Commit | Line | Data |
---|---|---|
1662094b BJ |
1 | .if \n(xx .bp |
2 | .if !\n(xx \{\ | |
3 | .so tmac.p \} | |
4 | 'if n 'ND | |
5 | .nr H1 1 | |
6 | .NH | |
7 | Basic UNIX Pascal | |
8 | .PP | |
9 | The following sections | |
10 | explain the basics of using | |
11 | .UP . | |
12 | In examples here we use the text editor | |
13 | .I ex | |
14 | (6). | |
15 | Users of the text editor | |
16 | .I ed | |
17 | should have little trouble following these examples, | |
18 | as | |
19 | .I ex | |
20 | is similar to | |
21 | .I ed . | |
22 | We use | |
23 | .I ex | |
24 | because it | |
25 | allows us to make clearer examples.\(dg | |
26 | .FS | |
27 | \(dg Users with \s-2CRT\s0 terminals should find the editor | |
28 | .I vi | |
29 | more pleasant to use; | |
30 | we do not show its use here because its display oriented nature | |
31 | makes it difficult to illustrate. | |
32 | .FE | |
33 | The new | |
34 | .UX | |
35 | user will find it helpful to read one of the text editor documents | |
36 | described in section 1.4 before continuing with this section. | |
37 | .NH 2 | |
38 | A first program | |
39 | .PP | |
40 | To prepare a program for | |
41 | .UP | |
42 | we first need to have an account on | |
43 | .UX | |
44 | and to `login' | |
45 | to the system on this account. | |
46 | These procedures are described in the documents | |
47 | .I "Communicating with UNIX" | |
48 | and | |
49 | .I "UNIX for Beginners". | |
50 | .PP | |
51 | Once we are logged in we need to choose a name for our program; | |
52 | let us call it `first' as this is the first example. | |
53 | We must also choose a name for the file in which the program will be stored. | |
54 | The | |
55 | .UP | |
56 | system requires that programs reside in files which have names ending with | |
57 | the sequence `.p' so we will call our file `first.p'. | |
58 | .PP | |
59 | A sample editing session to create this file would begin: | |
60 | .LS | |
61 | % \*bex first.p\fR | |
62 | "first.p" No such file or directory | |
63 | : | |
64 | .LE | |
65 | We didn't expect the file to exist, so the error diagnostic doesn't | |
66 | bother us. | |
67 | The editor now knows the name of the file we are creating. | |
68 | The `:' prompt indicates that it is ready for command input. | |
69 | We can add the text for our program using the `append' | |
70 | command as follows. | |
71 | .LS | |
72 | :\*bappend\fR | |
73 | .B | |
74 | program first(output) | |
75 | begin | |
76 | writeln('Hello, world!') | |
77 | end. | |
78 | \&. | |
79 | .R | |
80 | : | |
81 | .LE | |
82 | The line containing the single `\*b.\fR' character here indicated | |
83 | the end of the appended text. | |
84 | The `:' prompt indicates that | |
85 | .I ex | |
86 | is ready for another command. | |
87 | As the editor operates in a temporary work space we must now store the contents | |
88 | of this work space in the file `first.p' | |
89 | so we can use the Pascal | |
90 | translator and executor | |
91 | .IX | |
92 | on it. | |
93 | .LS | |
94 | :\*bwrite\fR | |
95 | "first.p" 4 lines, 59 characters | |
96 | :\*bquit\fR | |
97 | % | |
98 | .LE | |
99 | We wrote out the file from the edit buffer here with the | |
100 | `write' | |
101 | command, and | |
102 | .I ex | |
103 | indicated the number of lines and characters written. | |
104 | We then quit the editor, and now have a prompt from the shell.\(dd | |
105 | .FS | |
106 | \(dd Our examples here assume you are using | |
107 | .I csh. | |
108 | .FE | |
109 | .KS | |
110 | .PP | |
111 | We are ready to try | |
112 | to translate and execute our program. | |
113 | .DS | |
114 | .tr '\(aa^\(ua | |
115 | % \*bpix first.p\fR | |
116 | .so firstout | |
117 | .tr ''^^ | |
118 | % | |
119 | .DE | |
120 | .KE | |
121 | .PP | |
122 | The translator first printed a syntax error diagnostic. | |
123 | The number 2 here indicates that the rest of the line is an image | |
124 | of the second line of our program. | |
125 | The translator is saying that it expected to find a `;' before the | |
126 | keyword | |
127 | .B begin | |
128 | on this line. | |
129 | If we look at the Pascal syntax charts in the Jensen-Wirth | |
130 | .I "User Manual" , | |
131 | or at some of the sample programs therein, we will see that | |
132 | we have omitted the terminating `;' of the | |
133 | .B program | |
134 | statement on the first | |
135 | line of our program. | |
136 | .PP | |
137 | One other thing to notice about the error diagnostic is the letter `e' | |
138 | at the beginning. | |
139 | It stands for `error', | |
140 | indicating that our input was not legal Pascal. | |
141 | The fact that it is an `e' rather than an `E' | |
142 | indicates that the translator managed to recover from this error well | |
143 | enough that generation of code and execution could take place. | |
144 | Execution is possible whenever no fatal `E' errors | |
145 | occur during translation. | |
146 | The other classes of diagnostics are `w' warnings, | |
147 | which do not necessarily indicate errors in the program, | |
148 | but point out inconsistencies which are likely to be due to program bugs, | |
149 | and `s' standard-Pascal violations.\*(dg | |
150 | .FS | |
151 | \*(dgThe standard Pascal warnings occur only when the associated | |
152 | .B s | |
153 | translator option is enabled. | |
154 | The | |
155 | .B s | |
156 | option is discussed in sections 5.1 and A.6 below. | |
157 | Warning diagnostics are discussed at the end of section 3.2, | |
158 | the associated | |
159 | .B w | |
160 | option is described in section 5.2. | |
161 | .FE | |
162 | .PP | |
163 | After completing the translation of the program to interpretive code, | |
164 | the Pascal system indicates that execution of the translated program began. | |
165 | The output from the execution of the program then appeared. | |
166 | At program termination, the Pascal runtime system indicated the | |
167 | number of statements executed, and the amount of cpu time | |
168 | used, with the resolution of the latter being 1/60'th of a second. | |
169 | .PP | |
170 | Let us now fix the error in the program and translate it to a permanent | |
171 | object code file | |
172 | .I obj | |
173 | using | |
174 | .PI . | |
175 | The program | |
176 | .PI | |
177 | translates Pascal programs but stores the object code instead of executing it\*(dd. | |
178 | .FS | |
179 | \*(ddThis script indicates some other useful approaches to debugging | |
180 | Pascal programs. | |
181 | As in | |
182 | .I ed | |
183 | we can shorten commands in | |
184 | .I ex | |
185 | to an initial prefix of the command name as we did | |
186 | with the | |
187 | .I substitute | |
188 | command here. | |
189 | We have also used the `!' shell escape command here to execute other | |
190 | commands with a shell without leaving the editor. | |
191 | .FE | |
192 | .LS | |
193 | % \*bex first.p\fR | |
194 | "first.p" 4 lines, 59 characters | |
195 | :\*b1 print\fR | |
196 | program first(output) | |
197 | :\*bs/$/;\fR | |
198 | program first(output); | |
199 | :\*bwrite\fR | |
200 | "first.p" 4 lines, 60 characters | |
201 | :\*b!pi %\fR | |
202 | !pi first.p | |
203 | ! | |
204 | :\*bquit\fR | |
205 | % | |
206 | .LE | |
207 | The first command issued from | |
208 | .I ex | |
209 | with the `!' | |
210 | involved the use of the `%' character which stands in this command for | |
211 | the file we are editing. | |
212 | .I Ex | |
213 | made this substitution, and then echoed back the expanded line | |
214 | before executing the command. | |
215 | When the command finished, the editor echoed the character `!' | |
216 | so that we would know it was done. | |
217 | .PP | |
218 | If we now use the | |
219 | .UX | |
220 | .I ls | |
221 | list files command we can see what files we have: | |
222 | .LS | |
223 | % \*bls\fR | |
224 | first.p | |
225 | obj | |
226 | % | |
227 | .LE | |
228 | The file `obj' here contains the Pascal interpreter code. | |
229 | We can execute this by typing: | |
230 | .LS | |
231 | % \*bpx obj\fR | |
232 | .so firstobjout | |
233 | % | |
234 | .LE | |
235 | Alternatively, the command: | |
236 | .LS | |
237 | % \*bobj\fR | |
238 | .LE | |
239 | will have the same effect. | |
240 | Some examples of different ways to execute the program follow. | |
241 | .LS | |
242 | % \*bpx\fR | |
243 | .so firstobjout | |
244 | % \*bpi -p first.p\fR | |
245 | % \*bpx obj\fR | |
246 | .so firstobjout2 | |
247 | % \*bpix -p first.p\fR | |
248 | .so firstobjout2 | |
249 | % | |
250 | .LE | |
251 | .PP | |
252 | Note that | |
253 | .I px | |
254 | will assume that `obj' is the file we wish to execute | |
255 | if we don't tell it otherwise. | |
256 | The last two translations use the | |
257 | .B \-p | |
258 | no-post-mortem option to eliminate | |
259 | execution statistics and | |
260 | `Execution begins' | |
261 | and | |
262 | `Execution terminated' | |
263 | messages. | |
264 | See section 5.2 for more details. | |
265 | If we now look at the files in our directory we will see: | |
266 | .LS | |
267 | % \*bls\fR | |
268 | first.p | |
269 | obj | |
270 | % | |
271 | .LE | |
272 | We can give our object program a name other than `obj' by using the move | |
273 | command | |
274 | .I mv | |
275 | (1). | |
276 | Thus to name our program `hello': | |
277 | .LS | |
278 | % \*bmv obj hello\fR | |
279 | % \*bhello\fR | |
280 | Hello, world! | |
281 | % \*bls\fR | |
282 | first.p | |
283 | hello | |
284 | % | |
285 | .LE | |
286 | .KS | |
287 | Finally we can get rid of the Pascal object code by using the | |
288 | .I rm | |
289 | (1) remove file command, e.g.: | |
290 | .LS | |
291 | % \*brm hello\fR | |
292 | % \*bls\fR | |
293 | first.p | |
294 | % | |
295 | .LE | |
296 | .KE | |
297 | .PP | |
298 | For small programs which are being developed | |
299 | .IX | |
300 | tends to be more convenient to use than | |
301 | .PI | |
302 | and | |
303 | .X . | |
304 | Except for absence of the | |
305 | .I obj | |
306 | file after a | |
307 | .IX | |
308 | run, | |
309 | a | |
310 | .IX | |
311 | command is equivalent to a | |
312 | .PI | |
313 | command followed by a | |
314 | .X | |
315 | command. | |
316 | For larger programs, | |
317 | where a number of runs testing different parts of the program are | |
318 | to be made, | |
319 | .PI | |
320 | is useful as this | |
321 | .I obj | |
322 | file can be executed any desired number of times. | |
323 | .NH 2 | |
324 | A larger program | |
325 | .PP | |
326 | Suppose that we have used the editor to put a larger program | |
327 | in the file `bigger.p'. | |
328 | We can list this program with line numbers by using the program | |
329 | .I num | |
330 | i.e.: | |
331 | .LS | |
332 | % \*bnum bigger.p\fR | |
333 | .so bigger3.p | |
334 | % | |
335 | .LE | |
336 | This program is similar to program 4.9 on page 30 of the | |
337 | Jensen-Wirth | |
338 | .I "User Manual" . | |
339 | A number of problems have been introduced into this example for | |
340 | pedagogical reasons. | |
341 | .br | |
342 | .ne 8 | |
343 | .PP | |
344 | If we attempt to translate and execute the program using | |
345 | .IX | |
346 | we get the following response: | |
347 | .LS | |
348 | % \*bpix bigger.p\fR | |
349 | .so bigout1 | |
350 | % | |
351 | .LE | |
352 | .PP | |
353 | Since there were fatal `E' errors in our program, | |
354 | no code was generated and execution was necessarily suppressed. | |
355 | One thing which would be useful at this point is a listing of the | |
356 | program with the error messages. | |
357 | We can get this by using the command: | |
358 | .LS | |
359 | % \*bpi -l bigger.p\fR | |
360 | .LE | |
361 | There is no point in using | |
362 | .IX | |
363 | here, since we know there are fatal errors in the program. | |
364 | This command will produce the output at our terminal. | |
365 | If we are at a terminal which does not produce a hard copy | |
366 | we may wish to print this | |
367 | listing off-line on a line printer\*(dg. | |
368 | .FS | |
369 | \*(dgAt Berkeley, the line printer for the Cory Hall system is in Room 199B. | |
370 | The line printers for the Computer Center | |
371 | systems are in the basement of Evans Hall. | |
372 | .FE | |
373 | We can do this with the command: | |
374 | .LS | |
375 | % \*bpi -l bigger.p | lpr\fR | |
376 | .LE | |
377 | .PP | |
378 | In the next few sections we will illustrate various aspects of the | |
379 | Berkeley | |
380 | Pascal system by correcting this program. | |
381 | .NH 2 | |
382 | Correcting the first errors | |
383 | .PP | |
384 | Most of the errors which occurred in this program were | |
385 | .I syntactic | |
386 | errors, those in the format and structure of the program rather than | |
387 | its content. | |
388 | Syntax errors are flagged by printing the offending line, and then a line | |
389 | which flags the location at which an error was detected. | |
390 | The flag line also gives an explanation | |
391 | stating either a possible cause of the error, | |
392 | a simple action which can be taken to recover from the error so | |
393 | as to be able to continue the analysis, | |
394 | a symbol which was expected at the point of error, | |
395 | or an indication that the input was `malformed'. | |
396 | In the last case, the recovery may skip ahead in the input | |
397 | to a point where analysis of the program can continue. | |
398 | .PP | |
399 | In this example, | |
400 | the first error diagnostic indicates that the translator detected | |
401 | a comment within a comment. | |
402 | While this is not considered an error in `standard' | |
403 | Pascal, it usually corresponds to an error in the program which | |
404 | is being translated. | |
405 | In this case, we have accidentally omitted the trailing `*)' of the comment | |
406 | on line 8. | |
407 | We can begin an editor session to correct this problem by doing: | |
408 | .LS | |
409 | % \*bex bigger.p\fR | |
410 | "bigger.p" 24 lines, 512 characters | |
411 | :\*b8s/$/ *)\fR | |
412 | s = 32; (* 32 character width for interval [x, x+1] *) | |
413 | : | |
414 | .LE | |
415 | .PP | |
416 | The second diagnostic, given after line 16, | |
417 | indicates that the keyword | |
418 | .B do | |
419 | was expected before the keyword | |
420 | .B begin | |
421 | in the | |
422 | .B for | |
423 | statement. | |
424 | If we examine the | |
425 | .I statement | |
426 | syntax chart on page 118 of the | |
427 | Jensen-Wirth | |
428 | .I "User Manual" | |
429 | we will discover that | |
430 | .B do | |
431 | is a necessary part of the | |
432 | .B for | |
433 | statement. | |
434 | Similarly, we could have referred to section C.3 of the | |
435 | Jensen-Wirth | |
436 | .I "User Manual" | |
437 | to learn about the | |
438 | .B for | |
439 | statement and gotten the same information there. | |
440 | It is often useful to refer to these syntax charts and to the | |
441 | relevant sections of this book. | |
442 | .PP | |
443 | We can correct this problem by first scanning for the keyword | |
444 | .B for | |
445 | in the file and then substituting the keyword | |
446 | .B do | |
447 | to appear in front of the keyword | |
448 | .B begin | |
449 | there. | |
450 | Thus: | |
451 | .LS | |
452 | :\*b/for\fR | |
453 | for i := 0 to lim begin | |
454 | :\*bs/begin/do &\fR | |
455 | for i := 0 to lim do begin | |
456 | : | |
457 | .LE | |
458 | The next error in the program is easy to pinpoint. | |
459 | On line 18, we didn't hit the shift key and got a `9' | |
460 | instead of a `)'. | |
461 | The translator diagnosed that `x9' | |
462 | was an undefined variable and, later, | |
463 | that a `)' was missing in the statement. | |
464 | It should be stressed that | |
465 | .PI | |
466 | is not suggesting that you should insert a `)' before the `;'. | |
467 | It is only indicating that making this change will help it to be able to | |
468 | continue analyzing the program so as to be able to diagnose further | |
469 | errors. | |
470 | You must then determine the true cause of the error and make the | |
471 | appropriate correction to the source text. | |
472 | .PP | |
473 | This error also illustrates the fact that one error in the input may lead | |
474 | to multiple error diagnostics. | |
475 | .I Pi | |
476 | attempts | |
477 | to give only one diagnostic for each error, | |
478 | but single errors in the input sometimes appear to be more than | |
479 | one error. | |
480 | It is also the case that | |
481 | .PI | |
482 | may not detect an error when it occurs, but may detect it later in | |
483 | the input. | |
484 | This would have happened | |
485 | in this example if we had typed `x' instead of `x9'. | |
486 | .PP | |
487 | The translator next detected, on line 19, that the function | |
488 | .I Round | |
489 | and the variable | |
490 | .I h | |
491 | were undefined. | |
492 | It does not know about | |
493 | .I Round | |
494 | because | |
495 | .UP | |
496 | normally distinguishes between upper- and lower-case. | |
497 | On | |
498 | .UX | |
499 | lower-case is preferred\*(dg, | |
500 | .FS | |
501 | \*(dgOne good reason for using lower-case is that it is easier to type. | |
502 | .FE | |
503 | and all keywords and built-in | |
504 | .B procedure | |
505 | and | |
506 | .B function | |
507 | names are composed of lower-case letters, | |
508 | just as they are in the Jensen-Wirth | |
509 | .I "Pascal Report" . | |
510 | Thus we need to use the function | |
511 | .I round | |
512 | here. | |
513 | As far as | |
514 | .I h | |
515 | is concerned, | |
516 | we can see why it is undefined if we look back to line 9 | |
517 | and note that its definition was lost in the non-terminated | |
518 | comment. | |
519 | This diagnostic need not, therefore, concern us. | |
520 | .PP | |
521 | The next error which occurred in the program caused the translator | |
522 | to insert a `;' before the statement calling | |
523 | .I writeln | |
524 | on line 23. | |
525 | If we examine the program around the point of error we will see | |
526 | that the actual error is that the keyword | |
527 | .B until | |
528 | and an associated expression have been omitted here. | |
529 | Note that the diagnostic from the translator does not indicate the actual | |
530 | error, and is somewhat misleading. | |
531 | The translator made the correction which seemed to be most plausible. | |
532 | As the omission of a `;' character is a common mistake, | |
533 | the translator chose to indicate this as a possible fix here. | |
534 | It later detected that the keyword | |
535 | .B until | |
536 | was missing, but not until it saw the keyword | |
537 | .B end | |
538 | on line 24. | |
539 | The combination of these diagnostics indicate to us the true problem. | |
540 | .PP | |
541 | The final syntactic error message indicates that the translator needed an | |
542 | .B end | |
543 | keyword to match the | |
544 | .B begin | |
545 | at line 15. | |
546 | Since the | |
547 | .B end | |
548 | at line 24 is supposed to match this | |
549 | .B begin , | |
550 | we can infer that another | |
551 | .B begin | |
552 | must have been mismatched, and have matched this | |
553 | .B end . | |
554 | Thus we see that we need an | |
555 | .B end | |
556 | to match the | |
557 | .B begin | |
558 | at line 16, | |
559 | and to appear before the final | |
560 | .B end . | |
561 | We can make these corrections: | |
562 | .LS | |
563 | :\*b/x9/s//x)\fR | |
564 | y := exp(-x) * sin(i * x); | |
565 | :\*b+s/Round/round\fR | |
566 | n := round(s * y) + h; | |
567 | :\*b/write\fR | |
568 | write(' '); | |
569 | :\*b/\fR | |
570 | writeln('*') | |
571 | :\*binsert\fR | |
572 | \*buntil n = 0;\fR | |
573 | \&\*b.\fR | |
574 | :\*b$\fR | |
575 | end. | |
576 | :\*binsert\fR | |
577 | \*bend\fR | |
578 | \&\*b.\fR | |
579 | : | |
580 | .LE | |
581 | .PP | |
582 | At the end of each | |
583 | .B procedure | |
584 | or | |
585 | .B function | |
586 | and the end of the | |
587 | .B program | |
588 | the translator summarizes references to undefined variables | |
589 | and improper usages of variables. | |
590 | It also gives | |
591 | warnings about potential errors. | |
592 | In our program, the summary errors do not indicate any further problems | |
593 | but the warning that | |
594 | .I c | |
595 | is unused is somewhat suspicious. | |
596 | Examining the program we see that the constant was intended | |
597 | to be used in the expression which is an argument to | |
598 | .I sin , | |
599 | so we can correct this expression, and translate the program. | |
600 | We have now made a correction for each diagnosed error | |
601 | in our program. | |
602 | .LS | |
603 | :\*b?i ?s//c /\fR | |
604 | y := exp(-x) * sin(c * x); | |
605 | :\*bwrite\fR | |
606 | "bigger.p" 26 lines, 538 characters | |
607 | :\*b!pi %\fR | |
608 | !pi bigger.p | |
609 | ! | |
610 | :\*bquit\fR | |
611 | % | |
612 | .LE | |
613 | It should be noted that the translator suppresses warning | |
614 | diagnostics for a particular | |
615 | .B procedure , | |
616 | .B function | |
617 | or the main | |
618 | .B program | |
619 | when it finds severe syntax errors in that part of the source | |
620 | text. | |
621 | This is to prevent possibly confusing and | |
622 | incorrect warning diagnostics from being produced. | |
623 | Thus these warning diagnostics may not appear in a program with | |
624 | bad syntax errors until these errors are corrected. | |
625 | .KS | |
626 | .PP | |
627 | We are now ready to execute our program for the first | |
628 | time. | |
629 | We will do so in the next section after giving a listing | |
630 | of the corrected program for reference purposes. | |
631 | .ne 15 | |
632 | .LS | |
633 | % \*bnumber bigger.p\fR | |
634 | .so bigger6.p | |
635 | % | |
636 | .LE | |
637 | .NH 2 | |
638 | Executing the second example | |
639 | .PP | |
640 | We are now ready to execute the second example. | |
641 | The following output was produced by our first run. | |
642 | .LS | |
643 | % \*bpx\fR | |
644 | .so bigout2 | |
645 | % | |
646 | .LE | |
647 | Here the interpreter is presenting us with a runtime error diagnostic. | |
648 | It detected a `division by zero' at line 17. | |
649 | Examining line 17, we see that we have written | |
650 | the statement `x := d / i' instead of `x := d * i'. | |
651 | We can correct this and rerun the program: | |
652 | .ne 10 | |
653 | .LS | |
654 | % \*bex bigger.p\fR | |
655 | "bigger.p" 26 lines, 538 characters | |
656 | :\*b17\fR | |
657 | x := d / i | |
658 | :\*bs'/'*\fR | |
659 | x := d * i | |
660 | :\*bwrite\fR | |
661 | "bigger.p" 26 lines, 538 characters | |
662 | :\*b!pix %\fR | |
663 | !pix bigger.p | |
664 | .so bigout3 | |
665 | ! | |
666 | :\*bq\fR | |
667 | % | |
668 | .LE | |
669 | .KS | |
670 | .PP | |
671 | This appears to be the output we wanted. | |
672 | We could now save the output in a file if we wished by using the shell | |
673 | to redirect the output: | |
674 | .LS | |
675 | % \*bpx > graph\fR | |
676 | .LE | |
677 | .KE | |
678 | We can use | |
679 | .I cat | |
680 | (1) to see the contents of the file graph. | |
681 | We can also make a listing of the graph on the line printer without | |
682 | putting it into a file, e.g. | |
683 | .LS | |
684 | % \*bpx | lpr\fR | |
685 | .so bigout4 | |
686 | % | |
687 | .LE | |
688 | Note here that the statistics lines came out on our terminal. | |
689 | The statistics line comes out on the diagnostic output (unit 2.) | |
690 | There are two ways to get rid of the statistics line. | |
691 | We can redirect the statistics message to the printer using the | |
692 | syntax `|\|&' to the shell rather than `|', i.e.: | |
693 | .LS | |
694 | % \*bpx |\|& lpr\fR | |
695 | % | |
696 | .LE | |
697 | or we can translate the program with the | |
698 | .B p | |
699 | option disabled on the command line as we did above. | |
700 | This will disable all post-mortem dumping including the statistics line, | |
701 | thus: | |
702 | .LS | |
703 | % \*bpi -p bigger.p\fR | |
704 | % \*bpx | lpr\fR | |
705 | % | |
706 | .LE | |
707 | This option also disables the statement limit which normally guards | |
708 | against infinite looping. | |
709 | You should not use it until your program is debugged. | |
710 | Also if | |
711 | .B p | |
712 | is specified and an error occurs, you will | |
713 | not get run time diagnostic information to help you | |
714 | determine what the problem is. | |
715 | .NH 2 | |
716 | Formatting the program listing | |
717 | .PP | |
718 | It is possible to use special lines within the source text of a program | |
719 | to format the program listing. | |
720 | An empty line (one with no characters on it) corresponds to a | |
721 | `space' macro in an assembler, leaving a completely blank line | |
722 | without a line number. | |
723 | A line containing only a control-l (form-feed) character | |
724 | will cause a page eject in the listing with the corresponding line number | |
725 | suppressed. | |
726 | This corresponds to an `eject' pseudo-instruction. | |
727 | See also section 5.2 for details on the | |
728 | .B n | |
729 | and | |
730 | .B i | |
731 | options of | |
732 | .PI . | |
733 | .NH 2 | |
734 | Execution profiling | |
735 | .PP | |
736 | An execution profile consists of a structured listing of (all or part of) | |
737 | a program with information about the number of times each statement in | |
738 | the program was executed for a particular run of the program. | |
739 | These profiles can be used for several purposes. | |
740 | In a program which was abnormally terminated due to excessive looping | |
741 | or recursion or by a program fault, the counts can facilitate location | |
742 | of the error. | |
743 | Zero counts mark portions of the program which were not executed; | |
744 | during the early debugging stages they should prompt new test data or | |
745 | a re-examination of the program logic. | |
746 | The profile is perhaps most valuable, however, in drawing | |
747 | attention to the (typically small) | |
748 | portions of the program that dominate execution time. | |
749 | This information can be used for source level optimization. | |
750 | .SH | |
751 | An example | |
752 | .PP | |
753 | A prime number is a number which is divisible only by itself and the | |
754 | number one. | |
755 | The program | |
756 | .I primes , | |
757 | written by Niklaus Wirth, | |
758 | determines the first few prime numbers. | |
759 | In translating the program we have specified the | |
760 | .B z | |
761 | option to | |
762 | .IX . | |
763 | This option causes the translator to generate counters and count instructions | |
764 | sufficient in number to determine the number of times each statement in the | |
765 | program was executed.\*(dg | |
766 | .FS | |
767 | \*(dgThe counts | |
768 | are completely accurate only in the absence of runtime errors and nonlocal | |
769 | .B goto | |
770 | statements. | |
771 | This is not generally a problem, however, as in structured programs | |
772 | nonlocal | |
773 | .B goto | |
774 | statements occur infrequently, | |
775 | and counts are incorrect after abnormal termination only when the | |
776 | .I "upward look" | |
777 | described below to get a count passes a suspended call point. | |
778 | .FE | |
779 | When execution of the program completes, either normally or abnormally, | |
780 | this count data is written to the file | |
781 | .I pmon.out | |
782 | in the current directory.\*(dd | |
783 | .FS | |
784 | \*(dd\c | |
785 | .I Pmon.out | |
786 | has a name similar to | |
787 | .I mon.out | |
788 | the monitor file produced by the profiling facility of the C compiler | |
789 | .I cc | |
790 | (1). | |
791 | See | |
792 | .I prof | |
793 | (1) for a discussion of the C compiler profiling facilities. | |
794 | .FE | |
795 | It is then possible to prepare an execution profile by giving | |
796 | .XP | |
797 | the name of the file associated with this data, as was done in the following | |
798 | example. | |
799 | .LS | |
800 | % \*bpix -l -z primes.p\fR | |
801 | .so primeout1 | |
802 | % | |
803 | .LE | |
804 | .SH | |
805 | Discussion | |
806 | .PP | |
807 | The header lines of the outputs of | |
808 | .IX | |
809 | and | |
810 | .XP | |
811 | in this example indicate the version of the translator and execution | |
812 | profiler in use at the time this example was prepared. | |
813 | The time given with the file name (also on the header line) | |
814 | indicates the time of last modification of the program source file. | |
815 | This time serves to | |
816 | .I "version stamp" | |
817 | the input program. | |
818 | .I Pxp | |
819 | also indicates the time at which the profile data was gathered. | |
820 | .LS | |
821 | % \*bpxp -z primes.p\fR | |
822 | .so primeout2 | |
823 | % | |
824 | .LE | |
825 | .KE | |
826 | .PP | |
827 | To determine the number of times a statement was executed, | |
828 | one looks to the left of the statement and finds the corresponding | |
829 | vertical bar `|'. | |
830 | If this vertical bar is labelled with a count then that count gives the | |
831 | number of times the statement was executed. | |
832 | If the bar is not labelled, we look up in the listing to find the first | |
833 | `|' which directly above the original one which has a count and that | |
834 | is the answer. | |
835 | Thus, in our example, | |
836 | .I k | |
837 | was incremented 157 times on line 18, | |
838 | while the | |
839 | .I write | |
840 | procedure call on line 24 was executed 48 times as given by the count | |
841 | on the | |
842 | .B repeat . | |
843 | .PP | |
844 | More information on | |
845 | .I pxp | |
846 | can be found in its manual section | |
847 | .XP | |
848 | (6) | |
849 | and in sections 5.4, 5.5 and 5.10. |