarly, grep returns aliases into the original list, much as a for | |
2184 | loop's index variable aliases the list elements. That is, modifying an | |
2185 | element of a list returned by grep (for example, in a C<foreach>, C<map> | |
2186 | or another C<grep>) actually modifies the element in the original list. | |
2187 | This is usually something to be avoided when writing clear code. | |
2188 | ||
2189 | See also L</map> for a list composed of the results of the BLOCK or EXPR. | |
2190 | ||
2191 | =item hex EXPR | |
2192 | ||
2193 | =item hex | |
2194 | ||
2195 | Interprets EXPR as a hex string and returns the corresponding value. | |
2196 | (To convert strings that might start with either 0, 0x, or 0b, see | |
2197 | L</oct>.) If EXPR is omitted, uses C<$_>. | |
2198 | ||
2199 | print hex '0xAf'; # prints '175' | |
2200 | print hex 'aF'; # same | |
2201 | ||
2202 | Hex strings may only represent integers. Strings that would cause | |
2203 | integer overflow trigger a warning. Leading whitespace is not stripped, | |
2204 | unlike oct(). | |
2205 | ||
2206 | =item import | |
2207 | ||
2208 | There is no builtin C<import> function. It is just an ordinary | |
2209 | method (subroutine) defined (or inherited) by modules that wish to export | |
2210 | names to another module. The C<use> function calls the C<import> method | |
2211 | for the package used. See also L</use>, L<perlmod>, and L<Exporter>. | |
2212 | ||
2213 | =item index STR,SUBSTR,POSITION | |
2214 | ||
2215 | =item index STR,SUBSTR | |
2216 | ||
2217 | The index function searches for one string within another, but without | |
2218 | the wildcard-like behavior of a full regular-expression pattern match. | |
2219 | It returns the position of the first occurrence of SUBSTR in STR at | |
2220 | or after POSITION. If POSITION is omitted, starts searching from the | |
2221 | beginning of the string. The return value is based at C<0> (or whatever | |
2222 | you've set the C<$[> variable to--but don't do that). If the substring | |
2223 | is not found, returns one less than the base, ordinarily C<-1>. | |
2224 | ||
2225 | =item int EXPR | |
2226 | ||
2227 | =item int | |
2228 | ||
2229 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. | |
2230 | You should not use this function for rounding: one because it truncates | |
2231 | towards C<0>, and two because machine representations of floating point | |
2232 | numbers can sometimes produce counterintuitive results. For example, | |
2233 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2234 | because it's really more like -268.99999999999994315658 instead. Usually, | |
2235 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> | |
2236 | functions will serve you better than will int(). | |
2237 | ||
2238 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
2239 | ||
2240 | Implements the ioctl(2) function. You'll probably first have to say | |
2241 | ||
2242 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph | |
2243 | ||
2244 | to get the correct function definitions. If F<ioctl.ph> doesn't | |
2245 | exist or doesn't have the correct definitions you'll have to roll your | |
2246 | own, based on your C header files such as F<< <sys/ioctl.h> >>. | |
2247 | (There is a Perl script called B<h2ph> that comes with the Perl kit that | |
2248 | may help you in this, but it's nontrivial.) SCALAR will be read and/or | |
2249 | written depending on the FUNCTION--a pointer to the string value of SCALAR | |
2250 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR | |
2251 | has no string value but does have a numeric value, that value will be | |
2252 | passed rather than a pointer to the string value. To guarantee this to be | |
2253 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> | |
2254 | functions may be needed to manipulate the values of structures used by | |
2255 | C<ioctl>. | |
2256 | ||
2257 | The return value of C<ioctl> (and C<fcntl>) is as follows: | |
2258 | ||
2259 | if OS returns: then Perl returns: | |
2260 | -1 undefined value | |
2261 | 0 string "0 but true" | |
2262 | anything else that number | |
2263 | ||
2264 | Thus Perl returns true on success and false on failure, yet you can | |
2265 | still easily determine the actual value returned by the operating | |
2266 | system: | |
2267 | ||
2268 | $retval = ioctl(...) || -1; | |
2269 | printf "System returned %d\n", $retval; | |
2270 | ||
2271 | The special string "C<0> but true" is exempt from B<-w> complaints | |
2272 | about improper numeric conversions. | |
2273 | ||
2274 | Here's an example of setting a filehandle named C<REMOTE> to be | |
2275 | non-blocking at the system level. You'll have to negotiate C<$|> | |
2276 | on your own, though. | |
2277 | ||
2278 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
2279 | ||
2280 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
2281 | or die "Can't get flags for the socket: $!\n"; | |
2282 | ||
2283 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
2284 | or die "Can't set flags for the socket: $!\n"; | |
2285 | ||
2286 | =item join EXPR,LIST | |
2287 | ||
2288 | Joins the separate strings of LIST into a single string with fields | |
2289 | separated by the value of EXPR, and returns that new string. Example: | |
2290 | ||
2291 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); | |
2292 | ||
2293 | Beware that unlike C<split>, C<join> doesn't take a pattern as its | |
2294 | first argument. Compare L</split>. | |
2295 | ||
2296 | =item keys HASH | |
2297 | ||
2298 | Returns a list consisting of all the keys of the named hash. (In | |
2299 | scalar context, returns the number of keys.) The keys are returned in | |
2300 | an apparently random order. The actual random order is subject to | |
2301 | change in future versions of perl, but it is guaranteed to be the same | |
2302 | order as either the C<values> or C<each> function produces (given | |
2303 | that the hash has not been modified). As a side effect, it resets | |
2304 | HASH's iterator. | |
2305 | ||
2306 | Here is yet another way to print your environment: | |
2307 | ||
2308 | @keys = keys %ENV; | |
2309 | @values = values %ENV; | |
2310 | while (@keys) { | |
2311 | print pop(@keys), '=', pop(@values), "\n"; | |
2312 | } | |
2313 | ||
2314 | or how about sorted by key: | |
2315 | ||
2316 | foreach $key (sort(keys %ENV)) { | |
2317 | print $key, '=', $ENV{$key}, "\n"; | |
2318 | } | |
2319 | ||
2320 | The returned values are copies of the original keys in the hash, so | |
2321 | modifying them will not affect the original hash. Compare L</values>. | |
2322 | ||
2323 | To sort a hash by value, you'll need to use a C<sort> function. | |
2324 | Here's a descending numeric sort of a hash by its values: | |
2325 | ||
2326 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { | |
2327 | printf "%4d %s\n", $hash{$key}, $key; | |
2328 | } | |
2329 | ||
2330 | As an lvalue C<keys> allows you to increase the number of hash buckets | |
2331 | allocated for the given hash. This can gain you a measure of efficiency if | |
2332 | you know the hash is going to get big. (This is similar to pre-extending | |
2333 | an array by assigning a larger number to $#array.) If you say | |
2334 | ||
2335 | keys %hash = 200; | |
2336 | ||
2337 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, | |
2338 | in fact, since it rounds up to the next power of two. These | |
2339 | buckets will be retained even if you do C<%hash = ()>, use C<undef | |
2340 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2341 | You can't shrink the number of buckets allocated for the hash using | |
2342 | C<keys> in this way (but you needn't worry about doing this by accident, | |
2343 | as trying has no effect). | |
2344 | ||
2345 | See also C<each>, C<values> and C<sort>. | |
2346 | ||
2347 | =item kill SIGNAL, LIST | |
2348 | ||
2349 | Sends a signal to a list of processes. Returns the number of | |
2350 | processes successfully signaled (which is not necessarily the | |
2351 | same as the number actually killed). | |
2352 | ||
2353 | $cnt = kill 1, $child1, $child2; | |
2354 | kill 9, @goners; | |
2355 | ||
2356 | If SIGNAL is zero, no signal is sent to the process. This is a | |
2357 | useful way to check that the process is alive and hasn't changed | |
2358 | its UID. See L<perlport> for notes on the portability of this | |
2359 | construct. | |
2360 | ||
2361 | Unlike in the shell, if SIGNAL is negative, it kills | |
2362 | process groups instead of processes. (On System V, a negative I<PROCESS> | |
2363 | number will also kill process groups, but that's not portable.) That | |
2364 | means you usually want to use positive not negative signals. You may also | |
2365 | use a signal name in quotes. See L<perlipc/"Signals"> for details. | |
2366 | ||
2367 | =item last LABEL | |
2368 | ||
2369 | =item last | |
2370 | ||
2371 | The C<last> command is like the C<break> statement in C (as used in | |
2372 | loops); it immediately exits the loop in question. If the LABEL is | |
2373 | omitted, the command refers to the innermost enclosing loop. The | |
2374 | C<continue> block, if any, is not executed: | |
2375 | ||
2376 | LINE: while (<STDIN>) { | |
2377 | last LINE if /^$/; # exit when done with header | |
2378 | #... | |
2379 | } | |
2380 | ||
2381 | C<last> cannot be used to exit a block which returns a value such as | |
2382 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit | |
2383 | a grep() or map() operation. | |
2384 | ||
2385 | Note that a block by itself is semantically identical to a loop | |
2386 | that executes once. Thus C<last> can be used to effect an early | |
2387 | exit out of such a block. | |
2388 | ||
2389 | See also L</continue> for an illustration of how C<last>, C<next>, and | |
2390 | C<redo> work. | |
2391 | ||
2392 | =item lc EXPR | |
2393 | ||
2394 | =item lc | |
2395 | ||
2396 | Returns a lowercased version of EXPR. This is the internal function | |
2397 | implementing the C<\L> escape in double-quoted strings. Respects | |
2398 | current LC_CTYPE locale if C<use locale> in force. See L<perllocale> | |
2399 | and L<perlunicode> for more details about locale and Unicode support. | |
2400 | ||
2401 | If EXPR is omitted, uses C<$_>. | |
2402 | ||
2403 | =item lcfirst EXPR | |
2404 | ||
2405 | =item lcfirst | |
2406 | ||
2407 | Returns the value of EXPR with the first character lowercased. This | |
2408 | is the internal function implementing the C<\l> escape in | |
2409 | double-quoted strings. Respects current LC_CTYPE locale if C<use | |
2410 | locale> in force. See L<perllocale> and L<perlunicode> for more | |
2411 | details about locale and Unicode support. | |
2412 | ||
2413 | If EXPR is omitted, uses C<$_>. | |
2414 | ||
2415 | =item length EXPR | |
2416 | ||
2417 | =item length | |
2418 | ||
2419 | Returns the length in characters of the value of EXPR. If EXPR is | |
2420 | omitted, returns length of C<$_>. Note that this cannot be used on | |
2421 | an entire array or hash to find out how many elements these have. | |
2422 | For that, use C<scalar @array> and C<scalar keys %hash> respectively. | |
2423 | ||
2424 | =item link OLDFILE,NEWFILE | |
2425 | ||
2426 | Creates a new filename linked to the old filename. Returns true for | |
2427 | success, false otherwise. | |
2428 | ||
2429 | =item listen SOCKET,QUEUESIZE | |
2430 | ||
2431 | Does the same thing that the listen system call does. Returns true if | |
2432 | it succeeded, false otherwise. See the example in | |
2433 | L<perlipc/"Sockets: Client/Server Communication">. | |
2434 | ||
2435 | =item local EXPR | |
2436 | ||
2437 | You really probably want to be using C<my> instead, because C<local> isn't | |
2438 | what most people think of as "local". See | |
2439 | L<perlsub/"Private Variables via my()"> for details. | |
2440 | ||
2441 | A local modifies the listed variables to be local to the enclosing | |
2442 | block, file, or eval. If more than one value is listed, the list must | |
2443 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
2444 | for details, including issues with tied arrays and hashes. | |
2445 | ||
2446 | =item localtime EXPR | |
2447 | ||
2448 | Converts a time as returned by the time function to a 9-element list | |
2449 | with the time analyzed for the local time zone. Typically used as | |
2450 | follows: | |
2451 | ||
2452 | # 0 1 2 3 4 5 6 7 8 | |
2453 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = | |
2454 | localtime(time); | |
2455 | ||
2456 | All list elements are numeric, and come straight out of the C `struct | |
2457 | tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the | |
2458 | specified time. $mday is the day of the month, and $mon is the month | |
2459 | itself, in the range C<0..11> with 0 indicating January and 11 | |
2460 | indicating December. $year is the number of years since 1900. That | |
2461 | is, $year is C<123> in year 2023. $wday is the day of the week, with | |
2462 | 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of | |
2463 | the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst | |
2464 | is true if the specified time occurs during daylight savings time, | |
2465 | false otherwise. | |
2466 | ||
2467 | Note that the $year element is I<not> simply the last two digits of | |
2468 | the year. If you assume it is, then you create non-Y2K-compliant | |
2469 | programs--and you wouldn't want to do that, would you? | |
2470 | ||
2471 | The proper way to get a complete 4-digit year is simply: | |
2472 | ||
2473 | $year += 1900; | |
2474 | ||
2475 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
2476 | ||
2477 | $year = sprintf("%02d", $year % 100); | |
2478 | ||
2479 | If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). | |
2480 | ||
2481 | In scalar context, C<localtime()> returns the ctime(3) value: | |
2482 | ||
2483 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
2484 | ||
2485 | This scalar value is B<not> locale dependent, see L<perllocale>, but | |
2486 | instead a Perl builtin. Also see the C<Time::Local> module | |
2487 | (to convert the second, minutes, hours, ... back to seconds since the | |
2488 | stroke of midnight the 1st of January 1970, the value returned by | |
2489 | time()), and the strftime(3) and mktime(3) functions available via the | |
2490 | POSIX module. To get somewhat similar but locale dependent date | |
2491 | strings, set up your locale environment variables appropriately | |
2492 | (please see L<perllocale>) and try for example: | |
2493 | ||
2494 | use POSIX qw(strftime); | |
2495 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; | |
2496 | ||
2497 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
2498 | and the month of the year, may not necessarily be three characters wide. | |
2499 | ||
2500 | =item lock THING | |
2501 | ||
2502 | This function places an advisory lock on a shared variable, or referenced | |
2503 | object contained in I<THING> until the lock goes out of scope. | |
2504 | ||
2505 | lock() is a "weak keyword" : this means that if you've defined a function | |
2506 | by this name (before any calls to it), that function will be called | |
2507 | instead. (However, if you've said C<use threads>, lock() is always a | |
2508 | keyword.) See L<threads>. | |
2509 | ||
2510 | =item log EXPR | |
2511 | ||
2512 | =item log | |
2513 | ||
2514 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, | |
2515 | returns log of C<$_>. To get the log of another base, use basic algebra: | |
2516 | The base-N log of a number is equal to the natural log of that number | |
2517 | divided by the natural log of N. For example: | |
2518 | ||
2519 | sub log10 { | |
2520 | my $n = shift; | |
2521 | return log($n)/log(10); | |
2522 | } | |
2523 | ||
2524 | See also L</exp> for the inverse operation. | |
2525 | ||
2526 | =item lstat EXPR | |
2527 | ||
2528 | =item lstat | |
2529 | ||
2530 | Does the same thing as the C<stat> function (including setting the | |
2531 | special C<_> filehandle) but stats a symbolic link instead of the file | |
2532 | the symbolic link points to. If symbolic links are unimplemented on | |
2533 | your system, a normal C<stat> is done. | |
2534 | ||
2535 | If EXPR is omitted, stats C<$_>. | |
2536 | ||
2537 | =item m// | |
2538 | ||
2539 | The match operator. See L<perlop>. | |
2540 | ||
2541 | =item map BLOCK LIST | |
2542 | ||
2543 | =item map EXPR,LIST | |
2544 | ||
2545 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting | |
2546 | C<$_> to each element) and returns the list value composed of the | |
2547 | results of each such evaluation. In scalar context, returns the | |
2548 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
2549 | list context, so each element of LIST may produce zero, one, or | |
2550 | more elements in the returned value. | |
2551 | ||
2552 | @chars = map(chr, @nums); | |
2553 | ||
2554 | translates a list of numbers to the corresponding characters. And | |
2555 | ||
2556 | %hash = map { getkey($_) => $_ } @array; | |
2557 | ||
2558 | is just a funny way to write | |
2559 | ||
2560 | %hash = (); | |
2561 | foreach $_ (@array) { | |
2562 | $hash{getkey($_)} = $_; | |
2563 | } | |
2564 | ||
2565 | Note that C<$_> is an alias to the list value, so it can be used to | |
2566 | modify the elements of the LIST. While this is useful and supported, | |
2567 | it can cause bizarre results if the elements of LIST are not variables. | |
2568 | Using a regular C<foreach> loop for this purpose would be clearer in | |
2569 | most cases. See also L</grep> for an array composed of those items of | |
2570 | the original list for which the BLOCK or EXPR evaluates to true. | |
2571 | ||
2572 | C<{> starts both hash references and blocks, so C<map { ...> could be either | |
2573 | the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look | |
2574 | ahead for the closing C<}> it has to take a guess at which its dealing with | |
2575 | based what it finds just after the C<{>. Usually it gets it right, but if it | |
2576 | doesn't it won't realize something is wrong until it gets to the C<}> and | |
2577 | encounters the missing (or unexpected) comma. The syntax error will be | |
2578 | reported close to the C<}> but you'll need to change something near the C<{> | |
2579 | such as using a unary C<+> to give perl some help: | |
2580 | ||
2581 | %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong | |
2582 | %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right | |
2583 | %hash = map { ("\L$_", 1) } @array # this also works | |
2584 | %hash = map { lc($_), 1 } @array # as does this. | |
2585 | %hash = map +( lc($_), 1 ), @array # this is EXPR and works! | |
2586 | ||
2587 | %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) | |
2588 | ||
2589 | or to force an anon hash constructor use C<+{> | |
2590 | ||
2591 | @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end | |
2592 | ||
2593 | and you get list of anonymous hashes each with only 1 entry. | |
2594 | ||
2595 | =item mkdir FILENAME,MASK | |
2596 | ||
2597 | =item mkdir FILENAME | |
2598 | ||
2599 | Creates the directory specified by FILENAME, with permissions | |
2600 | specified by MASK (as modified by C<umask>). If it succeeds it | |
2601 | returns true, otherwise it returns false and sets C<$!> (errno). | |
2602 | If omitted, MASK defaults to 0777. | |
2603 | ||
2604 | In general, it is better to create directories with permissive MASK, | |
2605 | and let the user modify that with their C<umask>, than it is to supply | |
2606 | a restrictive MASK and give the user no way to be more permissive. | |
2607 | The exceptions to this rule are when the file or directory should be | |
2608 | kept private (mail files, for instance). The perlfunc(1) entry on | |
2609 | C<umask> discusses the choice of MASK in more detail. | |
2610 | ||
2611 | Note that according to the POSIX 1003.1-1996 the FILENAME may have any | |
2612 | number of trailing slashes. Some operating and filesystems do not get | |
2613 | this right, so Perl automatically removes all trailing slashes to keep | |
2614 | everyone happy. | |
2615 | ||
2616 | =item msgctl ID,CMD,ARG | |
2617 | ||
2618 | Calls the System V IPC function msgctl(2). You'll probably have to say | |
2619 | ||
2620 | use IPC::SysV; | |
2621 | ||
2622 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, | |
2623 | then ARG must be a variable which will hold the returned C<msqid_ds> | |
2624 | structure. Returns like C<ioctl>: the undefined value for error, | |
2625 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
2626 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. | |
2627 | ||
2628 | =item msgget KEY,FLAGS | |
2629 | ||
2630 | Calls the System V IPC function msgget(2). Returns the message queue | |
2631 | id, or the undefined value if there is an error. See also | |
2632 | L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. | |
2633 | ||
2634 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS | |
2635 | ||
2636 | Calls the System V IPC function msgrcv to receive a message from | |
2637 | message queue ID into variable VAR with a maximum message size of | |
2638 | SIZE. Note that when a message is received, the message type as a | |
2639 | native long integer will be the first thing in VAR, followed by the | |
2640 | actual message. This packing may be opened with C<unpack("l! a*")>. | |
2641 | Taints the variable. Returns true if successful, or false if there is | |
2642 | an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and | |
2643 | C<IPC::SysV::Msg> documentation. | |
2644 | ||
2645 | =item msgsnd ID,MSG,FLAGS | |
2646 | ||
2647 | Calls the System V IPC function msgsnd to send the message MSG to the | |
2648 | message queue ID. MSG must begin with the native long integer message | |
2649 | type, and be followed by the length of the actual message, and finally | |
2650 | the message itself. This kind of packing can be achieved with | |
2651 | C<pack("l! a*", $type, $message)>. Returns true if successful, | |
2652 | or false if there is an error. See also C<IPC::SysV> | |
2653 | and C<IPC::SysV::Msg> documentation. | |
2654 | ||
2655 | =item my EXPR | |
2656 | ||
2657 | =item my TYPE EXPR | |
2658 | ||
2659 | =item my EXPR : ATTRS | |
2660 | ||
2661 | =item my TYPE EXPR : ATTRS | |
2662 | ||
2663 | A C<my> declares the listed variables to be local (lexically) to the | |
2664 | enclosing block, file, or C<eval>. If more than one value is listed, | |
2665 | the list must be placed in parentheses. | |
2666 | ||
2667 | The exact semantics and interface of TYPE and ATTRS are still | |
2668 | evolving. TYPE is currently bound to the use of C<fields> pragma, | |
2669 | and attributes are handled using the C<attributes> pragma, or starting | |
2670 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
2671 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
2672 | L<attributes>, and L<Attribute::Handlers>. | |
2673 | ||
2674 | =item next LABEL | |
2675 | ||
2676 | =item next | |
2677 | ||
2678 | The C<next> command is like the C<continue> statement in C; it starts | |
2679 | the next iteration of the loop: | |
2680 | ||
2681 | LINE: while (<STDIN>) { | |
2682 | next LINE if /^#/; # discard comments | |
2683 | #... | |
2684 | } | |
2685 | ||
2686 | Note that if there were a C<continue> block on the above, it would get | |
2687 | executed even on discarded lines. If the LABEL is omitted, the command | |
2688 | refers to the innermost enclosing loop. | |
2689 | ||
2690 | C<next> cannot be used to exit a block which returns a value such as | |
2691 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit | |
2692 | a grep() or map() operation. | |
2693 | ||
2694 | Note that a block by itself is semantically identical to a loop | |
2695 | that executes once. Thus C<next> will exit such a block early. | |
2696 | ||
2697 | See also L</continue> for an illustration of how C<last>, C<next>, and | |
2698 | C<redo> work. | |
2699 | ||
2700 | =item no Module VERSION LIST | |
2701 | ||
2702 | =item no Module VERSION | |
2703 | ||
2704 | =item no Module LIST | |
2705 | ||
2706 | =item no Module | |
2707 | ||
2708 | See the L</use> function, which C<no> is the opposite of. | |
2709 | ||
2710 | =item oct EXPR | |
2711 | ||
2712 | =item oct | |
2713 | ||
2714 | Interprets EXPR as an octal string and returns the corresponding | |
2715 | value. (If EXPR happens to start off with C<0x>, interprets it as a | |
2716 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
2717 | binary string. Leading whitespace is ignored in all three cases.) | |
2718 | The following will handle decimal, binary, octal, and hex in the standard | |
2719 | Perl or C notation: | |
2720 | ||
2721 | $val = oct($val) if $val =~ /^0/; | |
2722 | ||
2723 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number | |
2724 | in octal), use sprintf() or printf(): | |
2725 | ||
2726 | $perms = (stat("filename"))[2] & 07777; | |
2727 | $oct_perms = sprintf "%lo", $perms; | |
2728 | ||
2729 | The oct() function is commonly used when a string such as C<644> needs | |
2730 | to be converted into a file mode, for example. (Although perl will | |
2731 | automatically convert strings into numbers as needed, this automatic | |
2732 | conversion assumes base 10.) | |
2733 | ||
2734 | =item open FILEHANDLE,EXPR | |
2735 | ||
2736 | =item open FILEHANDLE,MODE,EXPR | |
2737 | ||
2738 | =item open FILEHANDLE,MODE,EXPR,LIST | |
2739 | ||
2740 | =item open FILEHANDLE,MODE,REFERENCE | |
2741 | ||
2742 | =item open FILEHANDLE | |
2743 | ||
2744 | Opens the file whose filename is given by EXPR, and associates it with | |
2745 | FILEHANDLE. | |
2746 | ||
2747 | (The following is a comprehensive reference to open(): for a gentler | |
2748 | introduction you may consider L<perlopentut>.) | |
2749 | ||
2750 | If FILEHANDLE is an undefined lexical (C<my>) variable the variable is | |
2751 | assigned a reference to a new anonymous filehandle, otherwise if | |
2752 | FILEHANDLE is an expression, its value is used as the name of the real | |
2753 | filehandle wanted. (This is considered a symbolic reference, so C<use | |
2754 | strict 'refs'> should I<not> be in effect.) | |
2755 | ||
2756 | If EXPR is omitted, the scalar variable of the same name as the | |
2757 | FILEHANDLE contains the filename. (Note that lexical variables--those | |
2758 | declared with C<my>--will not work for this purpose; so if you're | |
2759 | using C<my>, specify EXPR in your call to open.) | |
2760 | ||
2761 | If three or more arguments are specified then the mode of opening and | |
2762 | the file name are separate. If MODE is C<< '<' >> or nothing, the file | |
2763 | is opened for input. If MODE is C<< '>' >>, the file is truncated and | |
2764 | opened for output, being created if necessary. If MODE is C<<< '>>' >>>, | |
2765 | the file is opened for appending, again being created if necessary. | |
2766 | ||
2767 | You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to | |
2768 | indicate that you want both read and write access to the file; thus | |
2769 | C<< '+<' >> is almost always preferred for read/write updates--the C<< | |
2770 | '+>' >> mode would clobber the file first. You can't usually use | |
2771 | either read-write mode for updating textfiles, since they have | |
2772 | variable length records. See the B<-i> switch in L<perlrun> for a | |
2773 | better approach. The file is created with permissions of C<0666> | |
2774 | modified by the process' C<umask> value. | |
2775 | ||
2776 | These various prefixes correspond to the fopen(3) modes of C<'r'>, | |
2777 | C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. | |
2778 | ||
2779 | In the 2-arguments (and 1-argument) form of the call the mode and | |
2780 | filename should be concatenated (in this order), possibly separated by | |
2781 | spaces. It is possible to omit the mode in these forms if the mode is | |
2782 | C<< '<' >>. | |
2783 | ||
2784 | If the filename begins with C<'|'>, the filename is interpreted as a | |
2785 | command to which output is to be piped, and if the filename ends with a | |
2786 | C<'|'>, the filename is interpreted as a command which pipes output to | |
2787 | us. See L<perlipc/"Using open() for IPC"> | |
2788 | for more examples of this. (You are not allowed to C<open> to a command | |
2789 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, | |
2790 | and L<perlipc/"Bidirectional Communication with Another Process"> | |
2791 | for alternatives.) | |
2792 | ||
2793 | For three or more arguments if MODE is C<'|-'>, the filename is | |
2794 | interpreted as a command to which output is to be piped, and if MODE | |
2795 | is C<'-|'>, the filename is interpreted as a command which pipes | |
2796 | output to us. In the 2-arguments (and 1-argument) form one should | |
2797 | replace dash (C<'-'>) with the command. | |
2798 | See L<perlipc/"Using open() for IPC"> for more examples of this. | |
2799 | (You are not allowed to C<open> to a command that pipes both in I<and> | |
2800 | out, but see L<IPC::Open2>, L<IPC::Open3>, and | |
2801 | L<perlipc/"Bidirectional Communication"> for alternatives.) | |
2802 | ||
2803 | In the three-or-more argument form of pipe opens, if LIST is specified | |
2804 | (extra arguments after the command name) then LIST becomes arguments | |
2805 | to the command invoked if the platform supports it. The meaning of | |
2806 | C<open> with more than three arguments for non-pipe modes is not yet | |
2807 | specified. Experimental "layers" may give extra LIST arguments | |
2808 | meaning. | |
2809 | ||
2810 | In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN | |
2811 | and opening C<< '>-' >> opens STDOUT. | |
2812 | ||
2813 | You may use the three-argument form of open to specify IO "layers" | |
2814 | (sometimes also referred to as "disciplines") to be applied to the handle | |
2815 | that affect how the input and output are processed (see L<open> and | |
2816 | L<PerlIO> for more details). For example | |
2817 | ||
2818 | open(FH, "<:utf8", "file") | |
2819 | ||
2820 | will open the UTF-8 encoded file containing Unicode characters, | |
2821 | see L<perluniintro>. (Note that if layers are specified in the | |
2822 | three-arg form then default layers set by the C<open> pragma are | |
2823 | ignored.) | |
2824 | ||
2825 | Open returns nonzero upon success, the undefined value otherwise. If | |
2826 | the C<open> involved a pipe, the return value happens to be the pid of | |
2827 | the subprocess. | |
2828 | ||
2829 | If you're running Perl on a system that distinguishes between text | |
2830 | files and binary files, then you should check out L</binmode> for tips | |
2831 | for dealing with this. The key distinction between systems that need | |
2832 | C<binmode> and those that don't is their text file formats. Systems | |
2833 | like Unix, Mac OS, and Plan 9, which delimit lines with a single | |
2834 | character, and which encode that character in C as C<"\n">, do not | |
2835 | need C<binmode>. The rest need it. | |
2836 | ||
2837 | When opening a file, it's usually a bad idea to continue normal execution | |
2838 | if the request failed, so C<open> is frequently used in connection with | |
2839 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, | |
2840 | where you want to make a nicely formatted error message (but there are | |
2841 | modules that can help with that problem)) you should always check | |
2842 | the return value from opening a file. The infrequent exception is when | |
2843 | working with an unopened filehandle is actually what you want to do. | |
2844 | ||
2845 | As a special case the 3 arg form with a read/write mode and the third | |
2846 | argument being C<undef>: | |
2847 | ||
2848 | open(TMP, "+>", undef) or die ... | |
2849 | ||
2850 | opens a filehandle to an anonymous temporary file. | |
2851 | ||
2852 | File handles can be opened to "in memory" files held in Perl scalars via: | |
2853 | ||
2854 | open($fh, '>', \$variable) || .. | |
2855 | ||
2856 | Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory" | |
2857 | file, you have to close it first: | |
2858 | ||
2859 | close STDOUT; | |
2860 | open STDOUT, '>', \$variable or die "Can't open STDOUT: $!"; | |
2861 | ||
2862 | Examples: | |
2863 | ||
2864 | $ARTICLE = 100; | |
2865 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; | |
2866 | while (<ARTICLE>) {... | |
2867 | ||
2868 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) | |
2869 | # if the open fails, output is discarded | |
2870 | ||
2871 | open(DBASE, '+<', 'dbase.mine') # open for update | |
2872 | or die "Can't open 'dbase.mine' for update: $!"; | |
2873 | ||
2874 | open(DBASE, '+<dbase.mine') # ditto | |
2875 | or die "Can't open 'dbase.mine' for update: $!"; | |
2876 | ||
2877 | open(ARTICLE, '-|', "caesar <$article") # decrypt article | |
2878 | or die "Can't start caesar: $!"; | |
2879 | ||
2880 | open(ARTICLE, "caesar <$article |") # ditto | |
2881 | or die "Can't start caesar: $!"; | |
2882 | ||
2883 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id | |
2884 | or die "Can't start sort: $!"; | |
2885 | ||
2886 | # in memory files | |
2887 | open(MEMORY,'>', \$var) | |
2888 | or die "Can't open memory file: $!"; | |
2889 | print MEMORY "foo!\n"; # output will end up in $var | |
2890 | ||
2891 | # process argument list of files along with any includes | |
2892 | ||
2893 | foreach $file (@ARGV) { | |
2894 | process($file, 'fh00'); | |
2895 | } | |
2896 | ||
2897 | sub process { | |
2898 | my($filename, $input) = @_; | |
2899 | $input++; # this is a string increment | |
2900 | unless (open($input, $filename)) { | |
2901 | print STDERR "Can't open $filename: $!\n"; | |
2902 | return; | |
2903 | } | |
2904 | ||
2905 | local $_; | |
2906 | while (<$input>) { # note use of indirection | |
2907 | if (/^#include "(.*)"/) { | |
2908 | process($1, $input); | |
2909 | next; | |
2910 | } | |
2911 | #... # whatever | |
2912 | } | |
2913 | } | |
2914 | ||
2915 | You may also, in the Bourne shell tradition, specify an EXPR beginning | |
2916 | with C<< '>&' >>, in which case the rest of the string is interpreted as the | |
2917 | name of a filehandle (or file descriptor, if numeric) to be | |
2918 | duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, | |
2919 | C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The | |
2920 | mode you specify should match the mode of the original filehandle. | |
2921 | (Duping a filehandle does not take into account any existing contents of | |
2922 | IO buffers.) If you use the 3 arg form then you can pass either a number, | |
2923 | the name of a filehandle or the normal "reference to a glob". | |
2924 | ||
2925 | Here is a script that saves, redirects, and restores C<STDOUT> and | |
2926 | C<STDERR> using various methods: | |
2927 | ||
2928 | #!/usr/bin/perl | |
2929 | open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!"; | |
2930 | open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!"; | |
2931 | ||
2932 | open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!"; | |
2933 | open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!"; | |
2934 | ||
2935 | select STDERR; $| = 1; # make unbuffered | |
2936 | select STDOUT; $| = 1; # make unbuffered | |
2937 | ||
2938 | print STDOUT "stdout 1\n"; # this works for | |
2939 | print STDERR "stderr 1\n"; # subprocesses too | |
2940 | ||
2941 | close STDOUT; | |
2942 | close STDERR; | |
2943 | ||
2944 | open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!"; | |
2945 | open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!"; | |
2946 | ||
2947 | print STDOUT "stdout 2\n"; | |
2948 | print STDERR "stderr 2\n"; | |
2949 | ||
2950 | If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will | |
2951 | do an equivalent of C's C<fdopen> of that file descriptor; this is | |
2952 | more parsimonious of file descriptors. For example: | |
2953 | ||
2954 | open(FILEHANDLE, "<&=$fd") | |
2955 | ||
2956 | or | |
2957 | ||
2958 | open(FILEHANDLE, "<&=", $fd) | |
2959 | ||
2960 | Note that if Perl is using the standard C libraries' fdopen() then on | |
2961 | many UNIX systems, fdopen() is known to fail when file descriptors | |
2962 | exceed a certain value, typically 255. If you need more file | |
2963 | descriptors than that, consider rebuilding Perl to use the C<PerlIO>. | |
2964 | ||
2965 | You can see whether Perl has been compiled with PerlIO or not by | |
2966 | running C<perl -V> and looking for C<useperlio=> line. If C<useperlio> | |
2967 | is C<define>, you have PerlIO, otherwise you don't. | |
2968 | ||
2969 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> | |
2970 | with 2-arguments (or 1-argument) form of open(), then | |
2971 | there is an implicit fork done, and the return value of open is the pid | |
2972 | of the child within the parent process, and C<0> within the child | |
2973 | process. (Use C<defined($pid)> to determine whether the open was successful.) | |
2974 | The filehandle behaves normally for the parent, but i/o to that | |
2975 | filehandle is piped from/to the STDOUT/STDIN of the child process. | |
2976 | In the child process the filehandle isn't opened--i/o happens from/to | |
2977 | the new STDOUT or STDIN. Typically this is used like the normal | |
2978 | piped open when you want to exercise more control over just how the | |
2979 | pipe command gets executed, such as when you are running setuid, and | |
2980 | don't want to have to scan shell commands for metacharacters. | |
2981 | The following triples are more or less equivalent: | |
2982 | ||
2983 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
2984 | open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); | |
2985 | open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; | |
2986 | open(FOO, '|-', "tr", '[a-z]', '[A-Z]'); | |
2987 | ||
2988 | open(FOO, "cat -n '$file'|"); | |
2989 | open(FOO, '-|', "cat -n '$file'"); | |
2990 | open(FOO, '-|') || exec 'cat', '-n', $file; | |
2991 | open(FOO, '-|', "cat", '-n', $file); | |
2992 | ||
2993 | The last example in each block shows the pipe as "list form", which is | |
2994 | not yet supported on all platforms. A good rule of thumb is that if | |
2995 | your platform has true C<fork()> (in other words, if your platform is | |
2996 | UNIX) you can use the list form. | |
2997 | ||
2998 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. | |
2999 | ||
3000 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
3001 | output before any operation that may do a fork, but this may not be | |
3002 | supported on some platforms (see L<perlport>). To be safe, you may need | |
3003 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
3004 | of C<IO::Handle> on any open handles. | |
3005 | ||
3006 | On systems that support a close-on-exec flag on files, the flag will | |
3007 | be set for the newly opened file descriptor as determined by the value | |
3008 | of $^F. See L<perlvar/$^F>. | |
3009 | ||
3010 | Closing any piped filehandle causes the parent process to wait for the | |
3011 | child to finish, and returns the status value in C<$?>. | |
3012 | ||
3013 | The filename passed to 2-argument (or 1-argument) form of open() will | |
3014 | have leading and trailing whitespace deleted, and the normal | |
3015 | redirection characters honored. This property, known as "magic open", | |
3016 | can often be used to good effect. A user could specify a filename of | |
3017 | F<"rsh cat file |">, or you could change certain filenames as needed: | |
3018 | ||
3019 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
3020 | open(FH, $filename) or die "Can't open $filename: $!"; | |
3021 | ||
3022 | Use 3-argument form to open a file with arbitrary weird characters in it, | |
3023 | ||
3024 | open(FOO, '<', $file); | |
3025 | ||
3026 | otherwise it's necessary to protect any leading and trailing whitespace: | |
3027 | ||
3028 | $file =~ s#^(\s)#./$1#; | |
3029 | open(FOO, "< $file\0"); | |
3030 | ||
3031 | (this may not work on some bizarre filesystems). One should | |
3032 | conscientiously choose between the I<magic> and 3-arguments form | |
3033 | of open(): | |
3034 | ||
3035 | open IN, $ARGV[0]; | |
3036 | ||
3037 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
3038 | but will not work on a filename which happens to have a trailing space, while | |
3039 | ||
3040 | open IN, '<', $ARGV[0]; | |
3041 | ||
3042 | will have exactly the opposite restrictions. | |
3043 | ||
3044 | If you want a "real" C C<open> (see L<open(2)> on your system), then you | |
3045 | should use the C<sysopen> function, which involves no such magic (but | |
3046 | may use subtly different filemodes than Perl open(), which is mapped | |
3047 | to C fopen()). This is | |
3048 | another way to protect your filenames from interpretation. For example: | |
3049 | ||
3050 | use IO::Handle; | |
3051 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
3052 | or die "sysopen $path: $!"; | |
3053 | $oldfh = select(HANDLE); $| = 1; select($oldfh); | |
3054 | print HANDLE "stuff $$\n"; | |
3055 | seek(HANDLE, 0, 0); | |
3056 | print "File contains: ", <HANDLE>; | |
3057 | ||
3058 | Using the constructor from the C<IO::Handle> package (or one of its | |
3059 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
3060 | filehandles that have the scope of whatever variables hold references to | |
3061 | them, and automatically close whenever and however you leave that scope: | |
3062 | ||
3063 | use IO::File; | |
3064 | #... | |
3065 | sub read_myfile_munged { | |
3066 | my $ALL = shift; | |
3067 | my $handle = new IO::File; | |
3068 | open($handle, "myfile") or die "myfile: $!"; | |
3069 | $first = <$handle> | |
3070 | or return (); # Automatically closed here. | |
3071 | mung $first or die "mung failed"; # Or here. | |
3072 | return $first, <$handle> if $ALL; # Or here. | |
3073 | $first; # Or here. | |
3074 | } | |
3075 | ||
3076 | See L</seek> for some details about mixing reading and writing. | |
3077 | ||
3078 | =item opendir DIRHANDLE,EXPR | |
3079 | ||
3080 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, | |
3081 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
3082 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. | |
3083 | ||
3084 | =item ord EXPR | |
3085 | ||
3086 | =item ord | |
3087 | ||
3088 | Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, | |
3089 | or Unicode) value of the first character of EXPR. If EXPR is omitted, | |
3090 | uses C<$_>. | |
3091 | ||
3092 | For the reverse, see L</chr>. | |
3093 | See L<perlunicode> and L<encoding> for more about Unicode. | |
3094 | ||
3095 | =item our EXPR | |
3096 | ||
3097 | =item our EXPR TYPE | |
3098 | ||
3099 | =item our EXPR : ATTRS | |
3100 | ||
3101 | =item our TYPE EXPR : ATTRS | |
3102 | ||
3103 | An C<our> declares the listed variables to be valid globals within | |
3104 | the enclosing block, file, or C<eval>. That is, it has the same | |
3105 | scoping rules as a "my" declaration, but does not create a local | |
3106 | variable. If more than one value is listed, the list must be placed | |
3107 | in parentheses. The C<our> declaration has no semantic effect unless | |
3108 | "use strict vars" is in effect, in which case it lets you use the | |
3109 | declared global variable without qualifying it with a package name. | |
3110 | (But only within the lexical scope of the C<our> declaration. In this | |
3111 | it differs from "use vars", which is package scoped.) | |
3112 | ||
3113 | An C<our> declaration declares a global variable that will be visible | |
3114 | across its entire lexical scope, even across package boundaries. The | |
3115 | package in which the variable is entered is determined at the point | |
3116 | of the declaration, not at the point of use. This means the following | |
3117 | behavior holds: | |
3118 | ||
3119 | package Foo; | |
3120 | our $bar; # declares $Foo::bar for rest of lexical scope | |
3121 | $bar = 20; | |
3122 | ||
3123 | package Bar; | |
3124 | print $bar; # prints 20 | |
3125 | ||
3126 | Multiple C<our> declarations in the same lexical scope are allowed | |
3127 | if they are in different packages. If they happened to be in the same | |
3128 | package, Perl will emit warnings if you have asked for them. | |
3129 | ||
3130 | use warnings; | |
3131 | package Foo; | |
3132 | our $bar; # declares $Foo::bar for rest of lexical scope | |
3133 | $bar = 20; | |
3134 | ||
3135 | package Bar; | |
3136 | our $bar = 30; # declares $Bar::bar for rest of lexical scope | |
3137 | print $bar; # prints 30 | |
3138 | ||
3139 | our $bar; # emits warning | |
3140 | ||
3141 | An C<our> declaration may also have a list of attributes associated | |
3142 | with it. | |
3143 | ||
3144 | The exact semantics and interface of TYPE and ATTRS are still | |
3145 | evolving. TYPE is currently bound to the use of C<fields> pragma, | |
3146 | and attributes are handled using the C<attributes> pragma, or starting | |
3147 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3148 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3149 | L<attributes>, and L<Attribute::Handlers>. | |
3150 | ||
3151 | The only currently recognized C<our()> attribute is C<unique> which | |
3152 | indicates that a single copy of the global is to be used by all | |
3153 | interpreters should the program happen to be running in a | |
3154 | multi-interpreter environment. (The default behaviour would be for | |
3155 | each interpreter to have its own copy of the global.) Examples: | |
3156 | ||
3157 | our @EXPORT : unique = qw(foo); | |
3158 | our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]); | |
3159 | our $VERSION : unique = "1.00"; | |
3160 | ||
3161 | Note that this attribute also has the effect of making the global | |
3162 | readonly when the first new interpreter is cloned (for example, | |
3163 | when the first new thread is created). | |
3164 | ||
3165 | Multi-interpreter environments can come to being either through the | |
3166 | fork() emulation on Windows platforms, or by embedding perl in a | |
3167 | multi-threaded application. The C<unique> attribute does nothing in | |
3168 | all other environments. | |
3169 | ||
3170 | =item pack TEMPLATE,LIST | |
3171 | ||
3172 | Takes a LIST of values and converts it into a string using the rules | |
3173 | given by the TEMPLATE. The resulting string is the concatenation of | |
3174 | the converted values. Typically, each converted value looks | |
3175 | like its machine-level representation. For example, on 32-bit machines | |
3176 | a converted integer may be represented by a sequence of 4 bytes. | |
3177 | ||
3178 | The TEMPLATE is a sequence of characters that give the order and type | |
3179 | of values, as follows: | |
3180 | ||
3181 | a A string with arbitrary binary data, will be null padded. | |
3182 | A A text (ASCII) string, will be space padded. | |
3183 | Z A null terminated (ASCIZ) string, will be null padded. | |
3184 | ||
3185 | b A bit string (ascending bit order inside each byte, like vec()). | |
3186 | B A bit string (descending bit order inside each byte). | |
3187 | h A hex string (low nybble first). | |
3188 | H A hex string (high nybble first). | |
3189 | ||
3190 | c A signed char value. | |
3191 | C An unsigned char value. Only does bytes. See U for Unicode. | |
3192 | ||
3193 | s A signed short value. | |
3194 | S An unsigned short value. | |
3195 | (This 'short' is _exactly_ 16 bits, which may differ from | |
3196 | what a local C compiler calls 'short'. If you want | |
3197 | native-length shorts, use the '!' suffix.) | |
3198 | ||
3199 | i A signed integer value. | |
3200 | I An unsigned integer value. | |
3201 | (This 'integer' is _at_least_ 32 bits wide. Its exact | |
3202 | size depends on what a local C compiler calls 'int', | |
3203 | and may even be larger than the 'long' described in | |
3204 | the next item.) | |
3205 | ||
3206 | l A signed long value. | |
3207 | L An unsigned long value. | |
3208 | (This 'long' is _exactly_ 32 bits, which may differ from | |
3209 | what a local C compiler calls 'long'. If you want | |
3210 | native-length longs, use the '!' suffix.) | |
3211 | ||
3212 | n An unsigned short in "network" (big-endian) order. | |
3213 | N An unsigned long in "network" (big-endian) order. | |
3214 | v An unsigned short in "VAX" (little-endian) order. | |
3215 | V An unsigned long in "VAX" (little-endian) order. | |
3216 | (These 'shorts' and 'longs' are _exactly_ 16 bits and | |
3217 | _exactly_ 32 bits, respectively.) | |
3218 | ||
3219 | q A signed quad (64-bit) value. | |
3220 | Q An unsigned quad value. | |
3221 | (Quads are available only if your system supports 64-bit | |
3222 | integer values _and_ if Perl has been compiled to support those. | |
3223 | Causes a fatal error otherwise.) | |
3224 | ||
3225 | j A signed integer value (a Perl internal integer, IV). | |
3226 | J An unsigned integer value (a Perl internal unsigned integer, UV). | |
3227 | ||
3228 | f A single-precision float in the native format. | |
3229 | d A double-precision float in the native format. | |
3230 | ||
3231 | F A floating point value in the native native format | |
3232 | (a Perl internal floating point value, NV). | |
3233 | D A long double-precision float in the native format. | |
3234 | (Long doubles are available only if your system supports long | |
3235 | double values _and_ if Perl has been compiled to support those. | |
3236 | Causes a fatal error otherwise.) | |
3237 | ||
3238 | p A pointer to a null-terminated string. | |
3239 | P A pointer to a structure (fixed-length string). | |
3240 | ||
3241 | u A uuencoded string. | |
3242 | U A Unicode character number. Encodes to UTF-8 internally | |
3243 | (or UTF-EBCDIC in EBCDIC platforms). | |
3244 | ||
3245 | w A BER compressed integer. Its bytes represent an unsigned | |
3246 | integer in base 128, most significant digit first, with as | |
3247 | few digits as possible. Bit eight (the high bit) is set | |
3248 | on each byte except the last. | |
3249 | ||
3250 | x A null byte. | |
3251 | X Back up a byte. | |
3252 | @ Null fill to absolute position. | |
3253 | ( Start of a ()-group. | |
3254 | ||
3255 | The following rules apply: | |
3256 | ||
3257 | =over 8 | |
3258 | ||
3259 | =item * | |
3260 | ||
3261 | Each letter may optionally be followed by a number giving a repeat | |
3262 | count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, | |
3263 | C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that | |
3264 | many values from the LIST. A C<*> for the repeat count means to use | |
3265 | however many items are left, except for C<@>, C<x>, C<X>, where it is | |
3266 | equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what | |
3267 | is the same). A numeric repeat count may optionally be enclosed in | |
3268 | brackets, as in C<pack 'C[80]', @arr>. | |
3269 | ||
3270 | One can replace the numeric repeat count by a template enclosed in brackets; | |
3271 | then the packed length of this template in bytes is used as a count. | |
3272 | For example, C<x[L]> skips a long (it skips the number of bytes in a long); | |
3273 | the template C<$t X[$t] $t> unpack()s twice what $t unpacks. | |
3274 | If the template in brackets contains alignment commands (such as C<x![d]>), | |
3275 | its packed length is calculated as if the start of the template has the maximal | |
3276 | possible alignment. | |
3277 | ||
3278 | When used with C<Z>, C<*> results in the addition of a trailing null | |
3279 | byte (so the packed result will be one longer than the byte C<length> | |
3280 | of the item). | |
3281 | ||
3282 | The repeat count for C<u> is interpreted as the maximal number of bytes | |
3283 | to encode per line of output, with 0 and 1 replaced by 45. | |
3284 | ||
3285 | =item * | |
3286 | ||
3287 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a | |
3288 | string of length count, padding with nulls or spaces as necessary. When | |
3289 | unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything | |
3290 | after the first null, and C<a> returns data verbatim. When packing, | |
3291 | C<a>, and C<Z> are equivalent. | |
3292 | ||
3293 | If the value-to-pack is too long, it is truncated. If too long and an | |
3294 | explicit count is provided, C<Z> packs only C<$count-1> bytes, followed | |
3295 | by a null byte. Thus C<Z> always packs a trailing null byte under | |
3296 | all circumstances. | |
3297 | ||
3298 | =item * | |
3299 | ||
3300 | Likewise, the C<b> and C<B> fields pack a string that many bits long. | |
3301 | Each byte of the input field of pack() generates 1 bit of the result. | |
3302 | Each result bit is based on the least-significant bit of the corresponding | |
3303 | input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and | |
3304 | C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. | |
3305 | ||
3306 | Starting from the beginning of the input string of pack(), each 8-tuple | |
3307 | of bytes is converted to 1 byte of output. With format C<b> | |
3308 | the first byte of the 8-tuple determines the least-significant bit of a | |
3309 | byte, and with format C<B> it determines the most-significant bit of | |
3310 | a byte. | |
3311 | ||
3312 | If the length of the input string is not exactly divisible by 8, the | |
3313 | remainder is packed as if the input string were padded by null bytes | |
3314 | at the end. Similarly, during unpack()ing the "extra" bits are ignored. | |
3315 | ||
3316 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
3317 | A C<*> for the repeat count of pack() means to use all the bytes of | |
3318 | the input field. On unpack()ing the bits are converted to a string | |
3319 | of C<"0">s and C<"1">s. | |
3320 | ||
3321 | =item * | |
3322 | ||
3323 | The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, | |
3324 | representable as hexadecimal digits, 0-9a-f) long. | |
3325 | ||
3326 | Each byte of the input field of pack() generates 4 bits of the result. | |
3327 | For non-alphabetical bytes the result is based on the 4 least-significant | |
3328 | bits of the input byte, i.e., on C<ord($byte)%16>. In particular, | |
3329 | bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
3330 | C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result | |
3331 | is compatible with the usual hexadecimal digits, so that C<"a"> and | |
3332 | C<"A"> both generate the nybble C<0xa==10>. The result for bytes | |
3333 | C<"g".."z"> and C<"G".."Z"> is not well-defined. | |
3334 | ||
3335 | Starting from the beginning of the input string of pack(), each pair | |
3336 | of bytes is converted to 1 byte of output. With format C<h> the | |
3337 | first byte of the pair determines the least-significant nybble of the | |
3338 | output byte, and with format C<H> it determines the most-significant | |
3339 | nybble. | |
3340 | ||
3341 | If the length of the input string is not even, it behaves as if padded | |
3342 | by a null byte at the end. Similarly, during unpack()ing the "extra" | |
3343 | nybbles are ignored. | |
3344 | ||
3345 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
3346 | A C<*> for the repeat count of pack() means to use all the bytes of | |
3347 | the input field. On unpack()ing the bits are converted to a string | |
3348 | of hexadecimal digits. | |
3349 | ||
3350 | =item * | |
3351 | ||
3352 | The C<p> type packs a pointer to a null-terminated string. You are | |
3353 | responsible for ensuring the string is not a temporary value (which can | |
3354 | potentially get deallocated before you get around to using the packed result). | |
3355 | The C<P> type packs a pointer to a structure of the size indicated by the | |
3356 | length. A NULL pointer is created if the corresponding value for C<p> or | |
3357 | C<P> is C<undef>, similarly for unpack(). | |
3358 | ||
3359 | =item * | |
3360 | ||
3361 | The C</> template character allows packing and unpacking of strings where | |
3362 | the packed structure contains a byte count followed by the string itself. | |
3363 | You write I<length-item>C</>I<string-item>. | |
3364 | ||
3365 | The I<length-item> can be any C<pack> template letter, and describes | |
3366 | how the length value is packed. The ones likely to be of most use are | |
3367 | integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or | |
3368 | SNMP) and C<N> (for Sun XDR). | |
3369 | ||
3370 | The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. | |
3371 | For C<unpack> the length of the string is obtained from the I<length-item>, | |
3372 | but if you put in the '*' it will be ignored. | |
3373 | ||
3374 | unpack 'C/a', "\04Gurusamy"; gives 'Guru' | |
3375 | unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') | |
3376 | pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" | |
3377 | ||
3378 | The I<length-item> is not returned explicitly from C<unpack>. | |
3379 | ||
3380 | Adding a count to the I<length-item> letter is unlikely to do anything | |
3381 | useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a | |
3382 | I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, | |
3383 | which Perl does not regard as legal in numeric strings. | |
3384 | ||
3385 | =item * | |
3386 | ||
3387 | The integer types C<s>, C<S>, C<l>, and C<L> may be | |
3388 | immediately followed by a C<!> suffix to signify native shorts or | |
3389 | longs--as you can see from above for example a bare C<l> does mean | |
3390 | exactly 32 bits, the native C<long> (as seen by the local C compiler) | |
3391 | may be larger. This is an issue mainly in 64-bit platforms. You can | |
3392 | see whether using C<!> makes any difference by | |
3393 | ||
3394 | print length(pack("s")), " ", length(pack("s!")), "\n"; | |
3395 | print length(pack("l")), " ", length(pack("l!")), "\n"; | |
3396 | ||
3397 | C<i!> and C<I!> also work but only because of completeness; | |
3398 | they are identical to C<i> and C<I>. | |
3399 | ||
3400 | The actual sizes (in bytes) of native shorts, ints, longs, and long | |
3401 | longs on the platform where Perl was built are also available via | |
3402 | L<Config>: | |
3403 | ||
3404 | use Config; | |
3405 | print $Config{shortsize}, "\n"; | |
3406 | print $Config{intsize}, "\n"; | |
3407 | print $Config{longsize}, "\n"; | |
3408 | print $Config{longlongsize}, "\n"; | |
3409 | ||
3410 | (The C<$Config{longlongsize}> will be undefine if your system does | |
3411 | not support long longs.) | |
3412 | ||
3413 | =item * | |
3414 | ||
3415 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> | |
3416 | are inherently non-portable between processors and operating systems | |
3417 | because they obey the native byteorder and endianness. For example a | |
3418 | 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively | |
3419 | (arranged in and handled by the CPU registers) into bytes as | |
3420 | ||
3421 | 0x12 0x34 0x56 0x78 # big-endian | |
3422 | 0x78 0x56 0x34 0x12 # little-endian | |
3423 | ||
3424 | Basically, the Intel and VAX CPUs are little-endian, while everybody | |
3425 | else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and | |
3426 | Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq | |
3427 | used/uses them in little-endian mode; SGI/Cray uses them in big-endian | |
3428 | mode. | |
3429 | ||
3430 | The names `big-endian' and `little-endian' are comic references to | |
3431 | the classic "Gulliver's Travels" (via the paper "On Holy Wars and a | |
3432 | Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and | |
3433 | the egg-eating habits of the Lilliputians. | |
3434 | ||
3435 | Some systems may have even weirder byte orders such as | |
3436 | ||
3437 | 0x56 0x78 0x12 0x34 | |
3438 | 0x34 0x12 0x78 0x56 | |
3439 | ||
3440 | You can see your system's preference with | |
3441 | ||
3442 | print join(" ", map { sprintf "%#02x", $_ } | |
3443 | unpack("C*",pack("L",0x12345678))), "\n"; | |
3444 | ||
3445 | The byteorder on the platform where Perl was built is also available | |
3446 | via L<Config>: | |
3447 | ||
3448 | use Config; | |
3449 | print $Config{byteorder}, "\n"; | |
3450 | ||
3451 | Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> | |
3452 | and C<'87654321'> are big-endian. | |
3453 | ||
3454 | If you want portable packed integers use the formats C<n>, C<N>, | |
3455 | C<v>, and C<V>, their byte endianness and size are known. | |
3456 | See also L<perlport>. | |
3457 | ||
3458 | =item * | |
3459 | ||
3460 | Real numbers (floats and doubles) are in the native machine format only; | |
3461 | due to the multiplicity of floating formats around, and the lack of a | |
3462 | standard "network" representation, no facility for interchange has been | |
3463 | made. This means that packed floating point data written on one machine | |
3464 | may not be readable on another - even if both use IEEE floating point | |
3465 | arithmetic (as the endian-ness of the memory representation is not part | |
3466 | of the IEEE spec). See also L<perlport>. | |
3467 | ||
3468 | Note that Perl uses doubles internally for all numeric calculation, and | |
3469 | converting from double into float and thence back to double again will | |
3470 | lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general | |
3471 | equal $foo). | |
3472 | ||
3473 | =item * | |
3474 | ||
3475 | If the pattern begins with a C<U>, the resulting string will be treated | |
3476 | as Unicode-encoded. You can force UTF8 encoding on in a string with an | |
3477 | initial C<U0>, and the bytes that follow will be interpreted as Unicode | |
3478 | characters. If you don't want this to happen, you can begin your pattern | |
3479 | with C<C0> (or anything else) to force Perl not to UTF8 encode your | |
3480 | string, and then follow this with a C<U*> somewhere in your pattern. | |
3481 | ||
3482 | =item * | |
3483 | ||
3484 | You must yourself do any alignment or padding by inserting for example | |
3485 | enough C<'x'>es while packing. There is no way to pack() and unpack() | |
3486 | could know where the bytes are going to or coming from. Therefore | |
3487 | C<pack> (and C<unpack>) handle their output and input as flat | |
3488 | sequences of bytes. | |
3489 | ||
3490 | =item * | |
3491 | ||
3492 | A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may | |
3493 | take a repeat count, both as postfix, and via the C</> template | |
3494 | character. | |
3495 | ||
3496 | =item * | |
3497 | ||
3498 | C<x> and C<X> accept C<!> modifier. In this case they act as | |
3499 | alignment commands: they jump forward/back to the closest position | |
3500 | aligned at a multiple of C<count> bytes. For example, to pack() or | |
3501 | unpack() C's C<struct {char c; double d; char cc[2]}> one may need to | |
3502 | use the template C<C x![d] d C[2]>; this assumes that doubles must be | |
3503 | aligned on the double's size. | |
3504 | ||
3505 | For alignment commands C<count> of 0 is equivalent to C<count> of 1; | |
3506 | both result in no-ops. | |
3507 | ||
3508 | =item * | |
3509 | ||
3510 | A comment in a TEMPLATE starts with C<#> and goes to the end of line. | |
3511 | ||
3512 | =item * | |
3513 | ||
3514 | If TEMPLATE requires more arguments to pack() than actually given, pack() | |
3515 | assumes additional C<""> arguments. If TEMPLATE requires less arguments | |
3516 | to pack() than actually given, extra arguments are ignored. | |
3517 | ||
3518 | =back | |
3519 | ||
3520 | Examples: | |
3521 | ||
3522 | $foo = pack("CCCC",65,66,67,68); | |
3523 | # foo eq "ABCD" | |
3524 | $foo = pack("C4",65,66,67,68); | |
3525 | # same thing | |
3526 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); | |
3527 | # same thing with Unicode circled letters | |
3528 | ||
3529 | $foo = pack("ccxxcc",65,66,67,68); | |
3530 | # foo eq "AB\0\0CD" | |
3531 | ||
3532 | # note: the above examples featuring "C" and "c" are true | |
3533 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 | |
3534 | # and UTF-8. In EBCDIC the first example would be | |
3535 | # $foo = pack("CCCC",193,194,195,196); | |
3536 | ||
3537 | $foo = pack("s2",1,2); | |
3538 | # "\1\0\2\0" on little-endian | |
3539 | # "\0\1\0\2" on big-endian | |
3540 | ||
3541 | $foo = pack("a4","abcd","x","y","z"); | |
3542 | # "abcd" | |
3543 | ||
3544 | $foo = pack("aaaa","abcd","x","y","z"); | |
3545 | # "axyz" | |
3546 | ||
3547 | $foo = pack("a14","abcdefg"); | |
3548 | # "abcdefg\0\0\0\0\0\0\0" | |
3549 | ||
3550 | $foo = pack("i9pl", gmtime); | |
3551 | # a real struct tm (on my system anyway) | |
3552 | ||
3553 | $utmp_template = "Z8 Z8 Z16 L"; | |
3554 | $utmp = pack($utmp_template, @utmp1); | |
3555 | # a struct utmp (BSDish) | |
3556 | ||
3557 | @utmp2 = unpack($utmp_template, $utmp); | |
3558 | # "@utmp1" eq "@utmp2" | |
3559 | ||
3560 | sub bintodec { | |
3561 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); | |
3562 | } | |
3563 | ||
3564 | $foo = pack('sx2l', 12, 34); | |
3565 | # short 12, two zero bytes padding, long 34 | |
3566 | $bar = pack('s@4l', 12, 34); | |
3567 | # short 12, zero fill to position 4, long 34 | |
3568 | # $foo eq $bar | |
3569 | ||
3570 | The same template may generally also be used in unpack(). | |
3571 | ||
3572 | =item package NAMESPACE | |
3573 | ||
3574 | =item package | |
3575 | ||
3576 | Declares the compilation unit as being in the given namespace. The scope | |
3577 | of the package declaration is from the declaration itself through the end | |
3578 | of the enclosing block, file, or eval (the same as the C<my> operator). | |
3579 | All further unqualified dynamic identifiers will be in this namespace. | |
3580 | A package statement affects only dynamic variables--including those | |
3581 | you've used C<local> on--but I<not> lexical variables, which are created | |
3582 | with C<my>. Typically it would be the first declaration in a file to | |
3583 | be included by the C<require> or C<use> operator. You can switch into a | |
3584 | package in more than one place; it merely influences which symbol table | |
3585 | is used by the compiler for the rest of that block. You can refer to | |
3586 | variables and filehandles in other packages by prefixing the identifier | |
3587 | with the package name and a double colon: C<$Package::Variable>. | |
3588 | If the package name is null, the C<main> package as assumed. That is, | |
3589 | C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, | |
3590 | still seen in older code). | |
3591 | ||
3592 | If NAMESPACE is omitted, then there is no current package, and all | |
3593 | identifiers must be fully qualified or lexicals. However, you are | |
3594 | strongly advised not to make use of this feature. Its use can cause | |
3595 | unexpected behaviour, even crashing some versions of Perl. It is | |
3596 | deprecated, and will be removed from a future release. | |
3597 | ||
3598 | See L<perlmod/"Packages"> for more information about packages, modules, | |
3599 | and classes. See L<perlsub> for other scoping issues. | |
3600 | ||
3601 | =item pipe READHANDLE,WRITEHANDLE | |
3602 | ||
3603 | Opens a pair of connected pipes like the corresponding system call. | |
3604 | Note that if you set up a loop of piped processes, deadlock can occur | |
3605 | unless you are very careful. In addition, note that Perl's pipes use | |
3606 | IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE | |
3607 | after each command, depending on the application. | |
3608 | ||
3609 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> | |
3610 | for examples of such things. | |
3611 | ||
3612 | On systems that support a close-on-exec flag on files, the flag will be set | |
3613 | for the newly opened file descriptors as determined by the value of $^F. | |
3614 | See L<perlvar/$^F>. | |
3615 | ||
3616 | =item pop ARRAY | |
3617 | ||
3618 | =item pop | |
3619 | ||
3620 | Pops and returns the last value of the array, shortening the array by | |
3621 | one element. Has an effect similar to | |
3622 | ||
3623 | $ARRAY[$#ARRAY--] | |
3624 | ||
3625 | If there are no elements in the array, returns the undefined value | |
3626 | (although this may happen at other times as well). If ARRAY is | |
3627 | omitted, pops the C<@ARGV> array in the main program, and the C<@_> | |
3628 | array in subroutines, just like C<shift>. | |
3629 | ||
3630 | =item pos SCALAR | |
3631 | ||
3632 | =item pos | |
3633 | ||
3634 | Returns the offset of where the last C<m//g> search left off for the variable | |
3635 | in question (C<$_> is used when the variable is not specified). May be | |
3636 | modified to change that offset. Such modification will also influence | |
3637 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and | |
3638 | L<perlop>. | |
3639 | ||
3640 | =item print FILEHANDLE LIST | |
3641 | ||
3642 | =item print LIST | |
3643 | ||
3644 | =item print | |
3645 | ||
3646 | Prints a string or a list of strings. Returns true if successful. | |
3647 | FILEHANDLE may be a scalar variable name, in which case the variable | |
3648 | contains the name of or a reference to the filehandle, thus introducing | |
3649 | one level of indirection. (NOTE: If FILEHANDLE is a variable and | |
3650 | the next token is a term, it may be misinterpreted as an operator | |
3651 | unless you interpose a C<+> or put parentheses around the arguments.) | |
3652 | If FILEHANDLE is omitted, prints by default to standard output (or | |
3653 | to the last selected output channel--see L</select>). If LIST is | |
3654 | also omitted, prints C<$_> to the currently selected output channel. | |
3655 | To set the default output channel to something other than STDOUT | |
3656 | use the select operation. The current value of C<$,> (if any) is | |
3657 | printed between each LIST item. The current value of C<$\> (if | |
3658 | any) is printed after the entire LIST has been printed. Because | |
3659 | print takes a LIST, anything in the LIST is evaluated in list | |
3660 | context, and any subroutine that you call will have one or more of | |
3661 | its expressions evaluated in list context. Also be careful not to | |
3662 | follow the print keyword with a left parenthesis unless you want | |
3663 | the corresponding right parenthesis to terminate the arguments to | |
3664 | the print--interpose a C<+> or put parentheses around all the | |
3665 | arguments. | |
3666 | ||
3667 | Note that if you're storing FILEHANDLES in an array or other expression, | |
3668 | you will have to use a block returning its value instead: | |
3669 | ||
3670 | print { $files[$i] } "stuff\n"; | |
3671 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
3672 | ||
3673 | =item printf FILEHANDLE FORMAT, LIST | |
3674 | ||
3675 | =item printf FORMAT, LIST | |
3676 | ||
3677 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> | |
3678 | (the output record separator) is not appended. The first argument | |
3679 | of the list will be interpreted as the C<printf> format. See C<sprintf> | |
3680 | for an explanation of the format argument. If C<use locale> is in effect, | |
3681 | the character used for the decimal point in formatted real numbers is | |
3682 | affected by the LC_NUMERIC locale. See L<perllocale>. | |
3683 | ||
3684 | Don't fall into the trap of using a C<printf> when a simple | |
3685 | C<print> would do. The C<print> is more efficient and less | |
3686 | error prone. | |
3687 | ||
3688 | =item prototype FUNCTION | |
3689 | ||
3690 | Returns the prototype of a function as a string (or C<undef> if the | |
3691 | function has no prototype). FUNCTION is a reference to, or the name of, | |
3692 | the function whose prototype you want to retrieve. | |
3693 | ||
3694 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a | |
3695 | name for Perl builtin. If the builtin is not I<overridable> (such as | |
3696 | C<qw//>) or its arguments cannot be expressed by a prototype (such as | |
3697 | C<system>) returns C<undef> because the builtin does not really behave | |
3698 | like a Perl function. Otherwise, the string describing the equivalent | |
3699 | prototype is returned. | |
3700 | ||
3701 | =item push ARRAY,LIST | |
3702 | ||
3703 | Treats ARRAY as a stack, and pushes the values of LIST | |
3704 | onto the end of ARRAY. The length of ARRAY increases by the length of | |
3705 | LIST. Has the same effect as | |
3706 | ||
3707 | for $value (LIST) { | |
3708 | $ARRAY[++$#ARRAY] = $value; | |
3709 | } | |
3710 | ||
3711 | but is more efficient. Returns the new number of elements in the array. | |
3712 | ||
3713 | =item q/STRING/ | |
3714 | ||
3715 | =item qq/STRING/ | |
3716 | ||
3717 | =item qr/STRING/ | |
3718 | ||
3719 | =item qx/STRING/ | |
3720 | ||
3721 | =item qw/STRING/ | |
3722 | ||
3723 | Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. | |
3724 | ||
3725 | =item quotemeta EXPR | |
3726 | ||
3727 | =item quotemeta | |
3728 | ||
3729 | Returns the value of EXPR with all non-"word" | |
3730 | characters backslashed. (That is, all characters not matching | |
3731 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
3732 | returned string, regardless of any locale settings.) | |
3733 | This is the internal function implementing | |
3734 | the C<\Q> escape in double-quoted strings. | |
3735 | ||
3736 | If EXPR is omitted, uses C<$_>. | |
3737 | ||
3738 | =item rand EXPR | |
3739 | ||
3740 | =item rand | |
3741 | ||
3742 | Returns a random fractional number greater than or equal to C<0> and less | |
3743 | than the value of EXPR. (EXPR should be positive.) If EXPR is | |
3744 | omitted, the value C<1> is used. Currently EXPR with the value C<0> is | |
3745 | also special-cased as C<1> - this has not been documented before perl 5.8.0 | |
3746 | and is subject to change in future versions of perl. Automatically calls | |
3747 | C<srand> unless C<srand> has already been called. See also C<srand>. | |
3748 | ||
3749 | Apply C<int()> to the value returned by C<rand()> if you want random | |
3750 | integers instead of random fractional numbers. For example, | |
3751 | ||
3752 | int(rand(10)) | |
3753 | ||
3754 | returns a random integer between C<0> and C<9>, inclusive. | |
3755 | ||
3756 | (Note: If your rand function consistently returns numbers that are too | |
3757 | large or too small, then your version of Perl was probably compiled | |
3758 | with the wrong number of RANDBITS.) | |
3759 | ||
3760 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET | |
3761 | ||
3762 | =item read FILEHANDLE,SCALAR,LENGTH | |
3763 | ||
3764 | Attempts to read LENGTH I<characters> of data into variable SCALAR | |
3765 | from the specified FILEHANDLE. Returns the number of characters | |
3766 | actually read, C<0> at end of file, or undef if there was an error. | |
3767 | SCALAR will be grown or shrunk to the length actually read. If SCALAR | |
3768 | needs growing, the new bytes will be zero bytes. An OFFSET may be | |
3769 | specified to place the read data into some other place in SCALAR than | |
3770 | the beginning. The call is actually implemented in terms of either | |
3771 | Perl's or system's fread() call. To get a true read(2) system call, | |
3772 | see C<sysread>. | |
3773 | ||
3774 | Note the I<characters>: depending on the status of the filehandle, | |
3775 | either (8-bit) bytes or characters are read. By default all | |
3776 | filehandles operate on bytes, but for example if the filehandle has | |
3777 | been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> | |
3778 | pragma, L<open>), the I/O will operate on characters, not bytes. | |
3779 | ||
3780 | =item readdir DIRHANDLE | |
3781 | ||
3782 | Returns the next directory entry for a directory opened by C<opendir>. | |
3783 | If used in list context, returns all the rest of the entries in the | |
3784 | directory. If there are no more entries, returns an undefined value in | |
3785 | scalar context or a null list in list context. | |
3786 | ||
3787 | If you're planning to filetest the return values out of a C<readdir>, you'd | |
3788 | better prepend the directory in question. Otherwise, because we didn't | |
3789 | C<chdir> there, it would have been testing the wrong file. | |
3790 | ||
3791 | opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; | |
3792 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); | |
3793 | closedir DIR; | |
3794 | ||
3795 | =item readline EXPR | |
3796 | ||
3797 | Reads from the filehandle whose typeglob is contained in EXPR. In scalar | |
3798 | context, each call reads and returns the next line, until end-of-file is | |
3799 | reached, whereupon the subsequent call returns undef. In list context, | |
3800 | reads until end-of-file is reached and returns a list of lines. Note that | |
3801 | the notion of "line" used here is however you may have defined it | |
3802 | with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. | |
3803 | ||
3804 | When C<$/> is set to C<undef>, when readline() is in scalar | |
3805 | context (i.e. file slurp mode), and when an empty file is read, it | |
3806 | returns C<''> the first time, followed by C<undef> subsequently. | |
3807 | ||
3808 | This is the internal function implementing the C<< <EXPR> >> | |
3809 | operator, but you can use it directly. The C<< <EXPR> >> | |
3810 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
3811 | ||
3812 | $line = <STDIN>; | |
3813 | $line = readline(*STDIN); # same thing | |
3814 | ||
3815 | =item readlink EXPR | |
3816 | ||
3817 | =item readlink | |
3818 | ||
3819 | Returns the value of a symbolic link, if symbolic links are | |
3820 | implemented. If not, gives a fatal error. If there is some system | |
3821 | error, returns the undefined value and sets C<$!> (errno). If EXPR is | |
3822 | omitted, uses C<$_>. | |
3823 | ||
3824 | =item readpipe EXPR | |
3825 | ||
3826 | EXPR is executed as a system command. | |
3827 | The collected standard output of the command is returned. | |
3828 | In scalar context, it comes back as a single (potentially | |
3829 | multi-line) string. In list context, returns a list of lines | |
3830 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). | |
3831 | This is the internal function implementing the C<qx/EXPR/> | |
3832 | operator, but you can use it directly. The C<qx/EXPR/> | |
3833 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
3834 | ||
3835 | =item recv SOCKET,SCALAR,LENGTH,FLAGS | |
3836 | ||
3837 | Receives a message on a socket. Attempts to receive LENGTH characters | |
3838 | of data into variable SCALAR from the specified SOCKET filehandle. | |
3839 | SCALAR will be grown or shrunk to the length actually read. Takes the | |
3840 | same flags as the system call of the same name. Returns the address | |
3841 | of the sender if SOCKET's protocol supports this; returns an empty | |
3842 | string otherwise. If there's an error, returns the undefined value. | |
3843 | This call is actually implemented in terms of recvfrom(2) system call. | |
3844 | See L<perlipc/"UDP: Message Passing"> for examples. | |
3845 | ||
3846 | Note the I<characters>: depending on the status of the socket, either | |
3847 | (8-bit) bytes or characters are received. By default all sockets | |
3848 | operate on bytes, but for example if the socket has been changed using | |
3849 | binmode() to operate with the C<:utf8> I/O layer (see the C<open> | |
3850 | pragma, L<open>), the I/O will operate on characters, not bytes. | |
3851 | ||
3852 | =item redo LABEL | |
3853 | ||
3854 | =item redo | |
3855 | ||
3856 | The C<redo> command restarts the loop block without evaluating the | |
3857 | conditional again. The C<continue> block, if any, is not executed. If | |
3858 | the LABEL is omitted, the command refers to the innermost enclosing | |
3859 | loop. This command is normally used by programs that want to lie to | |
3860 | themselves about what was just input: | |
3861 | ||
3862 | # a simpleminded Pascal comment stripper | |
3863 | # (warning: assumes no { or } in strings) | |
3864 | LINE: while (<STDIN>) { | |
3865 | while (s|({.*}.*){.*}|$1 |) {} | |
3866 | s|{.*}| |; | |
3867 | if (s|{.*| |) { | |
3868 | $front = $_; | |
3869 | while (<STDIN>) { | |
3870 | if (/}/) { # end of comment? | |
3871 | s|^|$front\{|; | |
3872 | redo LINE; | |
3873 | } | |
3874 | } | |
3875 | } | |
3876 | print; | |
3877 | } | |
3878 | ||
3879 | C<redo> cannot be used to retry a block which returns a value such as | |
3880 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit | |
3881 | a grep() or map() operation. | |
3882 | ||
3883 | Note that a block by itself is semantically identical to a loop | |
3884 | that executes once. Thus C<redo> inside such a block will effectively | |
3885 | turn it into a looping construct. | |
3886 | ||
3887 | See also L</continue> for an illustration of how C<last>, C<next>, and | |
3888 | C<redo> work. | |
3889 | ||
3890 | =item ref EXPR | |
3891 | ||
3892 | =item ref | |
3893 | ||
3894 | Returns a true value if EXPR is a reference, false otherwise. If EXPR | |
3895 | is not specified, C<$_> will be used. The value returned depends on the | |
3896 | type of thing the reference is a reference to. | |
3897 | Builtin types include: | |
3898 | ||
3899 | SCALAR | |
3900 | ARRAY | |
3901 | HASH | |
3902 | CODE | |
3903 | REF | |
3904 | GLOB | |
3905 | LVALUE | |
3906 | ||
3907 | If the referenced object has been blessed into a package, then that package | |
3908 | name is returned instead. You can think of C<ref> as a C<typeof> operator. | |
3909 | ||
3910 | if (ref($r) eq "HASH") { | |
3911 | print "r is a reference to a hash.\n"; | |
3912 | } | |
3913 | unless (ref($r)) { | |
3914 | print "r is not a reference at all.\n"; | |
3915 | } | |
3916 | if (UNIVERSAL::isa($r, "HASH")) { # for subclassing | |
3917 | print "r is a reference to something that isa hash.\n"; | |
3918 | } | |
3919 | ||
3920 | See also L<perlref>. | |
3921 | ||
3922 | =item rename OLDNAME,NEWNAME | |
3923 | ||
3924 | Changes the name of a file; an existing file NEWNAME will be | |
3925 | clobbered. Returns true for success, false otherwise. | |
3926 | ||
3927 | Behavior of this function varies wildly depending on your system | |
3928 | implementation. For example, it will usually not work across file system | |
3929 | boundaries, even though the system I<mv> command sometimes compensates | |
3930 | for this. Other restrictions include whether it works on directories, | |
3931 | open files, or pre-existing files. Check L<perlport> and either the | |
3932 | rename(2) manpage or equivalent system documentation for details. | |
3933 | ||
3934 | =item require VERSION | |
3935 | ||
3936 | =item require EXPR | |
3937 | ||
3938 | =item require | |
3939 | ||
3940 | Demands a version of Perl specified by VERSION, or demands some semantics | |
3941 | specified by EXPR or by C<$_> if EXPR is not supplied. | |
3942 | ||
3943 | VERSION may be either a numeric argument such as 5.006, which will be | |
3944 | compared to C<$]>, or a literal of the form v5.6.1, which will be compared | |
3945 | to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if | |
3946 | VERSION is greater than the version of the current Perl interpreter. | |
3947 | Compare with L</use>, which can do a similar check at compile time. | |
3948 | ||
3949 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
3950 | avoided, because it leads to misleading error messages under earlier | |
3951 | versions of Perl which do not support this syntax. The equivalent numeric | |
3952 | version should be used instead. | |
3953 | ||
3954 | require v5.6.1; # run time version check | |
3955 | require 5.6.1; # ditto | |
3956 | require 5.006_001; # ditto; preferred for backwards compatibility | |
3957 | ||
3958 | Otherwise, demands that a library file be included if it hasn't already | |
3959 | been included. The file is included via the do-FILE mechanism, which is | |
3960 | essentially just a variety of C<eval>. Has semantics similar to the following | |
3961 | subroutine: | |
3962 | ||
3963 | sub require { | |
3964 | my($filename) = @_; | |
3965 | return 1 if $INC{$filename}; | |
3966 | my($realfilename,$result); | |
3967 | ITER: { | |
3968 | foreach $prefix (@INC) { | |
3969 | $realfilename = "$prefix/$filename"; | |
3970 | if (-f $realfilename) { | |
3971 | $INC{$filename} = $realfilename; | |
3972 | $result = do $realfilename; | |
3973 | last ITER; | |
3974 | } | |
3975 | } | |
3976 | die "Can't find $filename in \@INC"; | |
3977 | } | |
3978 | delete $INC{$filename} if $@ || !$result; | |
3979 | die $@ if $@; | |
3980 | die "$filename did not return true value" unless $result; | |
3981 | return $result; | |
3982 | } | |
3983 | ||
3984 | Note that the file will not be included twice under the same specified | |
3985 | name. The file must return true as the last statement to indicate | |
3986 | successful execution of any initialization code, so it's customary to | |
3987 | end such a file with C<1;> unless you're sure it'll return true | |
3988 | otherwise. But it's better just to put the C<1;>, in case you add more | |
3989 | statements. | |
3990 | ||
3991 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and | |
3992 | replaces "F<::>" with "F</>" in the filename for you, | |
3993 | to make it easy to load standard modules. This form of loading of | |
3994 | modules does not risk altering your namespace. | |
3995 | ||
3996 | In other words, if you try this: | |
3997 | ||
3998 | require Foo::Bar; # a splendid bareword | |
3999 | ||
4000 | The require function will actually look for the "F<Foo/Bar.pm>" file in the | |
4001 | directories specified in the C<@INC> array. | |
4002 | ||
4003 | But if you try this: | |
4004 | ||
4005 | $class = 'Foo::Bar'; | |
4006 | require $class; # $class is not a bareword | |
4007 | #or | |
4008 | require "Foo::Bar"; # not a bareword because of the "" | |
4009 | ||
4010 | The require function will look for the "F<Foo::Bar>" file in the @INC array and | |
4011 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: | |
4012 | ||
4013 | eval "require $class"; | |
4014 | ||
4015 | You can also insert hooks into the import facility, by putting directly | |
4016 | Perl code into the @INC array. There are three forms of hooks: subroutine | |
4017 | references, array references and blessed objects. | |
4018 | ||
4019 | Subroutine references are the simplest case. When the inclusion system | |
4020 | walks through @INC and encounters a subroutine, this subroutine gets | |
4021 | called with two parameters, the first being a reference to itself, and the | |
4022 | second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The | |
4023 | subroutine should return C<undef> or a filehandle, from which the file to | |
4024 | include will be read. If C<undef> is returned, C<require> will look at | |
4025 | the remaining elements of @INC. | |
4026 | ||
4027 | If the hook is an array reference, its first element must be a subroutine | |
4028 | reference. This subroutine is called as above, but the first parameter is | |
4029 | the array reference. This enables to pass indirectly some arguments to | |
4030 | the subroutine. | |
4031 | ||
4032 | In other words, you can write: | |
4033 | ||
4034 | push @INC, \&my_sub; | |
4035 | sub my_sub { | |
4036 | my ($coderef, $filename) = @_; # $coderef is \&my_sub | |
4037 | ... | |
4038 | } | |
4039 | ||
4040 | or: | |
4041 | ||
4042 | push @INC, [ \&my_sub, $x, $y, ... ]; | |
4043 | sub my_sub { | |
4044 | my ($arrayref, $filename) = @_; | |
4045 | # Retrieve $x, $y, ... | |
4046 | my @parameters = @$arrayref[1..$#$arrayref]; | |
4047 | ... | |
4048 | } | |
4049 | ||
4050 | If the hook is an object, it must provide an INC method, that will be | |
4051 | called as above, the first parameter being the object itself. (Note that | |
4052 | you must fully qualify the sub's name, as it is always forced into package | |
4053 | C<main>.) Here is a typical code layout: | |
4054 | ||
4055 | # In Foo.pm | |
4056 | package Foo; | |
4057 | sub new { ... } | |
4058 | sub Foo::INC { | |
4059 | my ($self, $filename) = @_; | |
4060 | ... | |
4061 | } | |
4062 | ||
4063 | # In the main program | |
4064 | push @INC, new Foo(...); | |
4065 | ||
4066 | Note that these hooks are also permitted to set the %INC entry | |
4067 | corresponding to the files they have loaded. See L<perlvar/%INC>. | |
4068 | ||
4069 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. | |
4070 | ||
4071 | =item reset EXPR | |
4072 | ||
4073 | =item reset | |
4074 | ||
4075 | Generally used in a C<continue> block at the end of a loop to clear | |
4076 | variables and reset C<??> searches so that they work again. The | |
4077 | expression is interpreted as a list of single characters (hyphens | |
4078 | allowed for ranges). All variables and arrays beginning with one of | |
4079 | those letters are reset to their pristine state. If the expression is | |
4080 | omitted, one-match searches (C<?pattern?>) are reset to match again. Resets | |
4081 | only variables or searches in the current package. Always returns | |
4082 | 1. Examples: | |
4083 | ||
4084 | reset 'X'; # reset all X variables | |
4085 | reset 'a-z'; # reset lower case variables | |
4086 | reset; # just reset ?one-time? searches | |
4087 | ||
4088 | Resetting C<"A-Z"> is not recommended because you'll wipe out your | |
4089 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package | |
4090 | variables--lexical variables are unaffected, but they clean themselves | |
4091 | up on scope exit anyway, so you'll probably want to use them instead. | |
4092 | See L</my>. | |
4093 | ||
4094 | =item return EXPR | |
4095 | ||
4096 | =item return | |
4097 | ||
4098 | Returns from a subroutine, C<eval>, or C<do FILE> with the value | |
4099 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void | |
4100 | context, depending on how the return value will be used, and the context | |
4101 | may vary from one execution to the next (see C<wantarray>). If no EXPR | |
4102 | is given, returns an empty list in list context, the undefined value in | |
4103 | scalar context, and (of course) nothing at all in a void context. | |
4104 | ||
4105 | (Note that in the absence of an explicit C<return>, a subroutine, eval, | |
4106 | or do FILE will automatically return the value of the last expression | |
4107 | evaluated.) | |
4108 | ||
4109 | =item reverse LIST | |
4110 | ||
4111 | In list context, returns a list value consisting of the elements | |
4112 | of LIST in the opposite order. In scalar context, concatenates the | |
4113 | elements of LIST and returns a string value with all characters | |
4114 | in the opposite order. | |
4115 | ||
4116 | print reverse <>; # line tac, last line first | |
4117 | ||
4118 | undef $/; # for efficiency of <> | |
4119 | print scalar reverse <>; # character tac, last line tsrif | |
4120 | ||
4121 | This operator is also handy for inverting a hash, although there are some | |
4122 | caveats. If a value is duplicated in the original hash, only one of those | |
4123 | can be represented as a key in the inverted hash. Also, this has to | |
4124 | unwind one hash and build a whole new one, which may take some time | |
4125 | on a large hash, such as from a DBM file. | |
4126 | ||
4127 | %by_name = reverse %by_address; # Invert the hash | |
4128 | ||
4129 | =item rewinddir DIRHANDLE | |
4130 | ||
4131 | Sets the current position to the beginning of the directory for the | |
4132 | C<readdir> routine on DIRHANDLE. | |
4133 | ||
4134 | =item rindex STR,SUBSTR,POSITION | |
4135 | ||
4136 | =item rindex STR,SUBSTR | |
4137 | ||
4138 | Works just like index() except that it returns the position of the LAST | |
4139 | occurrence of SUBSTR in STR. If POSITION is specified, returns the | |
4140 | last occurrence at or before that position. | |
4141 | ||
4142 | =item rmdir FILENAME | |
4143 | ||
4144 | =item rmdir | |
4145 | ||
4146 | Deletes the directory specified by FILENAME if that directory is empty. If it | |
4147 | succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If | |
4148 | FILENAME is omitted, uses C<$_>. | |
4149 | ||
4150 | =item s/// | |
4151 | ||
4152 | The substitution operator. See L<perlop>. | |
4153 | ||
4154 | =item scalar EXPR | |
4155 | ||
4156 | Forces EXPR to be interpreted in scalar context and returns the value | |
4157 | of EXPR. | |
4158 | ||
4159 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
4160 | ||
4161 | There is no equivalent operator to force an expression to | |
4162 | be interpolated in list context because in practice, this is never | |
4163 | needed. If you really wanted to do so, however, you could use | |
4164 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
4165 | C<(some expression)> suffices. | |
4166 | ||
4167 | Because C<scalar> is unary operator, if you accidentally use for EXPR a | |
4168 | parenthesized list, this behaves as a scalar comma expression, evaluating | |
4169 | all but the last element in void context and returning the final element | |
4170 | evaluated in scalar context. This is seldom what you want. | |
4171 | ||
4172 | The following single statement: | |
4173 | ||
4174 | print uc(scalar(&foo,$bar)),$baz; | |
4175 | ||
4176 | is the moral equivalent of these two: | |
4177 | ||
4178 | &foo; | |
4179 | print(uc($bar),$baz); | |
4180 | ||
4181 | See L<perlop> for more details on unary operators and the comma operator. | |
4182 | ||
4183 | =item seek FILEHANDLE,POSITION,WHENCE | |
4184 | ||
4185 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. | |
4186 | FILEHANDLE may be an expression whose value gives the name of the | |
4187 | filehandle. The values for WHENCE are C<0> to set the new position | |
4188 | I<in bytes> to POSITION, C<1> to set it to the current position plus | |
4189 | POSITION, and C<2> to set it to EOF plus POSITION (typically | |
4190 | negative). For WHENCE you may use the constants C<SEEK_SET>, | |
4191 | C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end | |
4192 | of the file) from the Fcntl module. Returns C<1> upon success, C<0> | |
4193 | otherwise. | |
4194 | ||
4195 | Note the I<in bytes>: even if the filehandle has been set to | |
4196 | operate on characters (for example by using the C<:utf8> open | |
4197 | layer), tell() will return byte offsets, not character offsets | |
4198 | (because implementing that would render seek() and tell() rather slow). | |
4199 | ||
4200 | If you want to position file for C<sysread> or C<syswrite>, don't use | |
4201 | C<seek>--buffering makes its effect on the file's system position | |
4202 | unpredictable and non-portable. Use C<sysseek> instead. | |
4203 | ||
4204 | Due to the rules and rigors of ANSI C, on some systems you have to do a | |
4205 | seek whenever you switch between reading and writing. Amongst other | |
4206 | things, this may have the effect of calling stdio's clearerr(3). | |
4207 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: | |
4208 | ||
4209 | seek(TEST,0,1); | |
4210 | ||
4211 | This is also useful for applications emulating C<tail -f>. Once you hit | |
4212 | EOF on your read, and then sleep for a while, you might have to stick in a | |
4213 | seek() to reset things. The C<seek> doesn't change the current position, | |
4214 | but it I<does> clear the end-of-file condition on the handle, so that the | |
4215 | next C<< <FILE> >> makes Perl try again to read something. We hope. | |
4216 | ||
4217 | If that doesn't work (some IO implementations are particularly | |
4218 | cantankerous), then you may need something more like this: | |
4219 | ||
4220 | for (;;) { | |
4221 | for ($curpos = tell(FILE); $_ = <FILE>; | |
4222 | $curpos = tell(FILE)) { | |
4223 | # search for some stuff and put it into files | |
4224 | } | |
4225 | sleep($for_a_while); | |
4226 | seek(FILE, $curpos, 0); | |
4227 | } | |
4228 | ||
4229 | =item seekdir DIRHANDLE,POS | |
4230 | ||
4231 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS | |
4232 | must be a value returned by C<telldir>. Has the same caveats about | |
4233 | possible directory compaction as the corresponding system library | |
4234 | routine. | |
4235 | ||
4236 | =item select FILEHANDLE | |
4237 | ||
4238 | =item select | |
4239 | ||
4240 | Returns the currently selected filehandle. Sets the current default | |
4241 | filehandle for output, if FILEHANDLE is supplied. This has two | |
4242 | effects: first, a C<write> or a C<print> without a filehandle will | |
4243 | default to this FILEHANDLE. Second, references to variables related to | |
4244 | output will refer to this output channel. For example, if you have to | |
4245 | set the top of form format for more than one output channel, you might | |
4246 | do the following: | |
4247 | ||
4248 | select(REPORT1); | |
4249 | $^ = 'report1_top'; | |
4250 | select(REPORT2); | |
4251 | $^ = 'report2_top'; | |
4252 | ||
4253 | FILEHANDLE may be an expression whose value gives the name of the | |
4254 | actual filehandle. Thus: | |
4255 | ||
4256 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
4257 | ||
4258 | Some programmers may prefer to think of filehandles as objects with | |
4259 | methods, preferring to write the last example as: | |
4260 | ||
4261 | use IO::Handle; | |
4262 | STDERR->autoflush(1); | |
4263 | ||
4264 | =item select RBITS,WBITS,EBITS,TIMEOUT | |
4265 | ||
4266 | This calls the select(2) system call with the bit masks specified, which | |
4267 | can be constructed using C<fileno> and C<vec>, along these lines: | |
4268 | ||
4269 | $rin = $win = $ein = ''; | |
4270 | vec($rin,fileno(STDIN),1) = 1; | |
4271 | vec($win,fileno(STDOUT),1) = 1; | |
4272 | $ein = $rin | $win; | |
4273 | ||
4274 | If you want to select on many filehandles you might wish to write a | |
4275 | subroutine: | |
4276 | ||
4277 | sub fhbits { | |
4278 | my(@fhlist) = split(' ',$_[0]); | |
4279 | my($bits); | |
4280 | for (@fhlist) { | |
4281 | vec($bits,fileno($_),1) = 1; | |
4282 | } | |
4283 | $bits; | |
4284 | } | |
4285 | $rin = fhbits('STDIN TTY SOCK'); | |
4286 | ||
4287 | The usual idiom is: | |
4288 | ||
4289 | ($nfound,$timeleft) = | |
4290 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
4291 | ||
4292 | or to block until something becomes ready just do this | |
4293 | ||
4294 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
4295 | ||
4296 | Most systems do not bother to return anything useful in $timeleft, so | |
4297 | calling select() in scalar context just returns $nfound. | |
4298 | ||
4299 | Any of the bit masks can also be undef. The timeout, if specified, is | |
4300 | in seconds, which may be fractional. Note: not all implementations are | |
4301 | capable of returning the $timeleft. If not, they always return | |
4302 | $timeleft equal to the supplied $timeout. | |
4303 | ||
4304 | You can effect a sleep of 250 milliseconds this way: | |
4305 | ||
4306 | select(undef, undef, undef, 0.25); | |
4307 | ||
4308 | Note that whether C<select> gets restarted after signals (say, SIGALRM) | |
4309 | is implementation-dependent. | |
4310 | ||
4311 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> | |
4312 | or <FH>) with C<select>, except as permitted by POSIX, and even | |
4313 | then only on POSIX systems. You have to use C<sysread> instead. | |
4314 | ||
4315 | =item semctl ID,SEMNUM,CMD,ARG | |
4316 | ||
4317 | Calls the System V IPC function C<semctl>. You'll probably have to say | |
4318 | ||
4319 | use IPC::SysV; | |
4320 | ||
4321 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
4322 | GETALL, then ARG must be a variable which will hold the returned | |
4323 | semid_ds structure or semaphore value array. Returns like C<ioctl>: | |
4324 | the undefined value for error, "C<0 but true>" for zero, or the actual | |
4325 | return value otherwise. The ARG must consist of a vector of native | |
4326 | short integers, which may be created with C<pack("s!",(0)x$nsem)>. | |
4327 | See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> | |
4328 | documentation. | |
4329 | ||
4330 | =item semget KEY,NSEMS,FLAGS | |
4331 | ||
4332 | Calls the System V IPC function semget. Returns the semaphore id, or | |
4333 | the undefined value if there is an error. See also | |
4334 | L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> | |
4335 | documentation. | |
4336 | ||
4337 | =item semop KEY,OPSTRING | |
4338 | ||
4339 | Calls the System V IPC function semop to perform semaphore operations | |
4340 | such as signalling and waiting. OPSTRING must be a packed array of | |
4341 | semop structures. Each semop structure can be generated with | |
4342 | C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore | |
4343 | operations is implied by the length of OPSTRING. Returns true if | |
4344 | successful, or false if there is an error. As an example, the | |
4345 | following code waits on semaphore $semnum of semaphore id $semid: | |
4346 | ||
4347 | $semop = pack("s!3", $semnum, -1, 0); | |
4348 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); | |
4349 | ||
4350 | To signal the semaphore, replace C<-1> with C<1>. See also | |
4351 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> | |
4352 | documentation. | |
4353 | ||
4354 | =item send SOCKET,MSG,FLAGS,TO | |
4355 | ||
4356 | =item send SOCKET,MSG,FLAGS | |
4357 | ||
4358 | Sends a message on a socket. Attempts to send the scalar MSG to the | |
4359 | SOCKET filehandle. Takes the same flags as the system call of the | |
4360 | same name. On unconnected sockets you must specify a destination to | |
4361 | send TO, in which case it does a C C<sendto>. Returns the number of | |
4362 | characters sent, or the undefined value if there is an error. The C | |
4363 | system call sendmsg(2) is currently unimplemented. See | |
4364 | L<perlipc/"UDP: Message Passing"> for examples. | |
4365 | ||
4366 | Note the I<characters>: depending on the status of the socket, either | |
4367 | (8-bit) bytes or characters are sent. By default all sockets operate | |
4368 | on bytes, but for example if the socket has been changed using | |
4369 | binmode() to operate with the C<:utf8> I/O layer (see L</open>, or | |
4370 | the C<open> pragma, L<open>), the I/O will operate on characters, not | |
4371 | bytes. | |
4372 | ||
4373 | =item setpgrp PID,PGRP | |
4374 | ||
4375 | Sets the current process group for the specified PID, C<0> for the current | |
4376 | process. Will produce a fatal error if used on a machine that doesn't | |
4377 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, | |
4378 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not | |
4379 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also | |
4380 | C<POSIX::setsid()>. | |
4381 | ||
4382 | =item setpriority WHICH,WHO,PRIORITY | |
4383 | ||
4384 | Sets the current priority for a process, a process group, or a user. | |
4385 | (See setpriority(2).) Will produce a fatal error if used on a machine | |
4386 | that doesn't implement setpriority(2). | |
4387 | ||
4388 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL | |
4389 | ||
4390 | Sets the socket option requested. Returns undefined if there is an | |
4391 | error. OPTVAL may be specified as C<undef> if you don't want to pass an | |
4392 | argument. | |
4393 | ||
4394 | =item shift ARRAY | |
4395 | ||
4396 | =item shift | |
4397 | ||
4398 | Shifts the first value of the array off and returns it, shortening the | |
4399 | array by 1 and moving everything down. If there are no elements in the | |
4400 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
4401 | C<@_> array within the lexical scope of subroutines and formats, and the | |
4402 | C<@ARGV> array at file scopes or within the lexical scopes established by | |
4403 | the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> | |
4404 | constructs. | |
4405 | ||
4406 | See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the | |
4407 | same thing to the left end of an array that C<pop> and C<push> do to the | |
4408 | right end. | |
4409 | ||
4410 | =item shmctl ID,CMD,ARG | |
4411 | ||
4412 | Calls the System V IPC function shmctl. You'll probably have to say | |
4413 | ||
4414 | use IPC::SysV; | |
4415 | ||
4416 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, | |
4417 | then ARG must be a variable which will hold the returned C<shmid_ds> | |
4418 | structure. Returns like ioctl: the undefined value for error, "C<0> but | |
4419 | true" for zero, or the actual return value otherwise. | |
4420 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. | |
4421 | ||
4422 | =item shmget KEY,SIZE,FLAGS | |
4423 | ||
4424 | Calls the System V IPC function shmget. Returns the shared memory | |
4425 | segment id, or the undefined value if there is an error. | |
4426 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. | |
4427 | ||
4428 | =item shmread ID,VAR,POS,SIZE | |
4429 | ||
4430 | =item shmwrite ID,STRING,POS,SIZE | |
4431 | ||
4432 | Reads or writes the System V shared memory segment ID starting at | |
4433 | position POS for size SIZE by attaching to it, copying in/out, and | |
4434 | detaching from it. When reading, VAR must be a variable that will | |
4435 | hold the data read. When writing, if STRING is too long, only SIZE | |
4436 | bytes are used; if STRING is too short, nulls are written to fill out | |
4437 | SIZE bytes. Return true if successful, or false if there is an error. | |
4438 | shmread() taints the variable. See also L<perlipc/"SysV IPC">, | |
4439 | C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. | |
4440 | ||
4441 | =item shutdown SOCKET,HOW | |
4442 | ||
4443 | Shuts down a socket connection in the manner indicated by HOW, which | |
4444 | has the same interpretation as in the system call of the same name. | |
4445 | ||
4446 | shutdown(SOCKET, 0); # I/we have stopped reading data | |
4447 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
4448 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
4449 | ||
4450 | This is useful with sockets when you want to tell the other | |
4451 | side you're done writing but not done reading, or vice versa. | |
4452 | It's also a more insistent form of close because it also | |
4453 | disables the file descriptor in any forked copies in other | |
4454 | processes. | |
4455 | ||
4456 | =item sin EXPR | |
4457 | ||
4458 | =item sin | |
4459 | ||
4460 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, | |
4461 | returns sine of C<$_>. | |
4462 | ||
4463 | For the inverse sine operation, you may use the C<Math::Trig::asin> | |
4464 | function, or use this relation: | |
4465 | ||
4466 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
4467 | ||
4468 | =item sleep EXPR | |
4469 | ||
4470 | =item sleep | |
4471 | ||
4472 | Causes the script to sleep for EXPR seconds, or forever if no EXPR. | |
4473 | May be interrupted if the process receives a signal such as C<SIGALRM>. | |
4474 | Returns the number of seconds actually slept. You probably cannot | |
4475 | mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented | |
4476 | using C<alarm>. | |
4477 | ||
4478 | On some older systems, it may sleep up to a full second less than what | |
4479 | you requested, depending on how it counts seconds. Most modern systems | |
4480 | always sleep the full amount. They may appear to sleep longer than that, | |
4481 | however, because your process might not be scheduled right away in a | |
4482 | busy multitasking system. | |
4483 | ||
4484 | For delays of finer granularity than one second, you may use Perl's | |
4485 | C<syscall> interface to access setitimer(2) if your system supports | |
4486 | it, or else see L</select> above. The Time::HiRes module (from CPAN, | |
4487 | and starting from Perl 5.8 part of the standard distribution) may also | |
4488 | help. | |
4489 | ||
4490 | See also the POSIX module's C<pause> function. | |
4491 | ||
4492 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL | |
4493 | ||
4494 | Opens a socket of the specified kind and attaches it to filehandle | |
4495 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for | |
4496 | the system call of the same name. You should C<use Socket> first | |
4497 | to get the proper definitions imported. See the examples in | |
4498 | L<perlipc/"Sockets: Client/Server Communication">. | |
4499 | ||
4500 | On systems that support a close-on-exec flag on files, the flag will | |
4501 | be set for the newly opened file descriptor, as determined by the | |
4502 | value of $^F. See L<perlvar/$^F>. | |
4503 | ||
4504 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL | |
4505 | ||
4506 | Creates an unnamed pair of sockets in the specified domain, of the | |
4507 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as | |
4508 | for the system call of the same name. If unimplemented, yields a fatal | |
4509 | error. Returns true if successful. | |
4510 | ||
4511 | On systems that support a close-on-exec flag on files, the flag will | |
4512 | be set for the newly opened file descriptors, as determined by the value | |
4513 | of $^F. See L<perlvar/$^F>. | |
4514 | ||
4515 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call | |
4516 | to C<pipe(Rdr, Wtr)> is essentially: | |
4517 | ||
4518 | use Socket; | |
4519 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
4520 | shutdown(Rdr, 1); # no more writing for reader | |
4521 | shutdown(Wtr, 0); # no more reading for writer | |
4522 | ||
4523 | See L<perlipc> for an example of socketpair use. Perl 5.8 and later will | |
4524 | emulate socketpair using IP sockets to localhost if your system implements | |
4525 | sockets but not socketpair. | |
4526 | ||
4527 | =item sort SUBNAME LIST | |
4528 | ||
4529 | =item sort BLOCK LIST | |
4530 | ||
4531 | =item sort LIST | |
4532 | ||
4533 | In list context, this sorts the LIST and returns the sorted list value. | |
4534 | In scalar context, the behaviour of C<sort()> is undefined. | |
4535 | ||
4536 | If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison | |
4537 | order. If SUBNAME is specified, it gives the name of a subroutine | |
4538 | that returns an integer less than, equal to, or greater than C<0>, | |
4539 | depending on how the elements of the list are to be ordered. (The C<< | |
4540 | <=> >> and C<cmp> operators are extremely useful in such routines.) | |
4541 | SUBNAME may be a scalar variable name (unsubscripted), in which case | |
4542 | the value provides the name of (or a reference to) the actual | |
4543 | subroutine to use. In place of a SUBNAME, you can provide a BLOCK as | |
4544 | an anonymous, in-line sort subroutine. | |
4545 | ||
4546 | If the subroutine's prototype is C<($$)>, the elements to be compared | |
4547 | are passed by reference in C<@_>, as for a normal subroutine. This is | |
4548 | slower than unprototyped subroutines, where the elements to be | |
4549 | compared are passed into the subroutine | |
4550 | as the package global variables $a and $b (see example below). Note that | |
4551 | in the latter case, it is usually counter-productive to declare $a and | |
4552 | $b as lexicals. | |
4553 | ||
4554 | In either case, the subroutine may not be recursive. The values to be | |
4555 | compared are always passed by reference, so don't modify them. | |
4556 | ||
4557 | You also cannot exit out of the sort block or subroutine using any of the | |
4558 | loop control operators described in L<perlsyn> or with C<goto>. | |
4559 | ||
4560 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the | |
4561 | current collation locale. See L<perllocale>. | |
4562 | ||
4563 | Perl 5.6 and earlier used a quicksort algorithm to implement sort. | |
4564 | That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort | |
4565 | preserves the input order of elements that compare equal. Although | |
4566 | quicksort's run time is O(NlogN) when averaged over all arrays of | |
4567 | length N, the time can be O(N**2), I<quadratic> behavior, for some | |
4568 | inputs.) In 5.7, the quicksort implementation was replaced with | |
4569 | a stable mergesort algorithm whose worst case behavior is O(NlogN). | |
4570 | But benchmarks indicated that for some inputs, on some platforms, | |
4571 | the original quicksort was faster. 5.8 has a sort pragma for | |
4572 | limited control of the sort. Its rather blunt control of the | |
4573 | underlying algorithm may not persist into future perls, but the | |
4574 | ability to characterize the input or output in implementation | |
4575 | independent ways quite probably will. See L</use>. | |
4576 | ||
4577 | Examples: | |
4578 | ||
4579 | # sort lexically | |
4580 | @articles = sort @files; | |
4581 | ||
4582 | # same thing, but with explicit sort routine | |
4583 | @articles = sort {$a cmp $b} @files; | |
4584 | ||
4585 | # now case-insensitively | |
4586 | @articles = sort {uc($a) cmp uc($b)} @files; | |
4587 | ||
4588 | # same thing in reversed order | |
4589 | @articles = sort {$b cmp $a} @files; | |
4590 | ||
4591 | # sort numerically ascending | |
4592 | @articles = sort {$a <=> $b} @files; | |
4593 | ||
4594 | # sort numerically descending | |
4595 | @articles = sort {$b <=> $a} @files; | |
4596 | ||
4597 | # this sorts the %age hash by value instead of key | |
4598 | # using an in-line function | |
4599 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; | |
4600 | ||
4601 | # sort using explicit subroutine name | |
4602 | sub byage { | |
4603 | $age{$a} <=> $age{$b}; # presuming numeric | |
4604 | } | |
4605 | @sortedclass = sort byage @class; | |
4606 | ||
4607 | sub backwards { $b cmp $a } | |
4608 | @harry = qw(dog cat x Cain Abel); | |
4609 | @george = qw(gone chased yz Punished Axed); | |
4610 | print sort @harry; | |
4611 | # prints AbelCaincatdogx | |
4612 | print sort backwards @harry; | |
4613 | # prints xdogcatCainAbel | |
4614 | print sort @george, 'to', @harry; | |
4615 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz | |
4616 | ||
4617 | # inefficiently sort by descending numeric compare using | |
4618 | # the first integer after the first = sign, or the | |
4619 | # whole record case-insensitively otherwise | |
4620 | ||
4621 | @new = sort { | |
4622 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
4623 | || | |
4624 | uc($a) cmp uc($b) | |
4625 | } @old; | |
4626 | ||
4627 | # same thing, but much more efficiently; | |
4628 | # we'll build auxiliary indices instead | |
4629 | # for speed | |
4630 | @nums = @caps = (); | |
4631 | for (@old) { | |
4632 | push @nums, /=(\d+)/; | |
4633 | push @caps, uc($_); | |
4634 | } | |
4635 | ||
4636 | @new = @old[ sort { | |
4637 | $nums[$b] <=> $nums[$a] | |
4638 | || | |
4639 | $caps[$a] cmp $caps[$b] | |
4640 | } 0..$#old | |
4641 | ]; | |
4642 | ||
4643 | # same thing, but without any temps | |
4644 | @new = map { $_->[0] } | |
4645 | sort { $b->[1] <=> $a->[1] | |
4646 | || | |
4647 | $a->[2] cmp $b->[2] | |
4648 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
4649 | ||
4650 | # using a prototype allows you to use any comparison subroutine | |
4651 | # as a sort subroutine (including other package's subroutines) | |
4652 | package other; | |
4653 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here | |
4654 | ||
4655 | package main; | |
4656 | @new = sort other::backwards @old; | |
4657 | ||
4658 | # guarantee stability, regardless of algorithm | |
4659 | use sort 'stable'; | |
4660 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; | |
4661 | ||
4662 | # force use of mergesort (not portable outside Perl 5.8) | |
4663 | use sort '_mergesort'; # note discouraging _ | |
4664 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; | |
4665 | ||
4666 | If you're using strict, you I<must not> declare $a | |
4667 | and $b as lexicals. They are package globals. That means | |
4668 | if you're in the C<main> package and type | |
4669 | ||
4670 | @articles = sort {$b <=> $a} @files; | |
4671 | ||
4672 | then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), | |
4673 | but if you're in the C<FooPack> package, it's the same as typing | |
4674 | ||
4675 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
4676 | ||
4677 | The comparison function is required to behave. If it returns | |
4678 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and | |
4679 | sometimes saying the opposite, for example) the results are not | |
4680 | well-defined. | |
4681 | ||
4682 | =item splice ARRAY,OFFSET,LENGTH,LIST | |
4683 | ||
4684 | =item splice ARRAY,OFFSET,LENGTH | |
4685 | ||
4686 | =item splice ARRAY,OFFSET | |
4687 | ||
4688 | =item splice ARRAY | |
4689 | ||
4690 | Removes the elements designated by OFFSET and LENGTH from an array, and | |
4691 | replaces them with the elements of LIST, if any. In list context, | |
4692 | returns the elements removed from the array. In scalar context, | |
4693 | returns the last element removed, or C<undef> if no elements are | |
4694 | removed. The array grows or shrinks as necessary. | |
4695 | If OFFSET is negative then it starts that far from the end of the array. | |
4696 | If LENGTH is omitted, removes everything from OFFSET onward. | |
4697 | If LENGTH is negative, removes the elements from OFFSET onward | |
4698 | except for -LENGTH elements at the end of the array. | |
4699 | If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is | |
4700 | past the end of the array, perl issues a warning, and splices at the | |
4701 | end of the array. | |
4702 | ||
4703 | The following equivalences hold (assuming C<$[ == 0>): | |
4704 | ||
4705 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) | |
4706 | pop(@a) splice(@a,-1) | |
4707 | shift(@a) splice(@a,0,1) | |
4708 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
4709 | $a[$x] = $y splice(@a,$x,1,$y) | |
4710 | ||
4711 | Example, assuming array lengths are passed before arrays: | |
4712 | ||
4713 | sub aeq { # compare two list values | |
4714 | my(@a) = splice(@_,0,shift); | |
4715 | my(@b) = splice(@_,0,shift); | |
4716 | return 0 unless @a == @b; # same len? | |
4717 | while (@a) { | |
4718 | return 0 if pop(@a) ne pop(@b); | |
4719 | } | |
4720 | return 1; | |
4721 | } | |
4722 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
4723 | ||
4724 | =item split /PATTERN/,EXPR,LIMIT | |
4725 | ||
4726 | =item split /PATTERN/,EXPR | |
4727 | ||
4728 | =item split /PATTERN/ | |
4729 | ||
4730 | =item split | |
4731 | ||
4732 | Splits a string into a list of strings and returns that list. By default, | |
4733 | empty leading fields are preserved, and empty trailing ones are deleted. | |
4734 | ||
4735 | In scalar context, returns the number of fields found and splits into | |
4736 | the C<@_> array. Use of split in scalar context is deprecated, however, | |
4737 | because it clobbers your subroutine arguments. | |
4738 | ||
4739 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, | |
4740 | splits on whitespace (after skipping any leading whitespace). Anything | |
4741 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
4742 | that the delimiter may be longer than one character.) | |
4743 | ||
4744 | If LIMIT is specified and positive, it represents the maximum number | |
4745 | of fields the EXPR will be split into, though the actual number of | |
4746 | fields returned depends on the number of times PATTERN matches within | |
4747 | EXPR. If LIMIT is unspecified or zero, trailing null fields are | |
4748 | stripped (which potential users of C<pop> would do well to remember). | |
4749 | If LIMIT is negative, it is treated as if an arbitrarily large LIMIT | |
4750 | had been specified. Note that splitting an EXPR that evaluates to the | |
4751 | empty string always returns the empty list, regardless of the LIMIT | |
4752 | specified. | |
4753 | ||
4754 | A pattern matching the null string (not to be confused with | |
4755 | a null pattern C<//>, which is just one member of the set of patterns | |
4756 | matching a null string) will split the value of EXPR into separate | |
4757 | characters at each point it matches that way. For example: | |
4758 | ||
4759 | print join(':', split(/ */, 'hi there')); | |
4760 | ||
4761 | produces the output 'h:i:t:h:e:r:e'. | |
4762 | ||
4763 | Using the empty pattern C<//> specifically matches the null string, and is | |
4764 | not be confused with the use of C<//> to mean "the last successful pattern | |
4765 | match". | |
4766 | ||
4767 | Empty leading (or trailing) fields are produced when there are positive width | |
4768 | matches at the beginning (or end) of the string; a zero-width match at the | |
4769 | beginning (or end) of the string does not produce an empty field. For | |
4770 | example: | |
4771 | ||
4772 | print join(':', split(/(?=\w)/, 'hi there!')); | |
4773 | ||
4774 | produces the output 'h:i :t:h:e:r:e!'. | |
4775 | ||
4776 | The LIMIT parameter can be used to split a line partially | |
4777 | ||
4778 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
4779 | ||
4780 | When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT | |
4781 | one larger than the number of variables in the list, to avoid | |
4782 | unnecessary work. For the list above LIMIT would have been 4 by | |
4783 | default. In time critical applications it behooves you not to split | |
4784 | into more fields than you really need. | |
4785 | ||
4786 | If the PATTERN contains parentheses, additional list elements are | |
4787 | created from each matching substring in the delimiter. | |
4788 | ||
4789 | split(/([,-])/, "1-10,20", 3); | |
4790 | ||
4791 | produces the list value | |
4792 | ||
4793 | (1, '-', 10, ',', 20) | |
4794 | ||
4795 | If you had the entire header of a normal Unix email message in $header, | |
4796 | you could split it up into fields and their values this way: | |
4797 | ||
4798 | $header =~ s/\n\s+/ /g; # fix continuation lines | |
4799 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); | |
4800 | ||
4801 | The pattern C</PATTERN/> may be replaced with an expression to specify | |
4802 | patterns that vary at runtime. (To do runtime compilation only once, | |
4803 | use C</$variable/o>.) | |
4804 | ||
4805 | As a special case, specifying a PATTERN of space (C<' '>) will split on | |
4806 | white space just as C<split> with no arguments does. Thus, C<split(' ')> can | |
4807 | be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> | |
4808 | will give you as many null initial fields as there are leading spaces. | |
4809 | A C<split> on C</\s+/> is like a C<split(' ')> except that any leading | |
4810 | whitespace produces a null first field. A C<split> with no arguments | |
4811 | really does a C<split(' ', $_)> internally. | |
4812 | ||
4813 | A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't | |
4814 | much use otherwise. | |
4815 | ||
4816 | Example: | |
4817 | ||
4818 | open(PASSWD, '/etc/passwd'); | |
4819 | while (<PASSWD>) { | |
4820 | chomp; | |
4821 | ($login, $passwd, $uid, $gid, | |
4822 | $gcos, $home, $shell) = split(/:/); | |
4823 | #... | |
4824 | } | |
4825 | ||
4826 | As with regular pattern matching, any capturing parentheses that are not | |
4827 | matched in a C<split()> will be set to C<undef> when returned: | |
4828 | ||
4829 | @fields = split /(A)|B/, "1A2B3"; | |
4830 | # @fields is (1, 'A', 2, undef, 3) | |
4831 | ||
4832 | =item sprintf FORMAT, LIST | |
4833 | ||
4834 | Returns a string formatted by the usual C<printf> conventions of the C | |
4835 | library function C<sprintf>. See below for more details | |
4836 | and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of | |
4837 | the general principles. | |
4838 | ||
4839 | For example: | |
4840 | ||
4841 | # Format number with up to 8 leading zeroes | |
4842 | $result = sprintf("%08d", $number); | |
4843 | ||
4844 | # Round number to 3 digits after decimal point | |
4845 | $rounded = sprintf("%.3f", $number); | |
4846 | ||
4847 | Perl does its own C<sprintf> formatting--it emulates the C | |
4848 | function C<sprintf>, but it doesn't use it (except for floating-point | |
4849 | numbers, and even then only the standard modifiers are allowed). As a | |
4850 | result, any non-standard extensions in your local C<sprintf> are not | |
4851 | available from Perl. | |
4852 | ||
4853 | Unlike C<printf>, C<sprintf> does not do what you probably mean when you | |
4854 | pass it an array as your first argument. The array is given scalar context, | |
4855 | and instead of using the 0th element of the array as the format, Perl will | |
4856 | use the count of elements in the array as the format, which is almost never | |
4857 | useful. | |
4858 | ||
4859 | Perl's C<sprintf> permits the following universally-known conversions: | |
4860 | ||
4861 | %% a percent sign | |
4862 | %c a character with the given number | |
4863 | %s a string | |
4864 | %d a signed integer, in decimal | |
4865 | %u an unsigned integer, in decimal | |
4866 | %o an unsigned integer, in octal | |
4867 | %x an unsigned integer, in hexadecimal | |
4868 | %e a floating-point number, in scientific notation | |
4869 | %f a floating-point number, in fixed decimal notation | |
4870 | %g a floating-point number, in %e or %f notation | |
4871 | ||
4872 | In addition, Perl permits the following widely-supported conversions: | |
4873 | ||
4874 | %X like %x, but using upper-case letters | |
4875 | %E like %e, but using an upper-case "E" | |
4876 | %G like %g, but with an upper-case "E" (if applicable) | |
4877 | %b an unsigned integer, in binary | |
4878 | %p a pointer (outputs the Perl value's address in hexadecimal) | |
4879 | %n special: *stores* the number of characters output so far | |
4880 | into the next variable in the parameter list | |
4881 | ||
4882 | Finally, for backward (and we do mean "backward") compatibility, Perl | |
4883 | permits these unnecessary but widely-supported conversions: | |
4884 | ||
4885 | %i a synonym for %d | |
4886 | %D a synonym for %ld | |
4887 | %U a synonym for %lu | |
4888 | %O a synonym for %lo | |
4889 | %F a synonym for %f | |
4890 | ||
4891 | Note that the number of exponent digits in the scientific notation produced | |
4892 | by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the | |
4893 | exponent less than 100 is system-dependent: it may be three or less | |
4894 | (zero-padded as necessary). In other words, 1.23 times ten to the | |
4895 | 99th may be either "1.23e99" or "1.23e099". | |
4896 | ||
4897 | Between the C<%> and the format letter, you may specify a number of | |
4898 | additional attributes controlling the interpretation of the format. | |
4899 | In order, these are: | |
4900 | ||
4901 | =over 4 | |
4902 | ||
4903 | =item format parameter index | |
4904 | ||
4905 | An explicit format parameter index, such as C<2$>. By default sprintf | |
4906 | will format the next unused argument in the list, but this allows you | |
4907 | to take the arguments out of order. Eg: | |
4908 | ||
4909 | printf '%2$d %1$d', 12, 34; # prints "34 12" | |
4910 | printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" | |
4911 | ||
4912 | =item flags | |
4913 | ||
4914 | one or more of: | |
4915 | space prefix positive number with a space | |
4916 | + prefix positive number with a plus sign | |
4917 | - left-justify within the field | |
4918 | 0 use zeros, not spaces, to right-justify | |
4919 | # prefix non-zero octal with "0", non-zero hex with "0x", | |
4920 | non-zero binary with "0b" | |
4921 | ||
4922 | For example: | |
4923 | ||
4924 | printf '<% d>', 12; # prints "< 12>" | |
4925 | printf '<%+d>', 12; # prints "<+12>" | |
4926 | printf '<%6s>', 12; # prints "< 12>" | |
4927 | printf '<%-6s>', 12; # prints "<12 >" | |
4928 | printf '<%06s>', 12; # prints "<000012>" | |
4929 | printf '<%#x>', 12; # prints "<0xc>" | |
4930 | ||
4931 | =item vector flag | |
4932 | ||
4933 | The vector flag C<v>, optionally specifying the join string to use. | |
4934 | This flag tells perl to interpret the supplied string as a vector | |
4935 | of integers, one for each character in the string, separated by | |
4936 | a given string (a dot C<.> by default). This can be useful for | |
4937 | displaying ordinal values of characters in arbitrary strings: | |
4938 | ||
4939 | printf "version is v%vd\n", $^V; # Perl's version | |
4940 | ||
4941 | Put an asterisk C<*> before the C<v> to override the string to | |
4942 | use to separate the numbers: | |
4943 | ||
4944 | printf "address is %*vX\n", ":", $addr; # IPv6 address | |
4945 | printf "bits are %0*v8b\n", " ", $bits; # random bitstring | |
4946 | ||
4947 | You can also explicitly specify the argument number to use for | |
4948 | the join string using eg C<*2$v>: | |
4949 | ||
4950 | printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses | |
4951 | ||
4952 | =item (minimum) width | |
4953 | ||
4954 | Arguments are usually formatted to be only as wide as required to | |
4955 | display the given value. You can override the width by putting | |
4956 | a number here, or get the width from the next argument (with C<*>) | |
4957 | or from a specified argument (with eg C<*2$>): | |
4958 | ||
4959 | printf '<%s>', "a"; # prints "<a>" | |
4960 | printf '<%6s>', "a"; # prints "< a>" | |
4961 | printf '<%*s>', 6, "a"; # prints "< a>" | |
4962 | printf '<%*2$s>', "a", 6; # prints "< a>" | |
4963 | printf '<%2s>', "long"; # prints "<long>" (does not truncate) | |
4964 | ||
4965 | If a field width obtained through C<*> is negative, it has the same | |
4966 | effect as the C<-> flag: left-justification. | |
4967 | ||
4968 | =item precision, or maximum width | |
4969 | ||
4970 | You can specify a precision (for numeric conversions) or a maximum | |
4971 | width (for string conversions) by specifying a C<.> followed by a number. | |
4972 | For floating point formats, this specifies the number of decimal places | |
4973 | to show (the default being 6), eg: | |
4974 | ||
4975 | # these examples are subject to system-specific variation | |
4976 | printf '<%f>', 1; # prints "<1.000000>" | |
4977 | printf '<%.1f>', 1; # prints "<1.0>" | |
4978 | printf '<%.0f>', 1; # prints "<1>" | |
4979 | printf '<%e>', 10; # prints "<1.000000e+01>" | |
4980 | printf '<%.1e>', 10; # prints "<1.0e+01>" | |
4981 | ||
4982 | For integer conversions, specifying a precision implies that the | |
4983 | output of the number itself should be zero-padded to this width: | |
4984 | ||
4985 | printf '<%.6x>', 1; # prints "<000001>" | |
4986 | printf '<%#.6x>', 1; # prints "<0x000001>" | |
4987 | printf '<%-10.6x>', 1; # prints "<000001 >" | |
4988 | ||
4989 | For string conversions, specifying a precision truncates the string | |
4990 | to fit in the specified width: | |
4991 | ||
4992 | printf '<%.5s>', "truncated"; # prints "<trunc>" | |
4993 | printf '<%10.5s>', "truncated"; # prints "< trunc>" | |
4994 | ||
4995 | You can also get the precision from the next argument using C<.*>: | |
4996 | ||
4997 | printf '<%.6x>', 1; # prints "<000001>" | |
4998 | printf '<%.*x>', 6, 1; # prints "<000001>" | |
4999 | ||
5000 | You cannot currently get the precision from a specified number, | |
5001 | but it is intended that this will be possible in the future using | |
5002 | eg C<.*2$>: | |
5003 | ||
5004 | printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>" | |
5005 | ||
5006 | =item size | |
5007 | ||
5008 | For numeric conversions, you can specify the size to interpret the | |
5009 | number as using C<l>, C<h>, C<V>, C<q>, C<L> or C<ll>. For integer | |
5010 | conversions, numbers are usually assumed to be whatever the default | |
5011 | integer size is on your platform (usually 32 or 64 bits), but you | |
5012 | can override this to use instead one of the standard C types, as | |
5013 | supported by the compiler used to build Perl: | |
5014 | ||
5015 | l interpret integer as C type "long" or "unsigned long" | |
5016 | h interpret integer as C type "short" or "unsigned short" | |
5017 | q, L or ll interpret integer as C type "long long" or "unsigned long long" | |
5018 | (if your platform supports such a type, else it is an error) | |
5019 | ||
5020 | For floating point conversions, numbers are usually assumed to be | |
5021 | the default floating point size on your platform (double or long double), | |
5022 | but you can force 'long double' with C<q>, C<L> or C<ll> if your | |
5023 | platform supports them. | |
5024 | ||
5025 | The size specifier 'V' has no effect for Perl code, but it supported | |
5026 | for compatibility with XS code; it means 'use the standard size for | |
5027 | a Perl integer (or floating-point number)', which is already the | |
5028 | default for Perl code. | |
5029 | ||
5030 | =item order of arguments | |
5031 | ||
5032 | Normally, sprintf takes the next unused argument as the value to | |
5033 | format for each format specification. If the format specification | |
5034 | uses C<*> to require additional arguments, these are consumed from | |
5035 | the argument list in the order in which they appear in the format | |
5036 | specification I<before> the value to format. Where an argument is | |
5037 | specified using an explicit index, this does not affect the normal | |
5038 | order for the arguments (even when the explicitly specified index | |
5039 | would have been the next argument in any case). | |
5040 | ||
5041 | So: | |
5042 | ||
5043 | printf '<%*.*s>', $a, $b, $c; | |
5044 | ||
5045 | would use C<$a> for the width, C<$b> for the precision and C<$c> | |
5046 | as the value to format, while: | |
5047 | ||
5048 | print '<%*1$.*s>', $a, $b; | |
5049 | ||
5050 | would use C<$a> for the width and the precision, and C<$b> as the | |
5051 | value to format. | |
5052 | ||
5053 | Here are some more examples - beware that when using an explicit | |
5054 | index, the C<$> may need to be escaped: | |
5055 | ||
5056 | printf "%2\$d %d\n", 12, 34; # will print "34 12\n" | |
5057 | printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" | |
5058 | printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" | |
5059 | printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" | |
5060 | ||
5061 | =back | |
5062 | ||
5063 | If C<use locale> is in effect, the character used for the decimal | |
5064 | point in formatted real numbers is affected by the LC_NUMERIC locale. | |
5065 | See L<perllocale>. | |
5066 | ||
5067 | If Perl understands "quads" (64-bit integers) (this requires | |
5068 | either that the platform natively support quads or that Perl | |
5069 | be specifically compiled to support quads), the characters | |
5070 | ||
5071 | d u o x X b i D U O | |
5072 | ||
5073 | print quads, and they may optionally be preceded by | |
5074 | ||
5075 | ll L q | |
5076 | ||
5077 | For example | |
5078 | ||
5079 | %lld %16LX %qo | |
5080 | ||
5081 | You can find out whether your Perl supports quads via L<Config>: | |
5082 | ||
5083 | use Config; | |
5084 | ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) && | |
5085 | print "quads\n"; | |
5086 | ||
5087 | If Perl understands "long doubles" (this requires that the platform | |
5088 | support long doubles), the flags | |
5089 | ||
5090 | e f g E F G | |
5091 | ||
5092 | may optionally be preceded by | |
5093 | ||
5094 | ll L | |
5095 | ||
5096 | For example | |
5097 | ||
5098 | %llf %Lg | |
5099 | ||
5100 | You can find out whether your Perl supports long doubles via L<Config>: | |
5101 | ||
5102 | use Config; | |
5103 | $Config{d_longdbl} eq 'define' && print "long doubles\n"; | |
5104 | ||
5105 | =item sqrt EXPR | |
5106 | ||
5107 | =item sqrt | |
5108 | ||
5109 | Return the square root of EXPR. If EXPR is omitted, returns square | |
5110 | root of C<$_>. Only works on non-negative operands, unless you've | |
5111 | loaded the standard Math::Complex module. | |
5112 | ||
5113 | use Math::Complex; | |
5114 | print sqrt(-2); # prints 1.4142135623731i | |
5115 | ||
5116 | =item srand EXPR | |
5117 | ||
5118 | =item srand | |
5119 | ||
5120 | Sets the random number seed for the C<rand> operator. | |
5121 | ||
5122 | The point of the function is to "seed" the C<rand> function so that | |
5123 | C<rand> can produce a different sequence each time you run your | |
5124 | program. | |
5125 | ||
5126 | If srand() is not called explicitly, it is called implicitly at the | |
5127 | first use of the C<rand> operator. However, this was not the case in | |
5128 | versions of Perl before 5.004, so if your script will run under older | |
5129 | Perl versions, it should call C<srand>. | |
5130 | ||
5131 | Most programs won't even call srand() at all, except those that | |
5132 | need a cryptographically-strong starting point rather than the | |
5133 | generally acceptable default, which is based on time of day, | |
5134 | process ID, and memory allocation, or the F</dev/urandom> device, | |
5135 | if available. | |
5136 | ||
5137 | You can call srand($seed) with the same $seed to reproduce the | |
5138 | I<same> sequence from rand(), but this is usually reserved for | |
5139 | generating predictable results for testing or debugging. | |
5140 | Otherwise, don't call srand() more than once in your program. | |
5141 | ||
5142 | Do B<not> call srand() (i.e. without an argument) more than once in | |
5143 | a script. The internal state of the random number generator should | |
5144 | contain more entropy than can be provided by any seed, so calling | |
5145 | srand() again actually I<loses> randomness. | |
5146 | ||
5147 | Most implementations of C<srand> take an integer and will silently | |
5148 | truncate decimal numbers. This means C<srand(42)> will usually | |
5149 | produce the same results as C<srand(42.1)>. To be safe, always pass | |
5150 | C<srand> an integer. | |
5151 | ||
5152 | In versions of Perl prior to 5.004 the default seed was just the | |
5153 | current C<time>. This isn't a particularly good seed, so many old | |
5154 | programs supply their own seed value (often C<time ^ $$> or C<time ^ | |
5155 | ($$ + ($$ << 15))>), but that isn't necessary any more. | |
5156 | ||
5157 | Note that you need something much more random than the default seed for | |
5158 | cryptographic purposes. Checksumming the compressed output of one or more | |
5159 | rapidly changing operating system status programs is the usual method. For | |
5160 | example: | |
5161 | ||
5162 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); | |
5163 | ||
5164 | If you're particularly concerned with this, see the C<Math::TrulyRandom> | |
5165 | module in CPAN. | |
5166 | ||
5167 | Frequently called programs (like CGI scripts) that simply use | |
5168 | ||
5169 | time ^ $$ | |
5170 | ||
5171 | for a seed can fall prey to the mathematical property that | |
5172 | ||
5173 | a^b == (a+1)^(b+1) | |
5174 | ||
5175 | one-third of the time. So don't do that. | |
5176 | ||
5177 | =item stat FILEHANDLE | |
5178 | ||
5179 | =item stat EXPR | |
5180 | ||
5181 | =item stat | |
5182 | ||
5183 | Returns a 13-element list giving the status info for a file, either | |
5184 | the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, | |
5185 | it stats C<$_>. Returns a null list if the stat fails. Typically used | |
5186 | as follows: | |
5187 | ||
5188 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
5189 | $atime,$mtime,$ctime,$blksize,$blocks) | |
5190 | = stat($filename); | |
5191 | ||
5192 | Not all fields are supported on all filesystem types. Here are the | |
5193 | meaning of the fields: | |
5194 | ||
5195 | 0 dev device number of filesystem | |
5196 | 1 ino inode number | |
5197 | 2 mode file mode (type and permissions) | |
5198 | 3 nlink number of (hard) links to the file | |
5199 | 4 uid numeric user ID of file's owner | |
5200 | 5 gid numeric group ID of file's owner | |
5201 | 6 rdev the device identifier (special files only) | |
5202 | 7 size total size of file, in bytes | |
5203 | 8 atime last access time in seconds since the epoch | |
5204 | 9 mtime last modify time in seconds since the epoch | |
5205 | 10 ctime inode change time in seconds since the epoch (*) | |
5206 | 11 blksize preferred block size for file system I/O | |
5207 | 12 blocks actual number of blocks allocated | |
5208 | ||
5209 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
5210 | ||
5211 | (*) The ctime field is non-portable, in particular you cannot expect | |
5212 | it to be a "creation time", see L<perlport/"Files and Filesystems"> | |
5213 | for details. | |
5214 | ||
5215 | If stat is passed the special filehandle consisting of an underline, no | |
5216 | stat is done, but the current contents of the stat structure from the | |
5217 | last stat or filetest are returned. Example: | |
5218 | ||
5219 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
5220 | print "$file is executable NFS file\n"; | |
5221 | } | |
5222 | ||
5223 | (This works on machines only for which the device number is negative | |
5224 | under NFS.) | |
5225 | ||
5226 | Because the mode contains both the file type and its permissions, you | |
5227 | should mask off the file type portion and (s)printf using a C<"%o"> | |
5228 | if you want to see the real permissions. | |
5229 | ||
5230 | $mode = (stat($filename))[2]; | |
5231 | printf "Permissions are %04o\n", $mode & 07777; | |
5232 | ||
5233 | In scalar context, C<stat> returns a boolean value indicating success | |
5234 | or failure, and, if successful, sets the information associated with | |
5235 | the special filehandle C<_>. | |
5236 | ||
5237 | The File::stat module provides a convenient, by-name access mechanism: | |
5238 | ||
5239 | use File::stat; | |
5240 | $sb = stat($filename); | |
5241 | printf "File is %s, size is %s, perm %04o, mtime %s\n", | |
5242 | $filename, $sb->size, $sb->mode & 07777, | |
5243 | scalar localtime $sb->mtime; | |
5244 | ||
5245 | You can import symbolic mode constants (C<S_IF*>) and functions | |
5246 | (C<S_IS*>) from the Fcntl module: | |
5247 | ||
5248 | use Fcntl ':mode'; | |
5249 | ||
5250 | $mode = (stat($filename))[2]; | |
5251 | ||
5252 | $user_rwx = ($mode & S_IRWXU) >> 6; | |
5253 | $group_read = ($mode & S_IRGRP) >> 3; | |
5254 | $other_execute = $mode & S_IXOTH; | |
5255 | ||
5256 | printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; | |
5257 | ||
5258 | $is_setuid = $mode & S_ISUID; | |
5259 | $is_setgid = S_ISDIR($mode); | |
5260 | ||
5261 | You could write the last two using the C<-u> and C<-d> operators. | |
5262 | The commonly available S_IF* constants are | |
5263 | ||
5264 | # Permissions: read, write, execute, for user, group, others. | |
5265 | ||
5266 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR | |
5267 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP | |
5268 | S_IRWXO S_IROTH S_IWOTH S_IXOTH | |
5269 | ||
5270 | # Setuid/Setgid/Stickiness. | |
5271 | ||
5272 | S_ISUID S_ISGID S_ISVTX S_ISTXT | |
5273 | ||
5274 | # File types. Not necessarily all are available on your system. | |
5275 | ||
5276 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT | |
5277 | ||
5278 | # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. | |
5279 | ||
5280 | S_IREAD S_IWRITE S_IEXEC | |
5281 | ||
5282 | and the S_IF* functions are | |
5283 | ||
5284 | S_IFMODE($mode) the part of $mode containing the permission bits | |
5285 | and the setuid/setgid/sticky bits | |
5286 | ||
5287 | S_IFMT($mode) the part of $mode containing the file type | |
5288 | which can be bit-anded with e.g. S_IFREG | |
5289 | or with the following functions | |
5290 | ||
5291 | # The operators -f, -d, -l, -b, -c, -p, and -s. | |
5292 | ||
5293 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) | |
5294 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) | |
5295 | ||
5296 | # No direct -X operator counterpart, but for the first one | |
5297 | # the -g operator is often equivalent. The ENFMT stands for | |
5298 | # record flocking enforcement, a platform-dependent feature. | |
5299 | ||
5300 | S_ISENFMT($mode) S_ISWHT($mode) | |
5301 | ||
5302 | See your native chmod(2) and stat(2) documentation for more details | |
5303 | about the S_* constants. | |
5304 | ||
5305 | =item study SCALAR | |
5306 | ||
5307 | =item study | |
5308 | ||
5309 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of | |
5310 | doing many pattern matches on the string before it is next modified. | |
5311 | This may or may not save time, depending on the nature and number of | |
5312 | patterns you are searching on, and on the distribution of character | |
5313 | frequencies in the string to be searched--you probably want to compare | |
5314 | run times with and without it to see which runs faster. Those loops | |
5315 | which scan for many short constant strings (including the constant | |
5316 | parts of more complex patterns) will benefit most. You may have only | |
5317 | one C<study> active at a time--if you study a different scalar the first | |
5318 | is "unstudied". (The way C<study> works is this: a linked list of every | |
5319 | character in the string to be searched is made, so we know, for | |
5320 | example, where all the C<'k'> characters are. From each search string, | |
5321 | the rarest character is selected, based on some static frequency tables | |
5322 | constructed from some C programs and English text. Only those places | |
5323 | that contain this "rarest" character are examined.) | |
5324 | ||
5325 | For example, here is a loop that inserts index producing entries | |
5326 | before any line containing a certain pattern: | |
5327 | ||
5328 | while (<>) { | |
5329 | study; | |
5330 | print ".IX foo\n" if /\bfoo\b/; | |
5331 | print ".IX bar\n" if /\bbar\b/; | |
5332 | print ".IX blurfl\n" if /\bblurfl\b/; | |
5333 | # ... | |
5334 | print; | |
5335 | } | |
5336 | ||
5337 | In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> | |
5338 | will be looked at, because C<f> is rarer than C<o>. In general, this is | |
5339 | a big win except in pathological cases. The only question is whether | |
5340 | it saves you more time than it took to build the linked list in the | |
5341 | first place. | |
5342 | ||
5343 | Note that if you have to look for strings that you don't know till | |
5344 | runtime, you can build an entire loop as a string and C<eval> that to | |
5345 | avoid recompiling all your patterns all the time. Together with | |
5346 | undefining C<$/> to input entire files as one record, this can be very | |
5347 | fast, often faster than specialized programs like fgrep(1). The following | |
5348 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints | |
5349 | out the names of those files that contain a match: | |
5350 | ||
5351 | $search = 'while (<>) { study;'; | |
5352 | foreach $word (@words) { | |
5353 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; | |
5354 | } | |
5355 | $search .= "}"; | |
5356 | @ARGV = @files; | |
5357 | undef $/; | |
5358 | eval $search; # this screams | |
5359 | $/ = "\n"; # put back to normal input delimiter | |
5360 | foreach $file (sort keys(%seen)) { | |
5361 | print $file, "\n"; | |
5362 | } | |
5363 | ||
5364 | =item sub NAME BLOCK | |
5365 | ||
5366 | =item sub NAME (PROTO) BLOCK | |
5367 | ||
5368 | =item sub NAME : ATTRS BLOCK | |
5369 | ||
5370 | =item sub NAME (PROTO) : ATTRS BLOCK | |
5371 | ||
5372 | This is subroutine definition, not a real function I<per se>. | |
5373 | Without a BLOCK it's just a forward declaration. Without a NAME, | |
5374 | it's an anonymous function declaration, and does actually return | |
5375 | a value: the CODE ref of the closure you just created. | |
5376 | ||
5377 | See L<perlsub> and L<perlref> for details about subroutines and | |
5378 | references, and L<attributes> and L<Attribute::Handlers> for more | |
5379 | information about attributes. | |
5380 | ||
5381 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT | |
5382 | ||
5383 | =item substr EXPR,OFFSET,LENGTH | |
5384 | ||
5385 | =item substr EXPR,OFFSET | |
5386 | ||
5387 | Extracts a substring out of EXPR and returns it. First character is at | |
5388 | offset C<0>, or whatever you've set C<$[> to (but don't do that). | |
5389 | If OFFSET is negative (or more precisely, less than C<$[>), starts | |
5390 | that far from the end of the string. If LENGTH is omitted, returns | |
5391 | everything to the end of the string. If LENGTH is negative, leaves that | |
5392 | many characters off the end of the string. | |
5393 | ||
5394 | You can use the substr() function as an lvalue, in which case EXPR | |
5395 | must itself be an lvalue. If you assign something shorter than LENGTH, | |
5396 | the string will shrink, and if you assign something longer than LENGTH, | |
5397 | the string will grow to accommodate it. To keep the string the same | |
5398 | length you may need to pad or chop your value using C<sprintf>. | |
5399 | ||
5400 | If OFFSET and LENGTH specify a substring that is partly outside the | |
5401 | string, only the part within the string is returned. If the substring | |
5402 | is beyond either end of the string, substr() returns the undefined | |
5403 | value and produces a warning. When used as an lvalue, specifying a | |
5404 | substring that is entirely outside the string is a fatal error. | |
5405 | Here's an example showing the behavior for boundary cases: | |
5406 | ||
5407 | my $name = 'fred'; | |
5408 | substr($name, 4) = 'dy'; # $name is now 'freddy' | |
5409 | my $null = substr $name, 6, 2; # returns '' (no warning) | |
5410 | my $oops = substr $name, 7; # returns undef, with warning | |
5411 | substr($name, 7) = 'gap'; # fatal error | |
5412 | ||
5413 | An alternative to using substr() as an lvalue is to specify the | |
5414 | replacement string as the 4th argument. This allows you to replace | |
5415 | parts of the EXPR and return what was there before in one operation, | |
5416 | just as you can with splice(). | |
5417 | ||
5418 | =item symlink OLDFILE,NEWFILE | |
5419 | ||
5420 | Creates a new filename symbolically linked to the old filename. | |
5421 | Returns C<1> for success, C<0> otherwise. On systems that don't support | |
5422 | symbolic links, produces a fatal error at run time. To check for that, | |
5423 | use eval: | |
5424 | ||
5425 | $symlink_exists = eval { symlink("",""); 1 }; | |
5426 | ||
5427 | =item syscall LIST | |
5428 | ||
5429 | Calls the system call specified as the first element of the list, | |
5430 | passing the remaining elements as arguments to the system call. If | |
5431 | unimplemented, produces a fatal error. The arguments are interpreted | |
5432 | as follows: if a given argument is numeric, the argument is passed as | |
5433 | an int. If not, the pointer to the string value is passed. You are | |
5434 | responsible to make sure a string is pre-extended long enough to | |
5435 | receive any result that might be written into a string. You can't use a | |
5436 | string literal (or other read-only string) as an argument to C<syscall> | |
5437 | because Perl has to assume that any string pointer might be written | |
5438 | through. If your | |
5439 | integer arguments are not literals and have never been interpreted in a | |
5440 | numeric context, you may need to add C<0> to them to force them to look | |
5441 | like numbers. This emulates the C<syswrite> function (or vice versa): | |
5442 | ||
5443 | require 'syscall.ph'; # may need to run h2ph | |
5444 | $s = "hi there\n"; | |
5445 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
5446 | ||
5447 | Note that Perl supports passing of up to only 14 arguments to your system call, | |
5448 | which in practice should usually suffice. | |
5449 | ||
5450 | Syscall returns whatever value returned by the system call it calls. | |
5451 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). | |
5452 | Note that some system calls can legitimately return C<-1>. The proper | |
5453 | way to handle such calls is to assign C<$!=0;> before the call and | |
5454 | check the value of C<$!> if syscall returns C<-1>. | |
5455 | ||
5456 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
5457 | number of the read end of the pipe it creates. There is no way | |
5458 | to retrieve the file number of the other end. You can avoid this | |
5459 | problem by using C<pipe> instead. | |
5460 | ||
5461 | =item sysopen FILEHANDLE,FILENAME,MODE | |
5462 | ||
5463 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
5464 | ||
5465 | Opens the file whose filename is given by FILENAME, and associates it | |
5466 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as | |
5467 | the name of the real filehandle wanted. This function calls the | |
5468 | underlying operating system's C<open> function with the parameters | |
5469 | FILENAME, MODE, PERMS. | |
5470 | ||
5471 | The possible values and flag bits of the MODE parameter are | |
5472 | system-dependent; they are available via the standard module C<Fcntl>. | |
5473 | See the documentation of your operating system's C<open> to see which | |
5474 | values and flag bits are available. You may combine several flags | |
5475 | using the C<|>-operator. | |
5476 | ||
5477 | Some of the most common values are C<O_RDONLY> for opening the file in | |
5478 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, | |
5479 | and C<O_RDWR> for opening the file in read-write mode, and. | |
5480 | ||
5481 | For historical reasons, some values work on almost every system | |
5482 | supported by perl: zero means read-only, one means write-only, and two | |
5483 | means read/write. We know that these values do I<not> work under | |
5484 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to | |
5485 | use them in new code. | |
5486 | ||
5487 | If the file named by FILENAME does not exist and the C<open> call creates | |
5488 | it (typically because MODE includes the C<O_CREAT> flag), then the value of | |
5489 | PERMS specifies the permissions of the newly created file. If you omit | |
5490 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. | |
5491 | These permission values need to be in octal, and are modified by your | |
5492 | process's current C<umask>. | |
5493 | ||
5494 | In many systems the C<O_EXCL> flag is available for opening files in | |
5495 | exclusive mode. This is B<not> locking: exclusiveness means here that | |
5496 | if the file already exists, sysopen() fails. The C<O_EXCL> wins | |
5497 | C<O_TRUNC>. | |
5498 | ||
5499 | Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. | |
5500 | ||
5501 | You should seldom if ever use C<0644> as argument to C<sysopen>, because | |
5502 | that takes away the user's option to have a more permissive umask. | |
5503 | Better to omit it. See the perlfunc(1) entry on C<umask> for more | |
5504 | on this. | |
5505 | ||
5506 | Note that C<sysopen> depends on the fdopen() C library function. | |
5507 | On many UNIX systems, fdopen() is known to fail when file descriptors | |
5508 | exceed a certain value, typically 255. If you need more file | |
5509 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
5510 | library, or perhaps using the POSIX::open() function. | |
5511 | ||
5512 | See L<perlopentut> for a kinder, gentler explanation of opening files. | |
5513 | ||
5514 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET | |
5515 | ||
5516 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
5517 | ||
5518 | Attempts to read LENGTH I<characters> of data into variable SCALAR from | |
5519 | the specified FILEHANDLE, using the system call read(2). It bypasses | |
5520 | buffered IO, so mixing this with other kinds of reads, C<print>, | |
5521 | C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because | |
5522 | stdio usually buffers data. Returns the number of characters actually | |
5523 | read, C<0> at end of file, or undef if there was an error. SCALAR | |
5524 | will be grown or shrunk so that the last byte actually read is the | |
5525 | last byte of the scalar after the read. | |
5526 | ||
5527 | Note the I<characters>: depending on the status of the filehandle, | |
5528 | either (8-bit) bytes or characters are read. By default all | |
5529 | filehandles operate on bytes, but for example if the filehandle has | |
5530 | been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> | |
5531 | pragma, L<open>), the I/O will operate on characters, not bytes. | |
5532 | ||
5533 | An OFFSET may be specified to place the read data at some place in the | |
5534 | string other than the beginning. A negative OFFSET specifies | |
5535 | placement at that many characters counting backwards from the end of | |
5536 | the string. A positive OFFSET greater than the length of SCALAR | |
5537 | results in the string being padded to the required size with C<"\0"> | |
5538 | bytes before the result of the read is appended. | |
5539 | ||
5540 | There is no syseof() function, which is ok, since eof() doesn't work | |
5541 | very well on device files (like ttys) anyway. Use sysread() and check | |
5542 | for a return value for 0 to decide whether you're done. | |
5543 | ||
5544 | =item sysseek FILEHANDLE,POSITION,WHENCE | |
5545 | ||
5546 | Sets FILEHANDLE's system position I<in bytes> using the system call | |
5547 | lseek(2). FILEHANDLE may be an expression whose value gives the name | |
5548 | of the filehandle. The values for WHENCE are C<0> to set the new | |
5549 | position to POSITION, C<1> to set the it to the current position plus | |
5550 | POSITION, and C<2> to set it to EOF plus POSITION (typically | |
5551 | negative). | |
5552 | ||
5553 | Note the I<in bytes>: even if the filehandle has been set to operate | |
5554 | on characters (for example by using the C<:utf8> I/O layer), tell() | |
5555 | will return byte offsets, not character offsets (because implementing | |
5556 | that would render sysseek() very slow). | |
5557 | ||
5558 | sysseek() bypasses normal buffered io, so mixing this with reads (other | |
5559 | than C<sysread>, for example >< or read()) C<print>, C<write>, | |
5560 | C<seek>, C<tell>, or C<eof> may cause confusion. | |
5561 | ||
5562 | For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, | |
5563 | and C<SEEK_END> (start of the file, current position, end of the file) | |
5564 | from the Fcntl module. Use of the constants is also more portable | |
5565 | than relying on 0, 1, and 2. For example to define a "systell" function: | |
5566 | ||
5567 | use Fnctl 'SEEK_CUR'; | |
5568 | sub systell { sysseek($_[0], 0, SEEK_CUR) } | |
5569 | ||
5570 | Returns the new position, or the undefined value on failure. A position | |
5571 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns | |
5572 | true on success and false on failure, yet you can still easily determine | |
5573 | the new position. | |
5574 | ||
5575 | =item system LIST | |
5576 | ||
5577 | =item system PROGRAM LIST | |
5578 | ||
5579 | Does exactly the same thing as C<exec LIST>, except that a fork is | |
5580 | done first, and the parent process waits for the child process to | |
5581 | complete. Note that argument processing varies depending on the | |
5582 | number of arguments. If there is more than one argument in LIST, | |
5583 | or if LIST is an array with more than one value, starts the program | |
5584 | given by the first element of the list with arguments given by the | |
5585 | rest of the list. If there is only one scalar argument, the argument | |
5586 | is checked for shell metacharacters, and if there are any, the | |
5587 | entire argument is passed to the system's command shell for parsing | |
5588 | (this is C</bin/sh -c> on Unix platforms, but varies on other | |
5589 | platforms). If there are no shell metacharacters in the argument, | |
5590 | it is split into words and passed directly to C<execvp>, which is | |
5591 | more efficient. | |
5592 | ||
5593 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
5594 | output before any operation that may do a fork, but this may not be | |
5595 | supported on some platforms (see L<perlport>). To be safe, you may need | |
5596 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
5597 | of C<IO::Handle> on any open handles. | |
5598 | ||
5599 | The return value is the exit status of the program as returned by the | |
5600 | C<wait> call. To get the actual exit value shift right by eight (see below). | |
5601 | See also L</exec>. This is I<not> what you want to use to capture | |
5602 | the output from a command, for that you should use merely backticks or | |
5603 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 | |
5604 | indicates a failure to start the program (inspect $! for the reason). | |
5605 | ||
5606 | Like C<exec>, C<system> allows you to lie to a program about its name if | |
5607 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. | |
5608 | ||
5609 | Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, | |
5610 | killing the program they're running doesn't actually interrupt | |
5611 | your program. | |
5612 | ||
5613 | @args = ("command", "arg1", "arg2"); | |
5614 | system(@args) == 0 | |
5615 | or die "system @args failed: $?" | |
5616 | ||
5617 | You can check all the failure possibilities by inspecting | |
5618 | C<$?> like this: | |
5619 | ||
5620 | $exit_value = $? >> 8; | |
5621 | $signal_num = $? & 127; | |
5622 | $dumped_core = $? & 128; | |
5623 | ||
5624 | or more portably by using the W*() calls of the POSIX extension; | |
5625 | see L<perlport> for more information. | |
5626 | ||
5627 | When the arguments get executed via the system shell, results | |
5628 | and return codes will be subject to its quirks and capabilities. | |
5629 | See L<perlop/"`STRING`"> and L</exec> for details. | |
5630 | ||
5631 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET | |
5632 | ||
5633 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
5634 | ||
5635 | =item syswrite FILEHANDLE,SCALAR | |
5636 | ||
5637 | Attempts to write LENGTH characters of data from variable SCALAR to | |
5638 | the specified FILEHANDLE, using the system call write(2). If LENGTH | |
5639 | is not specified, writes whole SCALAR. It bypasses buffered IO, so | |
5640 | mixing this with reads (other than C<sysread())>, C<print>, C<write>, | |
5641 | C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually | |
5642 | buffers data. Returns the number of characters actually written, or | |
5643 | C<undef> if there was an error. If the LENGTH is greater than the | |
5644 | available data in the SCALAR after the OFFSET, only as much data as is | |
5645 | available will be written. | |
5646 | ||
5647 | An OFFSET may be specified to write the data from some part of the | |
5648 | string other than the beginning. A negative OFFSET specifies writing | |
5649 | that many characters counting backwards from the end of the string. | |
5650 | In the case the SCALAR is empty you can use OFFSET but only zero offset. | |
5651 | ||
5652 | Note the I<characters>: depending on the status of the filehandle, | |
5653 | either (8-bit) bytes or characters are written. By default all | |
5654 | filehandles operate on bytes, but for example if the filehandle has | |
5655 | been opened with the C<:utf8> I/O layer (see L</open>, and the open | |
5656 | pragma, L<open>), the I/O will operate on characters, not bytes. | |
5657 | ||
5658 | =item tell FILEHANDLE | |
5659 | ||
5660 | =item tell | |
5661 | ||
5662 | Returns the current position I<in bytes> for FILEHANDLE, or -1 on | |
5663 | error. FILEHANDLE may be an expression whose value gives the name of | |
5664 | the actual filehandle. If FILEHANDLE is omitted, assumes the file | |
5665 | last read. | |
5666 | ||
5667 | Note the I<in bytes>: even if the filehandle has been set to | |
5668 | operate on characters (for example by using the C<:utf8> open | |
5669 | layer), tell() will return byte offsets, not character offsets | |
5670 | (because that would render seek() and tell() rather slow). | |
5671 | ||
5672 | The return value of tell() for the standard streams like the STDIN | |
5673 | depends on the operating system: it may return -1 or something else. | |
5674 | tell() on pipes, fifos, and sockets usually returns -1. | |
5675 | ||
5676 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. | |
5677 | ||
5678 | Do not use tell() on a filehandle that has been opened using | |
5679 | sysopen(), use sysseek() for that as described above. Why? Because | |
5680 | sysopen() creates unbuffered, "raw", filehandles, while open() creates | |
5681 | buffered filehandles. sysseek() make sense only on the first kind, | |
5682 | tell() only makes sense on the second kind. | |
5683 | ||
5684 | =item telldir DIRHANDLE | |
5685 | ||
5686 | Returns the current position of the C<readdir> routines on DIRHANDLE. | |
5687 | Value may be given to C<seekdir> to access a particular location in a | |
5688 | directory. Has the same caveats about possible directory compaction as | |
5689 | the corresponding system library routine. | |
5690 | ||
5691 | =item tie VARIABLE,CLASSNAME,LIST | |
5692 | ||
5693 | This function binds a variable to a package class that will provide the | |
5694 | implementation for the variable. VARIABLE is the name of the variable | |
5695 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
5696 | of correct type. Any additional arguments are passed to the C<new> | |
5697 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, | |
5698 | or C<TIEHASH>). Typically these are arguments such as might be passed | |
5699 | to the C<dbm_open()> function of C. The object returned by the C<new> | |
5700 | method is also returned by the C<tie> function, which would be useful | |
5701 | if you want to access other methods in CLASSNAME. | |
5702 | ||
5703 | Note that functions such as C<keys> and C<values> may return huge lists | |
5704 | when used on large objects, like DBM files. You may prefer to use the | |
5705 | C<each> function to iterate over such. Example: | |
5706 | ||
5707 | # print out history file offsets | |
5708 | use NDBM_File; | |
5709 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); | |
5710 | while (($key,$val) = each %HIST) { | |
5711 | print $key, ' = ', unpack('L',$val), "\n"; | |
5712 | } | |
5713 | untie(%HIST); | |
5714 | ||
5715 | A class implementing a hash should have the following methods: | |
5716 | ||
5717 | TIEHASH classname, LIST | |
5718 | FETCH this, key | |
5719 | STORE this, key, value | |
5720 | DELETE this, key | |
5721 | CLEAR this | |
5722 | EXISTS this, key | |
5723 | FIRSTKEY this | |
5724 | NEXTKEY this, lastkey | |
5725 | DESTROY this | |
5726 | UNTIE this | |
5727 | ||
5728 | A class implementing an ordinary array should have the following methods: | |
5729 | ||
5730 | TIEARRAY classname, LIST | |
5731 | FETCH this, key | |
5732 | STORE this, key, value | |
5733 | FETCHSIZE this | |
5734 | STORESIZE this, count | |
5735 | CLEAR this | |
5736 | PUSH this, LIST | |
5737 | POP this | |
5738 | SHIFT this | |
5739 | UNSHIFT this, LIST | |
5740 | SPLICE this, offset, length, LIST | |
5741 | EXTEND this, count | |
5742 | DESTROY this | |
5743 | UNTIE this | |
5744 | ||
5745 | A class implementing a file handle should have the following methods: | |
5746 | ||
5747 | TIEHANDLE classname, LIST | |
5748 | READ this, scalar, length, offset | |
5749 | READLINE this | |
5750 | GETC this | |
5751 | WRITE this, scalar, length, offset | |
5752 | PRINT this, LIST | |
5753 | PRINTF this, format, LIST | |
5754 | BINMODE this | |
5755 | EOF this | |
5756 | FILENO this | |
5757 | SEEK this, position, whence | |
5758 | TELL this | |
5759 | OPEN this, mode, LIST | |
5760 | CLOSE this | |
5761 | DESTROY this | |
5762 | UNTIE this | |
5763 | ||
5764 | A class implementing a scalar should have the following methods: | |
5765 | ||
5766 | TIESCALAR classname, LIST | |
5767 | FETCH this, | |
5768 | STORE this, value | |
5769 | DESTROY this | |
5770 | UNTIE this | |
5771 | ||
5772 | Not all methods indicated above need be implemented. See L<perltie>, | |
5773 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. | |
5774 | ||
5775 | Unlike C<dbmopen>, the C<tie> function will not use or require a module | |
5776 | for you--you need to do that explicitly yourself. See L<DB_File> | |
5777 | or the F<Config> module for interesting C<tie> implementations. | |
5778 | ||
5779 | For further details see L<perltie>, L<"tied VARIABLE">. | |
5780 | ||
5781 | =item tied VARIABLE | |
5782 | ||
5783 | Returns a reference to the object underlying VARIABLE (the same value | |
5784 | that was originally returned by the C<tie> call that bound the variable | |
5785 | to a package.) Returns the undefined value if VARIABLE isn't tied to a | |
5786 | package. | |
5787 | ||
5788 | =item time | |
5789 | ||
5790 | Returns the number of non-leap seconds since whatever time the system | |
5791 | considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS, | |
5792 | and 00:00:00 UTC, January 1, 1970 for most other systems). | |
5793 | Suitable for feeding to C<gmtime> and C<localtime>. | |
5794 | ||
5795 | For measuring time in better granularity than one second, | |
5796 | you may use either the Time::HiRes module from CPAN, or | |
5797 | if you have gettimeofday(2), you may be able to use the | |
5798 | C<syscall> interface of Perl, see L<perlfaq8> for details. | |
5799 | ||
5800 | =item times | |
5801 | ||
5802 | Returns a four-element list giving the user and system times, in | |
5803 | seconds, for this process and the children of this process. | |
5804 | ||
5805 | ($user,$system,$cuser,$csystem) = times; | |
5806 | ||
5807 | In scalar context, C<times> returns C<$user>. | |
5808 | ||
5809 | =item tr/// | |
5810 | ||
5811 | The transliteration operator. Same as C<y///>. See L<perlop>. | |
5812 | ||
5813 | =item truncate FILEHANDLE,LENGTH | |
5814 | ||
5815 | =item truncate EXPR,LENGTH | |
5816 | ||
5817 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
5818 | specified length. Produces a fatal error if truncate isn't implemented | |
5819 | on your system. Returns true if successful, the undefined value | |
5820 | otherwise. | |
5821 | ||
5822 | The behavior is undefined if LENGTH is greater than the length of the | |
5823 | file. | |
5824 | ||
5825 | =item uc EXPR | |
5826 | ||
5827 | =item uc | |
5828 | ||
5829 | Returns an uppercased version of EXPR. This is the internal function | |
5830 | implementing the C<\U> escape in double-quoted strings. Respects | |
5831 | current LC_CTYPE locale if C<use locale> in force. See L<perllocale> | |
5832 | and L<perlunicode> for more details about locale and Unicode support. | |
5833 | It does not attempt to do titlecase mapping on initial letters. See | |
5834 | C<ucfirst> for that. | |
5835 | ||
5836 | If EXPR is omitted, uses C<$_>. | |
5837 | ||
5838 | =item ucfirst EXPR | |
5839 | ||
5840 | =item ucfirst | |
5841 | ||
5842 | Returns the value of EXPR with the first character in uppercase | |
5843 | (titlecase in Unicode). This is the internal function implementing | |
5844 | the C<\u> escape in double-quoted strings. Respects current LC_CTYPE | |
5845 | locale if C<use locale> in force. See L<perllocale> and L<perlunicode> | |
5846 | for more details about locale and Unicode support. | |
5847 | ||
5848 | If EXPR is omitted, uses C<$_>. | |
5849 | ||
5850 | =item umask EXPR | |
5851 | ||
5852 | =item umask | |
5853 | ||
5854 | Sets the umask for the process to EXPR and returns the previous value. | |
5855 | If EXPR is omitted, merely returns the current umask. | |
5856 | ||
5857 | The Unix permission C<rwxr-x---> is represented as three sets of three | |
5858 | bits, or three octal digits: C<0750> (the leading 0 indicates octal | |
5859 | and isn't one of the digits). The C<umask> value is such a number | |
5860 | representing disabled permissions bits. The permission (or "mode") | |
5861 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so | |
5862 | even if you tell C<sysopen> to create a file with permissions C<0777>, | |
5863 | if your umask is C<0022> then the file will actually be created with | |
5864 | permissions C<0755>. If your C<umask> were C<0027> (group can't | |
5865 | write; others can't read, write, or execute), then passing | |
5866 | C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ | |
5867 | 027> is C<0640>). | |
5868 | ||
5869 | Here's some advice: supply a creation mode of C<0666> for regular | |
5870 | files (in C<sysopen>) and one of C<0777> for directories (in | |
5871 | C<mkdir>) and executable files. This gives users the freedom of | |
5872 | choice: if they want protected files, they might choose process umasks | |
5873 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. | |
5874 | Programs should rarely if ever make policy decisions better left to | |
5875 | the user. The exception to this is when writing files that should be | |
5876 | kept private: mail files, web browser cookies, I<.rhosts> files, and | |
5877 | so on. | |
5878 | ||
5879 | If umask(2) is not implemented on your system and you are trying to | |
5880 | restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a | |
5881 | fatal error at run time. If umask(2) is not implemented and you are | |
5882 | not trying to restrict access for yourself, returns C<undef>. | |
5883 | ||
5884 | Remember that a umask is a number, usually given in octal; it is I<not> a | |
5885 | string of octal digits. See also L</oct>, if all you have is a string. | |
5886 | ||
5887 | =item undef EXPR | |
5888 | ||
5889 | =item undef | |
5890 | ||
5891 | Undefines the value of EXPR, which must be an lvalue. Use only on a | |
5892 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine | |
5893 | (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> | |
5894 | will probably not do what you expect on most predefined variables or | |
5895 | DBM list values, so don't do that; see L<delete>.) Always returns the | |
5896 | undefined value. You can omit the EXPR, in which case nothing is | |
5897 | undefined, but you still get an undefined value that you could, for | |
5898 | instance, return from a subroutine, assign to a variable or pass as a | |
5899 | parameter. Examples: | |
5900 | ||
5901 | undef $foo; | |
5902 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; | |
5903 | undef @ary; | |
5904 | undef %hash; | |
5905 | undef &mysub; | |
5906 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. | |
5907 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; | |
5908 | select undef, undef, undef, 0.25; | |
5909 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
5910 | ||
5911 | Note that this is a unary operator, not a list operator. | |
5912 | ||
5913 | =item unlink LIST | |
5914 | ||
5915 | =item unlink | |
5916 | ||
5917 | Deletes a list of files. Returns the number of files successfully | |
5918 | deleted. | |
5919 | ||
5920 | $cnt = unlink 'a', 'b', 'c'; | |
5921 | unlink @goners; | |
5922 | unlink <*.bak>; | |
5923 | ||
5924 | Note: C<unlink> will not delete directories unless you are superuser and | |
5925 | the B<-U> flag is supplied to Perl. Even if these conditions are | |
5926 | met, be warned that unlinking a directory can inflict damage on your | |
5927 | filesystem. Use C<rmdir> instead. | |
5928 | ||
5929 | If LIST is omitted, uses C<$_>. | |
5930 | ||
5931 | =item unpack TEMPLATE,EXPR | |
5932 | ||
5933 | C<unpack> does the reverse of C<pack>: it takes a string | |
5934 | and expands it out into a list of values. | |
5935 | (In scalar context, it returns merely the first value produced.) | |
5936 | ||
5937 | The string is broken into chunks described by the TEMPLATE. Each chunk | |
5938 | is converted separately to a value. Typically, either the string is a result | |
5939 | of C<pack>, or the bytes of the string represent a C structure of some | |
5940 | kind. | |
5941 | ||
5942 | The TEMPLATE has the same format as in the C<pack> function. | |
5943 | Here's a subroutine that does substring: | |
5944 | ||
5945 | sub substr { | |
5946 | my($what,$where,$howmuch) = @_; | |
5947 | unpack("x$where a$howmuch", $what); | |
5948 | } | |
5949 | ||
5950 | and then there's | |
5951 | ||
5952 | sub ordinal { unpack("c",$_[0]); } # same as ord() | |
5953 | ||
5954 | In addition to fields allowed in pack(), you may prefix a field with | |
5955 | a %<number> to indicate that | |
5956 | you want a <number>-bit checksum of the items instead of the items | |
5957 | themselves. Default is a 16-bit checksum. Checksum is calculated by | |
5958 | summing numeric values of expanded values (for string fields the sum of | |
5959 | C<ord($char)> is taken, for bit fields the sum of zeroes and ones). | |
5960 | ||
5961 | For example, the following | |
5962 | computes the same number as the System V sum program: | |
5963 | ||
5964 | $checksum = do { | |
5965 | local $/; # slurp! | |
5966 | unpack("%32C*",<>) % 65535; | |
5967 | }; | |
5968 | ||
5969 | The following efficiently counts the number of set bits in a bit vector: | |
5970 | ||
5971 | $setbits = unpack("%32b*", $selectmask); | |
5972 | ||
5973 | The C<p> and C<P> formats should be used with care. Since Perl | |
5974 | has no way of checking whether the value passed to C<unpack()> | |
5975 | corresponds to a valid memory location, passing a pointer value that's | |
5976 | not known to be valid is likely to have disastrous consequences. | |
5977 | ||
5978 | If the repeat count of a field is larger than what the remainder of | |
5979 | the input string allows, repeat count is decreased. If the input string | |
5980 | is longer than one described by the TEMPLATE, the rest is ignored. | |
5981 | ||
5982 | See L</pack> for more examples and notes. | |
5983 | ||
5984 | =item untie VARIABLE | |
5985 | ||
5986 | Breaks the binding between a variable and a package. (See C<tie>.) | |
5987 | Has no effect if the variable is not tied. | |
5988 | ||
5989 | =item unshift ARRAY,LIST | |
5990 | ||
5991 | Does the opposite of a C<shift>. Or the opposite of a C<push>, | |
5992 | depending on how you look at it. Prepends list to the front of the | |
5993 | array, and returns the new number of elements in the array. | |
5994 | ||
5995 | unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; | |
5996 | ||
5997 | Note the LIST is prepended whole, not one element at a time, so the | |
5998 | prepended elements stay in the same order. Use C<reverse> to do the | |
5999 | reverse. | |
6000 | ||
6001 | =item use Module VERSION LIST | |
6002 | ||
6003 | =item use Module VERSION | |
6004 | ||
6005 | =item use Module LIST | |
6006 | ||
6007 | =item use Module | |
6008 | ||
6009 | =item use VERSION | |
6010 | ||
6011 | Imports some semantics into the current package from the named module, | |
6012 | generally by aliasing certain subroutine or variable names into your | |
6013 | package. It is exactly equivalent to | |
6014 | ||
6015 | BEGIN { require Module; import Module LIST; } | |
6016 | ||
6017 | except that Module I<must> be a bareword. | |
6018 | ||
6019 | VERSION may be either a numeric argument such as 5.006, which will be | |
6020 | compared to C<$]>, or a literal of the form v5.6.1, which will be compared | |
6021 | to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is | |
6022 | greater than the version of the current Perl interpreter; Perl will not | |
6023 | attempt to parse the rest of the file. Compare with L</require>, which can | |
6024 | do a similar check at run time. | |
6025 | ||
6026 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
6027 | avoided, because it leads to misleading error messages under earlier | |
6028 | versions of Perl which do not support this syntax. The equivalent numeric | |
6029 | version should be used instead. | |
6030 | ||
6031 | use v5.6.1; # compile time version check | |
6032 | use 5.6.1; # ditto | |
6033 | use 5.006_001; # ditto; preferred for backwards compatibility | |
6034 | ||
6035 | This is often useful if you need to check the current Perl version before | |
6036 | C<use>ing library modules that have changed in incompatible ways from | |
6037 | older versions of Perl. (We try not to do this more than we have to.) | |
6038 | ||
6039 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The | |
6040 | C<require> makes sure the module is loaded into memory if it hasn't been | |
6041 | yet. The C<import> is not a builtin--it's just an ordinary static method | |
6042 | call into the C<Module> package to tell the module to import the list of | |
6043 | features back into the current package. The module can implement its | |
6044 | C<import> method any way it likes, though most modules just choose to | |
6045 | derive their C<import> method via inheritance from the C<Exporter> class that | |
6046 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> | |
6047 | method can be found then the call is skipped. | |
6048 | ||
6049 | If you do not want to call the package's C<import> method (for instance, | |
6050 | to stop your namespace from being altered), explicitly supply the empty list: | |
6051 | ||
6052 | use Module (); | |
6053 | ||
6054 | That is exactly equivalent to | |
6055 | ||
6056 | BEGIN { require Module } | |
6057 | ||
6058 | If the VERSION argument is present between Module and LIST, then the | |
6059 | C<use> will call the VERSION method in class Module with the given | |
6060 | version as an argument. The default VERSION method, inherited from | |
6061 | the UNIVERSAL class, croaks if the given version is larger than the | |
6062 | value of the variable C<$Module::VERSION>. | |
6063 | ||
6064 | Again, there is a distinction between omitting LIST (C<import> called | |
6065 | with no arguments) and an explicit empty LIST C<()> (C<import> not | |
6066 | called). Note that there is no comma after VERSION! | |
6067 | ||
6068 | Because this is a wide-open interface, pragmas (compiler directives) | |
6069 | are also implemented this way. Currently implemented pragmas are: | |
6070 | ||
6071 | use constant; | |
6072 | use diagnostics; | |
6073 | use integer; | |
6074 | use sigtrap qw(SEGV BUS); | |
6075 | use strict qw(subs vars refs); | |
6076 | use subs qw(afunc blurfl); | |
6077 | use warnings qw(all); | |
6078 | use sort qw(stable _quicksort _mergesort); | |
6079 | ||
6080 | Some of these pseudo-modules import semantics into the current | |
6081 | block scope (like C<strict> or C<integer>, unlike ordinary modules, | |
6082 | which import symbols into the current package (which are effective | |
6083 | through the end of the file). | |
6084 | ||
6085 | There's a corresponding C<no> command that unimports meanings imported | |
6086 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. | |
6087 | ||
6088 | no integer; | |
6089 | no strict 'refs'; | |
6090 | no warnings; | |
6091 | ||
6092 | See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> | |
6093 | for the C<-M> and C<-m> command-line options to perl that give C<use> | |
6094 | functionality from the command-line. | |
6095 | ||
6096 | =item utime LIST | |
6097 | ||
6098 | Changes the access and modification times on each file of a list of | |
6099 | files. The first two elements of the list must be the NUMERICAL access | |
6100 | and modification times, in that order. Returns the number of files | |
6101 | successfully changed. The inode change time of each file is set | |
6102 | to the current time. This code has the same effect as the C<touch> | |
6103 | command if the files already exist: | |
6104 | ||
6105 | #!/usr/bin/perl | |
6106 | $now = time; | |
6107 | utime $now, $now, @ARGV; | |
6108 | ||
6109 | If the first two elements of the list are C<undef>, then the utime(2) | |
6110 | function in the C library will be called with a null second argument. | |
6111 | On most systems, this will set the file's access and modification | |
6112 | times to the current time. (i.e. equivalent to the example above.) | |
6113 | ||
6114 | utime undef, undef, @ARGV; | |
6115 | ||
6116 | =item values HASH | |
6117 | ||
6118 | Returns a list consisting of all the values of the named hash. (In a | |
6119 | scalar context, returns the number of values.) The values are | |
6120 | returned in an apparently random order. The actual random order is | |
6121 | subject to change in future versions of perl, but it is guaranteed to | |
6122 | be the same order as either the C<keys> or C<each> function would | |
6123 | produce on the same (unmodified) hash. | |
6124 | ||
6125 | Note that the values are not copied, which means modifying them will | |
6126 | modify the contents of the hash: | |
6127 | ||
6128 | for (values %hash) { s/foo/bar/g } # modifies %hash values | |
6129 | for (@hash{keys %hash}) { s/foo/bar/g } # same | |
6130 | ||
6131 | As a side effect, calling values() resets the HASH's internal iterator. | |
6132 | See also C<keys>, C<each>, and C<sort>. | |
6133 | ||
6134 | =item vec EXPR,OFFSET,BITS | |
6135 | ||
6136 | Treats the string in EXPR as a bit vector made up of elements of | |
6137 | width BITS, and returns the value of the element specified by OFFSET | |
6138 | as an unsigned integer. BITS therefore specifies the number of bits | |
6139 | that are reserved for each element in the bit vector. This must | |
6140 | be a power of two from 1 to 32 (or 64, if your platform supports | |
6141 | that). | |
6142 | ||
6143 | If BITS is 8, "elements" coincide with bytes of the input string. | |
6144 | ||
6145 | If BITS is 16 or more, bytes of the input string are grouped into chunks | |
6146 | of size BITS/8, and each group is converted to a number as with | |
6147 | pack()/unpack() with big-endian formats C<n>/C<N> (and analogously | |
6148 | for BITS==64). See L<"pack"> for details. | |
6149 | ||
6150 | If bits is 4 or less, the string is broken into bytes, then the bits | |
6151 | of each byte are broken into 8/BITS groups. Bits of a byte are | |
6152 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, | |
6153 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, | |
6154 | breaking the single input byte C<chr(0x36)> into two groups gives a list | |
6155 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. | |
6156 | ||
6157 | C<vec> may also be assigned to, in which case parentheses are needed | |
6158 | to give the expression the correct precedence as in | |
6159 | ||
6160 | vec($image, $max_x * $x + $y, 8) = 3; | |
6161 | ||
6162 | If the selected element is outside the string, the value 0 is returned. | |
6163 | If an element off the end of the string is written to, Perl will first | |
6164 | extend the string with sufficiently many zero bytes. It is an error | |
6165 | to try to write off the beginning of the string (i.e. negative OFFSET). | |
6166 | ||
6167 | The string should not contain any character with the value > 255 (which | |
6168 | can only happen if you're using UTF8 encoding). If it does, it will be | |
6169 | treated as something which is not UTF8 encoded. When the C<vec> was | |
6170 | assigned to, other parts of your program will also no longer consider the | |
6171 | string to be UTF8 encoded. In other words, if you do have such characters | |
6172 | in your string, vec() will operate on the actual byte string, and not the | |
6173 | conceptual character string. | |
6174 | ||
6175 | Strings created with C<vec> can also be manipulated with the logical | |
6176 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit | |
6177 | vector operation is desired when both operands are strings. | |
6178 | See L<perlop/"Bitwise String Operators">. | |
6179 | ||
6180 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. | |
6181 | The comments show the string after each step. Note that this code works | |
6182 | in the same way on big-endian or little-endian machines. | |
6183 | ||
6184 | my $foo = ''; | |
6185 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' | |
6186 | ||
6187 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits | |
6188 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') | |
6189 | ||
6190 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' | |
6191 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
6192 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
6193 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
6194 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
6195 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' | |
6196 | # 'r' is "\x72" | |
6197 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" | |
6198 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
6199 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' | |
6200 | # 'l' is "\x6c" | |
6201 | ||
6202 | To transform a bit vector into a string or list of 0's and 1's, use these: | |
6203 | ||
6204 | $bits = unpack("b*", $vector); | |
6205 | @bits = split(//, unpack("b*", $vector)); | |
6206 | ||
6207 | If you know the exact length in bits, it can be used in place of the C<*>. | |
6208 | ||
6209 | Here is an example to illustrate how the bits actually fall in place: | |
6210 | ||
6211 | #!/usr/bin/perl -wl | |
6212 | ||
6213 | print <<'EOT'; | |
6214 | 0 1 2 3 | |
6215 | unpack("V",$_) 01234567890123456789012345678901 | |
6216 | ------------------------------------------------------------------ | |
6217 | EOT | |
6218 | ||
6219 | for $w (0..3) { | |
6220 | $width = 2**$w; | |
6221 | for ($shift=0; $shift < $width; ++$shift) { | |
6222 | for ($off=0; $off < 32/$width; ++$off) { | |
6223 | $str = pack("B*", "0"x32); | |
6224 | $bits = (1<<$shift); | |
6225 | vec($str, $off, $width) = $bits; | |
6226 | $res = unpack("b*",$str); | |
6227 | $val = unpack("V", $str); | |
6228 | write; | |
6229 | } | |
6230 | } | |
6231 | } | |
6232 | ||
6233 | format STDOUT = | |
6234 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> | |
6235 | $off, $width, $bits, $val, $res | |
6236 | . | |
6237 | __END__ | |
6238 | ||
6239 | Regardless of the machine architecture on which it is run, the above | |
6240 | example should print the following table: | |
6241 | ||
6242 | 0 1 2 3 | |
6243 | unpack("V",$_) 01234567890123456789012345678901 | |
6244 | ------------------------------------------------------------------ | |
6245 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 | |
6246 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 | |
6247 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 | |
6248 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 | |
6249 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 | |
6250 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 | |
6251 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 | |
6252 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 | |
6253 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 | |
6254 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 | |
6255 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 | |
6256 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 | |
6257 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 | |
6258 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 | |
6259 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 | |
6260 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 | |
6261 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 | |
6262 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 | |
6263 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 | |
6264 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 | |
6265 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 | |
6266 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 | |
6267 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 | |
6268 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 | |
6269 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 | |
6270 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 | |
6271 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 | |
6272 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 | |
6273 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 | |
6274 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 | |
6275 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 | |
6276 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 | |
6277 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 | |
6278 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 | |
6279 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 | |
6280 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 | |
6281 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 | |
6282 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 | |
6283 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 | |
6284 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 | |
6285 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 | |
6286 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 | |
6287 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 | |
6288 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 | |
6289 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 | |
6290 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 | |
6291 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 | |
6292 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 | |
6293 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 | |
6294 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 | |
6295 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 | |
6296 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 | |
6297 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 | |
6298 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 | |
6299 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 | |
6300 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 | |
6301 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 | |
6302 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 | |
6303 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 | |
6304 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 | |
6305 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 | |
6306 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 | |
6307 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 | |
6308 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 | |
6309 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 | |
6310 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 | |
6311 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 | |
6312 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 | |
6313 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 | |
6314 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 | |
6315 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 | |
6316 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 | |
6317 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 | |
6318 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 | |
6319 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 | |
6320 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 | |
6321 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 | |
6322 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 | |
6323 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 | |
6324 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 | |
6325 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 | |
6326 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 | |
6327 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 | |
6328 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 | |
6329 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 | |
6330 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 | |
6331 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 | |
6332 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 | |
6333 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 | |
6334 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 | |
6335 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 | |
6336 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 | |
6337 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 | |
6338 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 | |
6339 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 | |
6340 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 | |
6341 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 | |
6342 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 | |
6343 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 | |
6344 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 | |
6345 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 | |
6346 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 | |
6347 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 | |
6348 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 | |
6349 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 | |
6350 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 | |
6351 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 | |
6352 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 | |
6353 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 | |
6354 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 | |
6355 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 | |
6356 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 | |
6357 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 | |
6358 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 | |
6359 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 | |
6360 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 | |
6361 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 | |
6362 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 | |
6363 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 | |
6364 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 | |
6365 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 | |
6366 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 | |
6367 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 | |
6368 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 | |
6369 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 | |
6370 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 | |
6371 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 | |
6372 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 | |
6373 | ||
6374 | =item wait | |
6375 | ||
6376 | Behaves like the wait(2) system call on your system: it waits for a child | |
6377 | process to terminate and returns the pid of the deceased process, or | |
6378 | C<-1> if there are no child processes. The status is returned in C<$?>. | |
6379 | Note that a return value of C<-1> could mean that child processes are | |
6380 | being automatically reaped, as described in L<perlipc>. | |
6381 | ||
6382 | =item waitpid PID,FLAGS | |
6383 | ||
6384 | Waits for a particular child process to terminate and returns the pid of | |
6385 | the deceased process, or C<-1> if there is no such child process. On some | |
6386 | systems, a value of 0 indicates that there are processes still running. | |
6387 | The status is returned in C<$?>. If you say | |
6388 | ||
6389 | use POSIX ":sys_wait_h"; | |
6390 | #... | |
6391 | do { | |
6392 | $kid = waitpid(-1, WNOHANG); | |
6393 | } until $kid > 0; | |
6394 | ||
6395 | then you can do a non-blocking wait for all pending zombie processes. | |
6396 | Non-blocking wait is available on machines supporting either the | |
6397 | waitpid(2) or wait4(2) system calls. However, waiting for a particular | |
6398 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the | |
6399 | system call by remembering the status values of processes that have | |
6400 | exited but have not been harvested by the Perl script yet.) | |
6401 | ||
6402 | Note that on some systems, a return value of C<-1> could mean that child | |
6403 | processes are being automatically reaped. See L<perlipc> for details, | |
6404 | and for other examples. | |
6405 | ||
6406 | =item wantarray | |
6407 | ||
6408 | Returns true if the context of the currently executing subroutine is | |
6409 | looking for a list value. Returns false if the context is looking | |
6410 | for a scalar. Returns the undefined value if the context is looking | |
6411 | for no value (void context). | |
6412 | ||
6413 | return unless defined wantarray; # don't bother doing more | |
6414 | my @a = complex_calculation(); | |
6415 | return wantarray ? @a : "@a"; | |
6416 | ||
6417 | This function should have been named wantlist() instead. | |
6418 | ||
6419 | =item warn LIST | |
6420 | ||
6421 | Produces a message on STDERR just like C<die>, but doesn't exit or throw | |
6422 | an exception. | |
6423 | ||
6424 | If LIST is empty and C<$@> already contains a value (typically from a | |
6425 | previous eval) that value is used after appending C<"\t...caught"> | |
6426 | to C<$@>. This is useful for staying almost, but not entirely similar to | |
6427 | C<die>. | |
6428 | ||
6429 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. | |
6430 | ||
6431 | No message is printed if there is a C<$SIG{__WARN__}> handler | |
6432 | installed. It is the handler's responsibility to deal with the message | |
6433 | as it sees fit (like, for instance, converting it into a C<die>). Most | |
6434 | handlers must therefore make arrangements to actually display the | |
6435 | warnings that they are not prepared to deal with, by calling C<warn> | |
6436 | again in the handler. Note that this is quite safe and will not | |
6437 | produce an endless loop, since C<__WARN__> hooks are not called from | |
6438 | inside one. | |
6439 | ||
6440 | You will find this behavior is slightly different from that of | |
6441 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
6442 | instead call C<die> again to change it). | |
6443 | ||
6444 | Using a C<__WARN__> handler provides a powerful way to silence all | |
6445 | warnings (even the so-called mandatory ones). An example: | |
6446 | ||
6447 | # wipe out *all* compile-time warnings | |
6448 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
6449 | my $foo = 10; | |
6450 | my $foo = 20; # no warning about duplicate my $foo, | |
6451 | # but hey, you asked for it! | |
6452 | # no compile-time or run-time warnings before here | |
6453 | $DOWARN = 1; | |
6454 | ||
6455 | # run-time warnings enabled after here | |
6456 | warn "\$foo is alive and $foo!"; # does show up | |
6457 | ||
6458 | See L<perlvar> for details on setting C<%SIG> entries, and for more | |
6459 | examples. See the Carp module for other kinds of warnings using its | |
6460 | carp() and cluck() functions. | |
6461 | ||
6462 | =item write FILEHANDLE | |
6463 | ||
6464 | =item write EXPR | |
6465 | ||
6466 | =item write | |
6467 | ||
6468 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, | |
6469 | using the format associated with that file. By default the format for | |
6470 | a file is the one having the same name as the filehandle, but the | |
6471 | format for the current output channel (see the C<select> function) may be set | |
6472 | explicitly by assigning the name of the format to the C<$~> variable. | |
6473 | ||
6474 | Top of form processing is handled automatically: if there is | |
6475 | insufficient room on the current page for the formatted record, the | |
6476 | page is advanced by writing a form feed, a special top-of-page format | |
6477 | is used to format the new page header, and then the record is written. | |
6478 | By default the top-of-page format is the name of the filehandle with | |
6479 | "_TOP" appended, but it may be dynamically set to the format of your | |
6480 | choice by assigning the name to the C<$^> variable while the filehandle is | |
6481 | selected. The number of lines remaining on the current page is in | |
6482 | variable C<$->, which can be set to C<0> to force a new page. | |
6483 | ||
6484 | If FILEHANDLE is unspecified, output goes to the current default output | |
6485 | channel, which starts out as STDOUT but may be changed by the | |
6486 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression | |
6487 | is evaluated and the resulting string is used to look up the name of | |
6488 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
6489 | ||
6490 | Note that write is I<not> the opposite of C<read>. Unfortunately. | |
6491 | ||
6492 | =item y/// | |
6493 | ||
6494 | The transliteration operator. Same as C<tr///>. See L<perlop>. | |
6495 | ||
6496 | =back |