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| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLCALL 1" |
| 132 | .TH PERLCALL 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlcall \- Perl calling conventions from C |
| 135 | .SH "DESCRIPTION" |
| 136 | .IX Header "DESCRIPTION" |
| 137 | The purpose of this document is to show you how to call Perl subroutines |
| 138 | directly from C, i.e., how to write \fIcallbacks\fR. |
| 139 | .PP |
| 140 | Apart from discussing the C interface provided by Perl for writing |
| 141 | callbacks the document uses a series of examples to show how the |
| 142 | interface actually works in practice. In addition some techniques for |
| 143 | coding callbacks are covered. |
| 144 | .PP |
| 145 | Examples where callbacks are necessary include |
| 146 | .IP "\(bu An Error Handler" 5 |
| 147 | .IX Item "An Error Handler" |
| 148 | You have created an \s-1XSUB\s0 interface to an application's C \s-1API\s0. |
| 149 | .Sp |
| 150 | A fairly common feature in applications is to allow you to define a C |
| 151 | function that will be called whenever something nasty occurs. What we |
| 152 | would like is to be able to specify a Perl subroutine that will be |
| 153 | called instead. |
| 154 | .IP "\(bu An Event Driven Program" 5 |
| 155 | .IX Item "An Event Driven Program" |
| 156 | The classic example of where callbacks are used is when writing an |
| 157 | event driven program like for an X windows application. In this case |
| 158 | you register functions to be called whenever specific events occur, |
| 159 | e.g., a mouse button is pressed, the cursor moves into a window or a |
| 160 | menu item is selected. |
| 161 | .PP |
| 162 | Although the techniques described here are applicable when embedding |
| 163 | Perl in a C program, this is not the primary goal of this document. |
| 164 | There are other details that must be considered and are specific to |
| 165 | embedding Perl. For details on embedding Perl in C refer to |
| 166 | perlembed. |
| 167 | .PP |
| 168 | Before you launch yourself head first into the rest of this document, |
| 169 | it would be a good idea to have read the following two documents \- |
| 170 | perlxs and perlguts. |
| 171 | .SH "THE CALL_ FUNCTIONS" |
| 172 | .IX Header "THE CALL_ FUNCTIONS" |
| 173 | Although this stuff is easier to explain using examples, you first need |
| 174 | be aware of a few important definitions. |
| 175 | .PP |
| 176 | Perl has a number of C functions that allow you to call Perl |
| 177 | subroutines. They are |
| 178 | .PP |
| 179 | .Vb 4 |
| 180 | \& I32 call_sv(SV* sv, I32 flags) ; |
| 181 | \& I32 call_pv(char *subname, I32 flags) ; |
| 182 | \& I32 call_method(char *methname, I32 flags) ; |
| 183 | \& I32 call_argv(char *subname, I32 flags, register char **argv) ; |
| 184 | .Ve |
| 185 | .PP |
| 186 | The key function is \fIcall_sv\fR. All the other functions are |
| 187 | fairly simple wrappers which make it easier to call Perl subroutines in |
| 188 | special cases. At the end of the day they will all call \fIcall_sv\fR |
| 189 | to invoke the Perl subroutine. |
| 190 | .PP |
| 191 | All the \fIcall_*\fR functions have a \f(CW\*(C`flags\*(C'\fR parameter which is |
| 192 | used to pass a bit mask of options to Perl. This bit mask operates |
| 193 | identically for each of the functions. The settings available in the |
| 194 | bit mask are discussed in \*(L"\s-1FLAG\s0 \s-1VALUES\s0\*(R". |
| 195 | .PP |
| 196 | Each of the functions will now be discussed in turn. |
| 197 | .IP "call_sv" 5 |
| 198 | .IX Item "call_sv" |
| 199 | \&\fIcall_sv\fR takes two parameters, the first, \f(CW\*(C`sv\*(C'\fR, is an SV*. |
| 200 | This allows you to specify the Perl subroutine to be called either as a |
| 201 | C string (which has first been converted to an \s-1SV\s0) or a reference to a |
| 202 | subroutine. The section, \fIUsing call_sv\fR, shows how you can make |
| 203 | use of \fIcall_sv\fR. |
| 204 | .IP "call_pv" 5 |
| 205 | .IX Item "call_pv" |
| 206 | The function, \fIcall_pv\fR, is similar to \fIcall_sv\fR except it |
| 207 | expects its first parameter to be a C char* which identifies the Perl |
| 208 | subroutine you want to call, e.g., \f(CW\*(C`call_pv("fred", 0)\*(C'\fR. If the |
| 209 | subroutine you want to call is in another package, just include the |
| 210 | package name in the string, e.g., \f(CW"pkg::fred"\fR. |
| 211 | .IP "call_method" 5 |
| 212 | .IX Item "call_method" |
| 213 | The function \fIcall_method\fR is used to call a method from a Perl |
| 214 | class. The parameter \f(CW\*(C`methname\*(C'\fR corresponds to the name of the method |
| 215 | to be called. Note that the class that the method belongs to is passed |
| 216 | on the Perl stack rather than in the parameter list. This class can be |
| 217 | either the name of the class (for a static method) or a reference to an |
| 218 | object (for a virtual method). See perlobj for more information on |
| 219 | static and virtual methods and \*(L"Using call_method\*(R" for an example |
| 220 | of using \fIcall_method\fR. |
| 221 | .IP "call_argv" 5 |
| 222 | .IX Item "call_argv" |
| 223 | \&\fIcall_argv\fR calls the Perl subroutine specified by the C string |
| 224 | stored in the \f(CW\*(C`subname\*(C'\fR parameter. It also takes the usual \f(CW\*(C`flags\*(C'\fR |
| 225 | parameter. The final parameter, \f(CW\*(C`argv\*(C'\fR, consists of a \s-1NULL\s0 terminated |
| 226 | list of C strings to be passed as parameters to the Perl subroutine. |
| 227 | See \fIUsing call_argv\fR. |
| 228 | .PP |
| 229 | All the functions return an integer. This is a count of the number of |
| 230 | items returned by the Perl subroutine. The actual items returned by the |
| 231 | subroutine are stored on the Perl stack. |
| 232 | .PP |
| 233 | As a general rule you should \fIalways\fR check the return value from |
| 234 | these functions. Even if you are expecting only a particular number of |
| 235 | values to be returned from the Perl subroutine, there is nothing to |
| 236 | stop someone from doing something unexpected\*(--don't say you haven't |
| 237 | been warned. |
| 238 | .SH "FLAG VALUES" |
| 239 | .IX Header "FLAG VALUES" |
| 240 | The \f(CW\*(C`flags\*(C'\fR parameter in all the \fIcall_*\fR functions is a bit mask |
| 241 | which can consist of any combination of the symbols defined below, |
| 242 | \&\s-1OR\s0'ed together. |
| 243 | .Sh "G_VOID" |
| 244 | .IX Subsection "G_VOID" |
| 245 | Calls the Perl subroutine in a void context. |
| 246 | .PP |
| 247 | This flag has 2 effects: |
| 248 | .IP "1." 5 |
| 249 | It indicates to the subroutine being called that it is executing in |
| 250 | a void context (if it executes \fIwantarray\fR the result will be the |
| 251 | undefined value). |
| 252 | .IP "2." 5 |
| 253 | It ensures that nothing is actually returned from the subroutine. |
| 254 | .PP |
| 255 | The value returned by the \fIcall_*\fR function indicates how many |
| 256 | items have been returned by the Perl subroutine \- in this case it will |
| 257 | be 0. |
| 258 | .Sh "G_SCALAR" |
| 259 | .IX Subsection "G_SCALAR" |
| 260 | Calls the Perl subroutine in a scalar context. This is the default |
| 261 | context flag setting for all the \fIcall_*\fR functions. |
| 262 | .PP |
| 263 | This flag has 2 effects: |
| 264 | .IP "1." 5 |
| 265 | It indicates to the subroutine being called that it is executing in a |
| 266 | scalar context (if it executes \fIwantarray\fR the result will be false). |
| 267 | .IP "2." 5 |
| 268 | It ensures that only a scalar is actually returned from the subroutine. |
| 269 | The subroutine can, of course, ignore the \fIwantarray\fR and return a |
| 270 | list anyway. If so, then only the last element of the list will be |
| 271 | returned. |
| 272 | .PP |
| 273 | The value returned by the \fIcall_*\fR function indicates how many |
| 274 | items have been returned by the Perl subroutine \- in this case it will |
| 275 | be either 0 or 1. |
| 276 | .PP |
| 277 | If 0, then you have specified the G_DISCARD flag. |
| 278 | .PP |
| 279 | If 1, then the item actually returned by the Perl subroutine will be |
| 280 | stored on the Perl stack \- the section \fIReturning a Scalar\fR shows how |
| 281 | to access this value on the stack. Remember that regardless of how |
| 282 | many items the Perl subroutine returns, only the last one will be |
| 283 | accessible from the stack \- think of the case where only one value is |
| 284 | returned as being a list with only one element. Any other items that |
| 285 | were returned will not exist by the time control returns from the |
| 286 | \&\fIcall_*\fR function. The section \fIReturning a list in a scalar |
| 287 | context\fR shows an example of this behavior. |
| 288 | .Sh "G_ARRAY" |
| 289 | .IX Subsection "G_ARRAY" |
| 290 | Calls the Perl subroutine in a list context. |
| 291 | .PP |
| 292 | As with G_SCALAR, this flag has 2 effects: |
| 293 | .IP "1." 5 |
| 294 | It indicates to the subroutine being called that it is executing in a |
| 295 | list context (if it executes \fIwantarray\fR the result will be true). |
| 296 | .IP "2." 5 |
| 297 | It ensures that all items returned from the subroutine will be |
| 298 | accessible when control returns from the \fIcall_*\fR function. |
| 299 | .PP |
| 300 | The value returned by the \fIcall_*\fR function indicates how many |
| 301 | items have been returned by the Perl subroutine. |
| 302 | .PP |
| 303 | If 0, then you have specified the G_DISCARD flag. |
| 304 | .PP |
| 305 | If not 0, then it will be a count of the number of items returned by |
| 306 | the subroutine. These items will be stored on the Perl stack. The |
| 307 | section \fIReturning a list of values\fR gives an example of using the |
| 308 | G_ARRAY flag and the mechanics of accessing the returned items from the |
| 309 | Perl stack. |
| 310 | .Sh "G_DISCARD" |
| 311 | .IX Subsection "G_DISCARD" |
| 312 | By default, the \fIcall_*\fR functions place the items returned from |
| 313 | by the Perl subroutine on the stack. If you are not interested in |
| 314 | these items, then setting this flag will make Perl get rid of them |
| 315 | automatically for you. Note that it is still possible to indicate a |
| 316 | context to the Perl subroutine by using either G_SCALAR or G_ARRAY. |
| 317 | .PP |
| 318 | If you do not set this flag then it is \fIvery\fR important that you make |
| 319 | sure that any temporaries (i.e., parameters passed to the Perl |
| 320 | subroutine and values returned from the subroutine) are disposed of |
| 321 | yourself. The section \fIReturning a Scalar\fR gives details of how to |
| 322 | dispose of these temporaries explicitly and the section \fIUsing Perl to |
| 323 | dispose of temporaries\fR discusses the specific circumstances where you |
| 324 | can ignore the problem and let Perl deal with it for you. |
| 325 | .Sh "G_NOARGS" |
| 326 | .IX Subsection "G_NOARGS" |
| 327 | Whenever a Perl subroutine is called using one of the \fIcall_*\fR |
| 328 | functions, it is assumed by default that parameters are to be passed to |
| 329 | the subroutine. If you are not passing any parameters to the Perl |
| 330 | subroutine, you can save a bit of time by setting this flag. It has |
| 331 | the effect of not creating the \f(CW@_\fR array for the Perl subroutine. |
| 332 | .PP |
| 333 | Although the functionality provided by this flag may seem |
| 334 | straightforward, it should be used only if there is a good reason to do |
| 335 | so. The reason for being cautious is that even if you have specified |
| 336 | the G_NOARGS flag, it is still possible for the Perl subroutine that |
| 337 | has been called to think that you have passed it parameters. |
| 338 | .PP |
| 339 | In fact, what can happen is that the Perl subroutine you have called |
| 340 | can access the \f(CW@_\fR array from a previous Perl subroutine. This will |
| 341 | occur when the code that is executing the \fIcall_*\fR function has |
| 342 | itself been called from another Perl subroutine. The code below |
| 343 | illustrates this |
| 344 | .PP |
| 345 | .Vb 2 |
| 346 | \& sub fred |
| 347 | \& { print "@_\en" } |
| 348 | .Ve |
| 349 | .PP |
| 350 | .Vb 2 |
| 351 | \& sub joe |
| 352 | \& { &fred } |
| 353 | .Ve |
| 354 | .PP |
| 355 | .Vb 1 |
| 356 | \& &joe(1,2,3) ; |
| 357 | .Ve |
| 358 | .PP |
| 359 | This will print |
| 360 | .PP |
| 361 | .Vb 1 |
| 362 | \& 1 2 3 |
| 363 | .Ve |
| 364 | .PP |
| 365 | What has happened is that \f(CW\*(C`fred\*(C'\fR accesses the \f(CW@_\fR array which |
| 366 | belongs to \f(CW\*(C`joe\*(C'\fR. |
| 367 | .Sh "G_EVAL" |
| 368 | .IX Subsection "G_EVAL" |
| 369 | It is possible for the Perl subroutine you are calling to terminate |
| 370 | abnormally, e.g., by calling \fIdie\fR explicitly or by not actually |
| 371 | existing. By default, when either of these events occurs, the |
| 372 | process will terminate immediately. If you want to trap this |
| 373 | type of event, specify the G_EVAL flag. It will put an \fIeval { }\fR |
| 374 | around the subroutine call. |
| 375 | .PP |
| 376 | Whenever control returns from the \fIcall_*\fR function you need to |
| 377 | check the \f(CW$@\fR variable as you would in a normal Perl script. |
| 378 | .PP |
| 379 | The value returned from the \fIcall_*\fR function is dependent on |
| 380 | what other flags have been specified and whether an error has |
| 381 | occurred. Here are all the different cases that can occur: |
| 382 | .IP "\(bu" 5 |
| 383 | If the \fIcall_*\fR function returns normally, then the value |
| 384 | returned is as specified in the previous sections. |
| 385 | .IP "\(bu" 5 |
| 386 | If G_DISCARD is specified, the return value will always be 0. |
| 387 | .IP "\(bu" 5 |
| 388 | If G_ARRAY is specified \fIand\fR an error has occurred, the return value |
| 389 | will always be 0. |
| 390 | .IP "\(bu" 5 |
| 391 | If G_SCALAR is specified \fIand\fR an error has occurred, the return value |
| 392 | will be 1 and the value on the top of the stack will be \fIundef\fR. This |
| 393 | means that if you have already detected the error by checking \f(CW$@\fR and |
| 394 | you want the program to continue, you must remember to pop the \fIundef\fR |
| 395 | from the stack. |
| 396 | .PP |
| 397 | See \fIUsing G_EVAL\fR for details on using G_EVAL. |
| 398 | .Sh "G_KEEPERR" |
| 399 | .IX Subsection "G_KEEPERR" |
| 400 | You may have noticed that using the G_EVAL flag described above will |
| 401 | \&\fBalways\fR clear the \f(CW$@\fR variable and set it to a string describing |
| 402 | the error iff there was an error in the called code. This unqualified |
| 403 | resetting of \f(CW$@\fR can be problematic in the reliable identification of |
| 404 | errors using the \f(CW\*(C`eval {}\*(C'\fR mechanism, because the possibility exists |
| 405 | that perl will call other code (end of block processing code, for |
| 406 | example) between the time the error causes \f(CW$@\fR to be set within |
| 407 | \&\f(CW\*(C`eval {}\*(C'\fR, and the subsequent statement which checks for the value of |
| 408 | \&\f(CW$@\fR gets executed in the user's script. |
| 409 | .PP |
| 410 | This scenario will mostly be applicable to code that is meant to be |
| 411 | called from within destructors, asynchronous callbacks, signal |
| 412 | handlers, \f(CW\*(C`_\|_DIE_\|_\*(C'\fR or \f(CW\*(C`_\|_WARN_\|_\*(C'\fR hooks, and \f(CW\*(C`tie\*(C'\fR functions. In |
| 413 | such situations, you will not want to clear \f(CW$@\fR at all, but simply to |
| 414 | append any new errors to any existing value of \f(CW$@\fR. |
| 415 | .PP |
| 416 | The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in |
| 417 | \&\fIcall_*\fR functions that are used to implement such code. This flag |
| 418 | has no effect when G_EVAL is not used. |
| 419 | .PP |
| 420 | When G_KEEPERR is used, any errors in the called code will be prefixed |
| 421 | with the string \*(L"\et(in cleanup)\*(R", and appended to the current value |
| 422 | of \f(CW$@\fR. |
| 423 | .PP |
| 424 | The G_KEEPERR flag was introduced in Perl version 5.002. |
| 425 | .PP |
| 426 | See \fIUsing G_KEEPERR\fR for an example of a situation that warrants the |
| 427 | use of this flag. |
| 428 | .Sh "Determining the Context" |
| 429 | .IX Subsection "Determining the Context" |
| 430 | As mentioned above, you can determine the context of the currently |
| 431 | executing subroutine in Perl with \fIwantarray\fR. The equivalent test |
| 432 | can be made in C by using the \f(CW\*(C`GIMME_V\*(C'\fR macro, which returns |
| 433 | \&\f(CW\*(C`G_ARRAY\*(C'\fR if you have been called in a list context, \f(CW\*(C`G_SCALAR\*(C'\fR if |
| 434 | in a scalar context, or \f(CW\*(C`G_VOID\*(C'\fR if in a void context (i.e. the |
| 435 | return value will not be used). An older version of this macro is |
| 436 | called \f(CW\*(C`GIMME\*(C'\fR; in a void context it returns \f(CW\*(C`G_SCALAR\*(C'\fR instead of |
| 437 | \&\f(CW\*(C`G_VOID\*(C'\fR. An example of using the \f(CW\*(C`GIMME_V\*(C'\fR macro is shown in |
| 438 | section \fIUsing \s-1GIMME_V\s0\fR. |
| 439 | .SH "KNOWN PROBLEMS" |
| 440 | .IX Header "KNOWN PROBLEMS" |
| 441 | This section outlines all known problems that exist in the |
| 442 | \&\fIcall_*\fR functions. |
| 443 | .IP "1." 5 |
| 444 | If you are intending to make use of both the G_EVAL and G_SCALAR flags |
| 445 | in your code, use a version of Perl greater than 5.000. There is a bug |
| 446 | in version 5.000 of Perl which means that the combination of these two |
| 447 | flags will not work as described in the section \fI\s-1FLAG\s0 \s-1VALUES\s0\fR. |
| 448 | .Sp |
| 449 | Specifically, if the two flags are used when calling a subroutine and |
| 450 | that subroutine does not call \fIdie\fR, the value returned by |
| 451 | \&\fIcall_*\fR will be wrong. |
| 452 | .IP "2." 5 |
| 453 | In Perl 5.000 and 5.001 there is a problem with using \fIcall_*\fR if |
| 454 | the Perl sub you are calling attempts to trap a \fIdie\fR. |
| 455 | .Sp |
| 456 | The symptom of this problem is that the called Perl sub will continue |
| 457 | to completion, but whenever it attempts to pass control back to the |
| 458 | \&\s-1XSUB\s0, the program will immediately terminate. |
| 459 | .Sp |
| 460 | For example, say you want to call this Perl sub |
| 461 | .Sp |
| 462 | .Vb 6 |
| 463 | \& sub fred |
| 464 | \& { |
| 465 | \& eval { die "Fatal Error" ; } |
| 466 | \& print "Trapped error: $@\en" |
| 467 | \& if $@ ; |
| 468 | \& } |
| 469 | .Ve |
| 470 | .Sp |
| 471 | via this \s-1XSUB\s0 |
| 472 | .Sp |
| 473 | .Vb 6 |
| 474 | \& void |
| 475 | \& Call_fred() |
| 476 | \& CODE: |
| 477 | \& PUSHMARK(SP) ; |
| 478 | \& call_pv("fred", G_DISCARD|G_NOARGS) ; |
| 479 | \& fprintf(stderr, "back in Call_fred\en") ; |
| 480 | .Ve |
| 481 | .Sp |
| 482 | When \f(CW\*(C`Call_fred\*(C'\fR is executed it will print |
| 483 | .Sp |
| 484 | .Vb 1 |
| 485 | \& Trapped error: Fatal Error |
| 486 | .Ve |
| 487 | .Sp |
| 488 | As control never returns to \f(CW\*(C`Call_fred\*(C'\fR, the \f(CW"back in Call_fred"\fR |
| 489 | string will not get printed. |
| 490 | .Sp |
| 491 | To work around this problem, you can either upgrade to Perl 5.002 or |
| 492 | higher, or use the G_EVAL flag with \fIcall_*\fR as shown below |
| 493 | .Sp |
| 494 | .Vb 6 |
| 495 | \& void |
| 496 | \& Call_fred() |
| 497 | \& CODE: |
| 498 | \& PUSHMARK(SP) ; |
| 499 | \& call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ; |
| 500 | \& fprintf(stderr, "back in Call_fred\en") ; |
| 501 | .Ve |
| 502 | .SH "EXAMPLES" |
| 503 | .IX Header "EXAMPLES" |
| 504 | Enough of the definition talk, let's have a few examples. |
| 505 | .PP |
| 506 | Perl provides many macros to assist in accessing the Perl stack. |
| 507 | Wherever possible, these macros should always be used when interfacing |
| 508 | to Perl internals. We hope this should make the code less vulnerable |
| 509 | to any changes made to Perl in the future. |
| 510 | .PP |
| 511 | Another point worth noting is that in the first series of examples I |
| 512 | have made use of only the \fIcall_pv\fR function. This has been done |
| 513 | to keep the code simpler and ease you into the topic. Wherever |
| 514 | possible, if the choice is between using \fIcall_pv\fR and |
| 515 | \&\fIcall_sv\fR, you should always try to use \fIcall_sv\fR. See |
| 516 | \&\fIUsing call_sv\fR for details. |
| 517 | .Sh "No Parameters, Nothing returned" |
| 518 | .IX Subsection "No Parameters, Nothing returned" |
| 519 | This first trivial example will call a Perl subroutine, \fIPrintUID\fR, to |
| 520 | print out the \s-1UID\s0 of the process. |
| 521 | .PP |
| 522 | .Vb 4 |
| 523 | \& sub PrintUID |
| 524 | \& { |
| 525 | \& print "UID is $<\en" ; |
| 526 | \& } |
| 527 | .Ve |
| 528 | .PP |
| 529 | and here is a C function to call it |
| 530 | .PP |
| 531 | .Vb 4 |
| 532 | \& static void |
| 533 | \& call_PrintUID() |
| 534 | \& { |
| 535 | \& dSP ; |
| 536 | .Ve |
| 537 | .PP |
| 538 | .Vb 3 |
| 539 | \& PUSHMARK(SP) ; |
| 540 | \& call_pv("PrintUID", G_DISCARD|G_NOARGS) ; |
| 541 | \& } |
| 542 | .Ve |
| 543 | .PP |
| 544 | Simple, eh. |
| 545 | .PP |
| 546 | A few points to note about this example. |
| 547 | .IP "1." 5 |
| 548 | Ignore \f(CW\*(C`dSP\*(C'\fR and \f(CW\*(C`PUSHMARK(SP)\*(C'\fR for now. They will be discussed in |
| 549 | the next example. |
| 550 | .IP "2." 5 |
| 551 | We aren't passing any parameters to \fIPrintUID\fR so G_NOARGS can be |
| 552 | specified. |
| 553 | .IP "3." 5 |
| 554 | We aren't interested in anything returned from \fIPrintUID\fR, so |
| 555 | G_DISCARD is specified. Even if \fIPrintUID\fR was changed to |
| 556 | return some value(s), having specified G_DISCARD will mean that they |
| 557 | will be wiped by the time control returns from \fIcall_pv\fR. |
| 558 | .IP "4." 5 |
| 559 | As \fIcall_pv\fR is being used, the Perl subroutine is specified as a |
| 560 | C string. In this case the subroutine name has been 'hard\-wired' into the |
| 561 | code. |
| 562 | .IP "5." 5 |
| 563 | Because we specified G_DISCARD, it is not necessary to check the value |
| 564 | returned from \fIcall_pv\fR. It will always be 0. |
| 565 | .Sh "Passing Parameters" |
| 566 | .IX Subsection "Passing Parameters" |
| 567 | Now let's make a slightly more complex example. This time we want to |
| 568 | call a Perl subroutine, \f(CW\*(C`LeftString\*(C'\fR, which will take 2 parameters\*(--a |
| 569 | string ($s) and an integer ($n). The subroutine will simply |
| 570 | print the first \f(CW$n\fR characters of the string. |
| 571 | .PP |
| 572 | So the Perl subroutine would look like this |
| 573 | .PP |
| 574 | .Vb 5 |
| 575 | \& sub LeftString |
| 576 | \& { |
| 577 | \& my($s, $n) = @_ ; |
| 578 | \& print substr($s, 0, $n), "\en" ; |
| 579 | \& } |
| 580 | .Ve |
| 581 | .PP |
| 582 | The C function required to call \fILeftString\fR would look like this. |
| 583 | .PP |
| 584 | .Vb 6 |
| 585 | \& static void |
| 586 | \& call_LeftString(a, b) |
| 587 | \& char * a ; |
| 588 | \& int b ; |
| 589 | \& { |
| 590 | \& dSP ; |
| 591 | .Ve |
| 592 | .PP |
| 593 | .Vb 2 |
| 594 | \& ENTER ; |
| 595 | \& SAVETMPS ; |
| 596 | .Ve |
| 597 | .PP |
| 598 | .Vb 4 |
| 599 | \& PUSHMARK(SP) ; |
| 600 | \& XPUSHs(sv_2mortal(newSVpv(a, 0))); |
| 601 | \& XPUSHs(sv_2mortal(newSViv(b))); |
| 602 | \& PUTBACK ; |
| 603 | .Ve |
| 604 | .PP |
| 605 | .Vb 1 |
| 606 | \& call_pv("LeftString", G_DISCARD); |
| 607 | .Ve |
| 608 | .PP |
| 609 | .Vb 3 |
| 610 | \& FREETMPS ; |
| 611 | \& LEAVE ; |
| 612 | \& } |
| 613 | .Ve |
| 614 | .PP |
| 615 | Here are a few notes on the C function \fIcall_LeftString\fR. |
| 616 | .IP "1." 5 |
| 617 | Parameters are passed to the Perl subroutine using the Perl stack. |
| 618 | This is the purpose of the code beginning with the line \f(CW\*(C`dSP\*(C'\fR and |
| 619 | ending with the line \f(CW\*(C`PUTBACK\*(C'\fR. The \f(CW\*(C`dSP\*(C'\fR declares a local copy |
| 620 | of the stack pointer. This local copy should \fBalways\fR be accessed |
| 621 | as \f(CW\*(C`SP\*(C'\fR. |
| 622 | .IP "2." 5 |
| 623 | If you are going to put something onto the Perl stack, you need to know |
| 624 | where to put it. This is the purpose of the macro \f(CW\*(C`dSP\*(C'\fR\-\-it declares |
| 625 | and initializes a \fIlocal\fR copy of the Perl stack pointer. |
| 626 | .Sp |
| 627 | All the other macros which will be used in this example require you to |
| 628 | have used this macro. |
| 629 | .Sp |
| 630 | The exception to this rule is if you are calling a Perl subroutine |
| 631 | directly from an \s-1XSUB\s0 function. In this case it is not necessary to |
| 632 | use the \f(CW\*(C`dSP\*(C'\fR macro explicitly\*(--it will be declared for you |
| 633 | automatically. |
| 634 | .IP "3." 5 |
| 635 | Any parameters to be pushed onto the stack should be bracketed by the |
| 636 | \&\f(CW\*(C`PUSHMARK\*(C'\fR and \f(CW\*(C`PUTBACK\*(C'\fR macros. The purpose of these two macros, in |
| 637 | this context, is to count the number of parameters you are |
| 638 | pushing automatically. Then whenever Perl is creating the \f(CW@_\fR array for the |
| 639 | subroutine, it knows how big to make it. |
| 640 | .Sp |
| 641 | The \f(CW\*(C`PUSHMARK\*(C'\fR macro tells Perl to make a mental note of the current |
| 642 | stack pointer. Even if you aren't passing any parameters (like the |
| 643 | example shown in the section \fINo Parameters, Nothing returned\fR) you |
| 644 | must still call the \f(CW\*(C`PUSHMARK\*(C'\fR macro before you can call any of the |
| 645 | \&\fIcall_*\fR functions\*(--Perl still needs to know that there are no |
| 646 | parameters. |
| 647 | .Sp |
| 648 | The \f(CW\*(C`PUTBACK\*(C'\fR macro sets the global copy of the stack pointer to be |
| 649 | the same as our local copy. If we didn't do this \fIcall_pv\fR |
| 650 | wouldn't know where the two parameters we pushed were\*(--remember that |
| 651 | up to now all the stack pointer manipulation we have done is with our |
| 652 | local copy, \fInot\fR the global copy. |
| 653 | .IP "4." 5 |
| 654 | Next, we come to XPUSHs. This is where the parameters actually get |
| 655 | pushed onto the stack. In this case we are pushing a string and an |
| 656 | integer. |
| 657 | .Sp |
| 658 | See \*(L"XSUBs and the Argument Stack\*(R" in perlguts for details |
| 659 | on how the \s-1XPUSH\s0 macros work. |
| 660 | .IP "5." 5 |
| 661 | Because we created temporary values (by means of \fIsv_2mortal()\fR calls) |
| 662 | we will have to tidy up the Perl stack and dispose of mortal SVs. |
| 663 | .Sp |
| 664 | This is the purpose of |
| 665 | .Sp |
| 666 | .Vb 2 |
| 667 | \& ENTER ; |
| 668 | \& SAVETMPS ; |
| 669 | .Ve |
| 670 | .Sp |
| 671 | at the start of the function, and |
| 672 | .Sp |
| 673 | .Vb 2 |
| 674 | \& FREETMPS ; |
| 675 | \& LEAVE ; |
| 676 | .Ve |
| 677 | .Sp |
| 678 | at the end. The \f(CW\*(C`ENTER\*(C'\fR/\f(CW\*(C`SAVETMPS\*(C'\fR pair creates a boundary for any |
| 679 | temporaries we create. This means that the temporaries we get rid of |
| 680 | will be limited to those which were created after these calls. |
| 681 | .Sp |
| 682 | The \f(CW\*(C`FREETMPS\*(C'\fR/\f(CW\*(C`LEAVE\*(C'\fR pair will get rid of any values returned by |
| 683 | the Perl subroutine (see next example), plus it will also dump the |
| 684 | mortal SVs we have created. Having \f(CW\*(C`ENTER\*(C'\fR/\f(CW\*(C`SAVETMPS\*(C'\fR at the |
| 685 | beginning of the code makes sure that no other mortals are destroyed. |
| 686 | .Sp |
| 687 | Think of these macros as working a bit like using \f(CW\*(C`{\*(C'\fR and \f(CW\*(C`}\*(C'\fR in Perl |
| 688 | to limit the scope of local variables. |
| 689 | .Sp |
| 690 | See the section \fIUsing Perl to dispose of temporaries\fR for details of |
| 691 | an alternative to using these macros. |
| 692 | .IP "6." 5 |
| 693 | Finally, \fILeftString\fR can now be called via the \fIcall_pv\fR function. |
| 694 | The only flag specified this time is G_DISCARD. Because we are passing |
| 695 | 2 parameters to the Perl subroutine this time, we have not specified |
| 696 | G_NOARGS. |
| 697 | .Sh "Returning a Scalar" |
| 698 | .IX Subsection "Returning a Scalar" |
| 699 | Now for an example of dealing with the items returned from a Perl |
| 700 | subroutine. |
| 701 | .PP |
| 702 | Here is a Perl subroutine, \fIAdder\fR, that takes 2 integer parameters |
| 703 | and simply returns their sum. |
| 704 | .PP |
| 705 | .Vb 5 |
| 706 | \& sub Adder |
| 707 | \& { |
| 708 | \& my($a, $b) = @_ ; |
| 709 | \& $a + $b ; |
| 710 | \& } |
| 711 | .Ve |
| 712 | .PP |
| 713 | Because we are now concerned with the return value from \fIAdder\fR, the C |
| 714 | function required to call it is now a bit more complex. |
| 715 | .PP |
| 716 | .Vb 7 |
| 717 | \& static void |
| 718 | \& call_Adder(a, b) |
| 719 | \& int a ; |
| 720 | \& int b ; |
| 721 | \& { |
| 722 | \& dSP ; |
| 723 | \& int count ; |
| 724 | .Ve |
| 725 | .PP |
| 726 | .Vb 2 |
| 727 | \& ENTER ; |
| 728 | \& SAVETMPS; |
| 729 | .Ve |
| 730 | .PP |
| 731 | .Vb 4 |
| 732 | \& PUSHMARK(SP) ; |
| 733 | \& XPUSHs(sv_2mortal(newSViv(a))); |
| 734 | \& XPUSHs(sv_2mortal(newSViv(b))); |
| 735 | \& PUTBACK ; |
| 736 | .Ve |
| 737 | .PP |
| 738 | .Vb 1 |
| 739 | \& count = call_pv("Adder", G_SCALAR); |
| 740 | .Ve |
| 741 | .PP |
| 742 | .Vb 1 |
| 743 | \& SPAGAIN ; |
| 744 | .Ve |
| 745 | .PP |
| 746 | .Vb 2 |
| 747 | \& if (count != 1) |
| 748 | \& croak("Big trouble\en") ; |
| 749 | .Ve |
| 750 | .PP |
| 751 | .Vb 1 |
| 752 | \& printf ("The sum of %d and %d is %d\en", a, b, POPi) ; |
| 753 | .Ve |
| 754 | .PP |
| 755 | .Vb 4 |
| 756 | \& PUTBACK ; |
| 757 | \& FREETMPS ; |
| 758 | \& LEAVE ; |
| 759 | \& } |
| 760 | .Ve |
| 761 | .PP |
| 762 | Points to note this time are |
| 763 | .IP "1." 5 |
| 764 | The only flag specified this time was G_SCALAR. That means the \f(CW@_\fR |
| 765 | array will be created and that the value returned by \fIAdder\fR will |
| 766 | still exist after the call to \fIcall_pv\fR. |
| 767 | .IP "2." 5 |
| 768 | The purpose of the macro \f(CW\*(C`SPAGAIN\*(C'\fR is to refresh the local copy of the |
| 769 | stack pointer. This is necessary because it is possible that the memory |
| 770 | allocated to the Perl stack has been reallocated whilst in the |
| 771 | \&\fIcall_pv\fR call. |
| 772 | .Sp |
| 773 | If you are making use of the Perl stack pointer in your code you must |
| 774 | always refresh the local copy using \s-1SPAGAIN\s0 whenever you make use |
| 775 | of the \fIcall_*\fR functions or any other Perl internal function. |
| 776 | .IP "3." 5 |
| 777 | Although only a single value was expected to be returned from \fIAdder\fR, |
| 778 | it is still good practice to check the return code from \fIcall_pv\fR |
| 779 | anyway. |
| 780 | .Sp |
| 781 | Expecting a single value is not quite the same as knowing that there |
| 782 | will be one. If someone modified \fIAdder\fR to return a list and we |
| 783 | didn't check for that possibility and take appropriate action the Perl |
| 784 | stack would end up in an inconsistent state. That is something you |
| 785 | \&\fIreally\fR don't want to happen ever. |
| 786 | .IP "4." 5 |
| 787 | The \f(CW\*(C`POPi\*(C'\fR macro is used here to pop the return value from the stack. |
| 788 | In this case we wanted an integer, so \f(CW\*(C`POPi\*(C'\fR was used. |
| 789 | .Sp |
| 790 | Here is the complete list of \s-1POP\s0 macros available, along with the types |
| 791 | they return. |
| 792 | .Sp |
| 793 | .Vb 5 |
| 794 | \& POPs SV |
| 795 | \& POPp pointer |
| 796 | \& POPn double |
| 797 | \& POPi integer |
| 798 | \& POPl long |
| 799 | .Ve |
| 800 | .IP "5." 5 |
| 801 | The final \f(CW\*(C`PUTBACK\*(C'\fR is used to leave the Perl stack in a consistent |
| 802 | state before exiting the function. This is necessary because when we |
| 803 | popped the return value from the stack with \f(CW\*(C`POPi\*(C'\fR it updated only our |
| 804 | local copy of the stack pointer. Remember, \f(CW\*(C`PUTBACK\*(C'\fR sets the global |
| 805 | stack pointer to be the same as our local copy. |
| 806 | .Sh "Returning a list of values" |
| 807 | .IX Subsection "Returning a list of values" |
| 808 | Now, let's extend the previous example to return both the sum of the |
| 809 | parameters and the difference. |
| 810 | .PP |
| 811 | Here is the Perl subroutine |
| 812 | .PP |
| 813 | .Vb 5 |
| 814 | \& sub AddSubtract |
| 815 | \& { |
| 816 | \& my($a, $b) = @_ ; |
| 817 | \& ($a+$b, $a-$b) ; |
| 818 | \& } |
| 819 | .Ve |
| 820 | .PP |
| 821 | and this is the C function |
| 822 | .PP |
| 823 | .Vb 7 |
| 824 | \& static void |
| 825 | \& call_AddSubtract(a, b) |
| 826 | \& int a ; |
| 827 | \& int b ; |
| 828 | \& { |
| 829 | \& dSP ; |
| 830 | \& int count ; |
| 831 | .Ve |
| 832 | .PP |
| 833 | .Vb 2 |
| 834 | \& ENTER ; |
| 835 | \& SAVETMPS; |
| 836 | .Ve |
| 837 | .PP |
| 838 | .Vb 4 |
| 839 | \& PUSHMARK(SP) ; |
| 840 | \& XPUSHs(sv_2mortal(newSViv(a))); |
| 841 | \& XPUSHs(sv_2mortal(newSViv(b))); |
| 842 | \& PUTBACK ; |
| 843 | .Ve |
| 844 | .PP |
| 845 | .Vb 1 |
| 846 | \& count = call_pv("AddSubtract", G_ARRAY); |
| 847 | .Ve |
| 848 | .PP |
| 849 | .Vb 1 |
| 850 | \& SPAGAIN ; |
| 851 | .Ve |
| 852 | .PP |
| 853 | .Vb 2 |
| 854 | \& if (count != 2) |
| 855 | \& croak("Big trouble\en") ; |
| 856 | .Ve |
| 857 | .PP |
| 858 | .Vb 2 |
| 859 | \& printf ("%d - %d = %d\en", a, b, POPi) ; |
| 860 | \& printf ("%d + %d = %d\en", a, b, POPi) ; |
| 861 | .Ve |
| 862 | .PP |
| 863 | .Vb 4 |
| 864 | \& PUTBACK ; |
| 865 | \& FREETMPS ; |
| 866 | \& LEAVE ; |
| 867 | \& } |
| 868 | .Ve |
| 869 | .PP |
| 870 | If \fIcall_AddSubtract\fR is called like this |
| 871 | .PP |
| 872 | .Vb 1 |
| 873 | \& call_AddSubtract(7, 4) ; |
| 874 | .Ve |
| 875 | .PP |
| 876 | then here is the output |
| 877 | .PP |
| 878 | .Vb 2 |
| 879 | \& 7 - 4 = 3 |
| 880 | \& 7 + 4 = 11 |
| 881 | .Ve |
| 882 | .PP |
| 883 | Notes |
| 884 | .IP "1." 5 |
| 885 | We wanted list context, so G_ARRAY was used. |
| 886 | .IP "2." 5 |
| 887 | Not surprisingly \f(CW\*(C`POPi\*(C'\fR is used twice this time because we were |
| 888 | retrieving 2 values from the stack. The important thing to note is that |
| 889 | when using the \f(CW\*(C`POP*\*(C'\fR macros they come off the stack in \fIreverse\fR |
| 890 | order. |
| 891 | .Sh "Returning a list in a scalar context" |
| 892 | .IX Subsection "Returning a list in a scalar context" |
| 893 | Say the Perl subroutine in the previous section was called in a scalar |
| 894 | context, like this |
| 895 | .PP |
| 896 | .Vb 8 |
| 897 | \& static void |
| 898 | \& call_AddSubScalar(a, b) |
| 899 | \& int a ; |
| 900 | \& int b ; |
| 901 | \& { |
| 902 | \& dSP ; |
| 903 | \& int count ; |
| 904 | \& int i ; |
| 905 | .Ve |
| 906 | .PP |
| 907 | .Vb 2 |
| 908 | \& ENTER ; |
| 909 | \& SAVETMPS; |
| 910 | .Ve |
| 911 | .PP |
| 912 | .Vb 4 |
| 913 | \& PUSHMARK(SP) ; |
| 914 | \& XPUSHs(sv_2mortal(newSViv(a))); |
| 915 | \& XPUSHs(sv_2mortal(newSViv(b))); |
| 916 | \& PUTBACK ; |
| 917 | .Ve |
| 918 | .PP |
| 919 | .Vb 1 |
| 920 | \& count = call_pv("AddSubtract", G_SCALAR); |
| 921 | .Ve |
| 922 | .PP |
| 923 | .Vb 1 |
| 924 | \& SPAGAIN ; |
| 925 | .Ve |
| 926 | .PP |
| 927 | .Vb 1 |
| 928 | \& printf ("Items Returned = %d\en", count) ; |
| 929 | .Ve |
| 930 | .PP |
| 931 | .Vb 2 |
| 932 | \& for (i = 1 ; i <= count ; ++i) |
| 933 | \& printf ("Value %d = %d\en", i, POPi) ; |
| 934 | .Ve |
| 935 | .PP |
| 936 | .Vb 4 |
| 937 | \& PUTBACK ; |
| 938 | \& FREETMPS ; |
| 939 | \& LEAVE ; |
| 940 | \& } |
| 941 | .Ve |
| 942 | .PP |
| 943 | The other modification made is that \fIcall_AddSubScalar\fR will print the |
| 944 | number of items returned from the Perl subroutine and their value (for |
| 945 | simplicity it assumes that they are integer). So if |
| 946 | \&\fIcall_AddSubScalar\fR is called |
| 947 | .PP |
| 948 | .Vb 1 |
| 949 | \& call_AddSubScalar(7, 4) ; |
| 950 | .Ve |
| 951 | .PP |
| 952 | then the output will be |
| 953 | .PP |
| 954 | .Vb 2 |
| 955 | \& Items Returned = 1 |
| 956 | \& Value 1 = 3 |
| 957 | .Ve |
| 958 | .PP |
| 959 | In this case the main point to note is that only the last item in the |
| 960 | list is returned from the subroutine, \fIAddSubtract\fR actually made it back to |
| 961 | \&\fIcall_AddSubScalar\fR. |
| 962 | .Sh "Returning Data from Perl via the parameter list" |
| 963 | .IX Subsection "Returning Data from Perl via the parameter list" |
| 964 | It is also possible to return values directly via the parameter list \- |
| 965 | whether it is actually desirable to do it is another matter entirely. |
| 966 | .PP |
| 967 | The Perl subroutine, \fIInc\fR, below takes 2 parameters and increments |
| 968 | each directly. |
| 969 | .PP |
| 970 | .Vb 5 |
| 971 | \& sub Inc |
| 972 | \& { |
| 973 | \& ++ $_[0] ; |
| 974 | \& ++ $_[1] ; |
| 975 | \& } |
| 976 | .Ve |
| 977 | .PP |
| 978 | and here is a C function to call it. |
| 979 | .PP |
| 980 | .Vb 9 |
| 981 | \& static void |
| 982 | \& call_Inc(a, b) |
| 983 | \& int a ; |
| 984 | \& int b ; |
| 985 | \& { |
| 986 | \& dSP ; |
| 987 | \& int count ; |
| 988 | \& SV * sva ; |
| 989 | \& SV * svb ; |
| 990 | .Ve |
| 991 | .PP |
| 992 | .Vb 2 |
| 993 | \& ENTER ; |
| 994 | \& SAVETMPS; |
| 995 | .Ve |
| 996 | .PP |
| 997 | .Vb 2 |
| 998 | \& sva = sv_2mortal(newSViv(a)) ; |
| 999 | \& svb = sv_2mortal(newSViv(b)) ; |
| 1000 | .Ve |
| 1001 | .PP |
| 1002 | .Vb 4 |
| 1003 | \& PUSHMARK(SP) ; |
| 1004 | \& XPUSHs(sva); |
| 1005 | \& XPUSHs(svb); |
| 1006 | \& PUTBACK ; |
| 1007 | .Ve |
| 1008 | .PP |
| 1009 | .Vb 1 |
| 1010 | \& count = call_pv("Inc", G_DISCARD); |
| 1011 | .Ve |
| 1012 | .PP |
| 1013 | .Vb 3 |
| 1014 | \& if (count != 0) |
| 1015 | \& croak ("call_Inc: expected 0 values from 'Inc', got %d\en", |
| 1016 | \& count) ; |
| 1017 | .Ve |
| 1018 | .PP |
| 1019 | .Vb 2 |
| 1020 | \& printf ("%d + 1 = %d\en", a, SvIV(sva)) ; |
| 1021 | \& printf ("%d + 1 = %d\en", b, SvIV(svb)) ; |
| 1022 | .Ve |
| 1023 | .PP |
| 1024 | .Vb 3 |
| 1025 | \& FREETMPS ; |
| 1026 | \& LEAVE ; |
| 1027 | \& } |
| 1028 | .Ve |
| 1029 | .PP |
| 1030 | To be able to access the two parameters that were pushed onto the stack |
| 1031 | after they return from \fIcall_pv\fR it is necessary to make a note |
| 1032 | of their addresses\*(--thus the two variables \f(CW\*(C`sva\*(C'\fR and \f(CW\*(C`svb\*(C'\fR. |
| 1033 | .PP |
| 1034 | The reason this is necessary is that the area of the Perl stack which |
| 1035 | held them will very likely have been overwritten by something else by |
| 1036 | the time control returns from \fIcall_pv\fR. |
| 1037 | .Sh "Using G_EVAL" |
| 1038 | .IX Subsection "Using G_EVAL" |
| 1039 | Now an example using G_EVAL. Below is a Perl subroutine which computes |
| 1040 | the difference of its 2 parameters. If this would result in a negative |
| 1041 | result, the subroutine calls \fIdie\fR. |
| 1042 | .PP |
| 1043 | .Vb 3 |
| 1044 | \& sub Subtract |
| 1045 | \& { |
| 1046 | \& my ($a, $b) = @_ ; |
| 1047 | .Ve |
| 1048 | .PP |
| 1049 | .Vb 1 |
| 1050 | \& die "death can be fatal\en" if $a < $b ; |
| 1051 | .Ve |
| 1052 | .PP |
| 1053 | .Vb 2 |
| 1054 | \& $a - $b ; |
| 1055 | \& } |
| 1056 | .Ve |
| 1057 | .PP |
| 1058 | and some C to call it |
| 1059 | .PP |
| 1060 | .Vb 7 |
| 1061 | \& static void |
| 1062 | \& call_Subtract(a, b) |
| 1063 | \& int a ; |
| 1064 | \& int b ; |
| 1065 | \& { |
| 1066 | \& dSP ; |
| 1067 | \& int count ; |
| 1068 | .Ve |
| 1069 | .PP |
| 1070 | .Vb 2 |
| 1071 | \& ENTER ; |
| 1072 | \& SAVETMPS; |
| 1073 | .Ve |
| 1074 | .PP |
| 1075 | .Vb 4 |
| 1076 | \& PUSHMARK(SP) ; |
| 1077 | \& XPUSHs(sv_2mortal(newSViv(a))); |
| 1078 | \& XPUSHs(sv_2mortal(newSViv(b))); |
| 1079 | \& PUTBACK ; |
| 1080 | .Ve |
| 1081 | .PP |
| 1082 | .Vb 1 |
| 1083 | \& count = call_pv("Subtract", G_EVAL|G_SCALAR); |
| 1084 | .Ve |
| 1085 | .PP |
| 1086 | .Vb 1 |
| 1087 | \& SPAGAIN ; |
| 1088 | .Ve |
| 1089 | .PP |
| 1090 | .Vb 12 |
| 1091 | \& /* Check the eval first */ |
| 1092 | \& if (SvTRUE(ERRSV)) |
| 1093 | \& { |
| 1094 | \& STRLEN n_a; |
| 1095 | \& printf ("Uh oh - %s\en", SvPV(ERRSV, n_a)) ; |
| 1096 | \& POPs ; |
| 1097 | \& } |
| 1098 | \& else |
| 1099 | \& { |
| 1100 | \& if (count != 1) |
| 1101 | \& croak("call_Subtract: wanted 1 value from 'Subtract', got %d\en", |
| 1102 | \& count) ; |
| 1103 | .Ve |
| 1104 | .PP |
| 1105 | .Vb 2 |
| 1106 | \& printf ("%d - %d = %d\en", a, b, POPi) ; |
| 1107 | \& } |
| 1108 | .Ve |
| 1109 | .PP |
| 1110 | .Vb 4 |
| 1111 | \& PUTBACK ; |
| 1112 | \& FREETMPS ; |
| 1113 | \& LEAVE ; |
| 1114 | \& } |
| 1115 | .Ve |
| 1116 | .PP |
| 1117 | If \fIcall_Subtract\fR is called thus |
| 1118 | .PP |
| 1119 | .Vb 1 |
| 1120 | \& call_Subtract(4, 5) |
| 1121 | .Ve |
| 1122 | .PP |
| 1123 | the following will be printed |
| 1124 | .PP |
| 1125 | .Vb 1 |
| 1126 | \& Uh oh - death can be fatal |
| 1127 | .Ve |
| 1128 | .PP |
| 1129 | Notes |
| 1130 | .IP "1." 5 |
| 1131 | We want to be able to catch the \fIdie\fR so we have used the G_EVAL |
| 1132 | flag. Not specifying this flag would mean that the program would |
| 1133 | terminate immediately at the \fIdie\fR statement in the subroutine |
| 1134 | \&\fISubtract\fR. |
| 1135 | .IP "2." 5 |
| 1136 | The code |
| 1137 | .Sp |
| 1138 | .Vb 6 |
| 1139 | \& if (SvTRUE(ERRSV)) |
| 1140 | \& { |
| 1141 | \& STRLEN n_a; |
| 1142 | \& printf ("Uh oh - %s\en", SvPV(ERRSV, n_a)) ; |
| 1143 | \& POPs ; |
| 1144 | \& } |
| 1145 | .Ve |
| 1146 | .Sp |
| 1147 | is the direct equivalent of this bit of Perl |
| 1148 | .Sp |
| 1149 | .Vb 1 |
| 1150 | \& print "Uh oh - $@\en" if $@ ; |
| 1151 | .Ve |
| 1152 | .Sp |
| 1153 | \&\f(CW\*(C`PL_errgv\*(C'\fR is a perl global of type \f(CW\*(C`GV *\*(C'\fR that points to the |
| 1154 | symbol table entry containing the error. \f(CW\*(C`ERRSV\*(C'\fR therefore |
| 1155 | refers to the C equivalent of \f(CW$@\fR. |
| 1156 | .IP "3." 5 |
| 1157 | Note that the stack is popped using \f(CW\*(C`POPs\*(C'\fR in the block where |
| 1158 | \&\f(CW\*(C`SvTRUE(ERRSV)\*(C'\fR is true. This is necessary because whenever a |
| 1159 | \&\fIcall_*\fR function invoked with G_EVAL|G_SCALAR returns an error, |
| 1160 | the top of the stack holds the value \fIundef\fR. Because we want the |
| 1161 | program to continue after detecting this error, it is essential that |
| 1162 | the stack is tidied up by removing the \fIundef\fR. |
| 1163 | .Sh "Using G_KEEPERR" |
| 1164 | .IX Subsection "Using G_KEEPERR" |
| 1165 | Consider this rather facetious example, where we have used an \s-1XS\s0 |
| 1166 | version of the call_Subtract example above inside a destructor: |
| 1167 | .PP |
| 1168 | .Vb 9 |
| 1169 | \& package Foo; |
| 1170 | \& sub new { bless {}, $_[0] } |
| 1171 | \& sub Subtract { |
| 1172 | \& my($a,$b) = @_; |
| 1173 | \& die "death can be fatal" if $a < $b ; |
| 1174 | \& $a - $b; |
| 1175 | \& } |
| 1176 | \& sub DESTROY { call_Subtract(5, 4); } |
| 1177 | \& sub foo { die "foo dies"; } |
| 1178 | .Ve |
| 1179 | .PP |
| 1180 | .Vb 3 |
| 1181 | \& package main; |
| 1182 | \& eval { Foo->new->foo }; |
| 1183 | \& print "Saw: $@" if $@; # should be, but isn't |
| 1184 | .Ve |
| 1185 | .PP |
| 1186 | This example will fail to recognize that an error occurred inside the |
| 1187 | \&\f(CW\*(C`eval {}\*(C'\fR. Here's why: the call_Subtract code got executed while perl |
| 1188 | was cleaning up temporaries when exiting the eval block, and because |
| 1189 | call_Subtract is implemented with \fIcall_pv\fR using the G_EVAL |
| 1190 | flag, it promptly reset \f(CW$@\fR. This results in the failure of the |
| 1191 | outermost test for \f(CW$@\fR, and thereby the failure of the error trap. |
| 1192 | .PP |
| 1193 | Appending the G_KEEPERR flag, so that the \fIcall_pv\fR call in |
| 1194 | call_Subtract reads: |
| 1195 | .PP |
| 1196 | .Vb 1 |
| 1197 | \& count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); |
| 1198 | .Ve |
| 1199 | .PP |
| 1200 | will preserve the error and restore reliable error handling. |
| 1201 | .Sh "Using call_sv" |
| 1202 | .IX Subsection "Using call_sv" |
| 1203 | In all the previous examples I have 'hard\-wired' the name of the Perl |
| 1204 | subroutine to be called from C. Most of the time though, it is more |
| 1205 | convenient to be able to specify the name of the Perl subroutine from |
| 1206 | within the Perl script. |
| 1207 | .PP |
| 1208 | Consider the Perl code below |
| 1209 | .PP |
| 1210 | .Vb 4 |
| 1211 | \& sub fred |
| 1212 | \& { |
| 1213 | \& print "Hello there\en" ; |
| 1214 | \& } |
| 1215 | .Ve |
| 1216 | .PP |
| 1217 | .Vb 1 |
| 1218 | \& CallSubPV("fred") ; |
| 1219 | .Ve |
| 1220 | .PP |
| 1221 | Here is a snippet of \s-1XSUB\s0 which defines \fICallSubPV\fR. |
| 1222 | .PP |
| 1223 | .Vb 6 |
| 1224 | \& void |
| 1225 | \& CallSubPV(name) |
| 1226 | \& char * name |
| 1227 | \& CODE: |
| 1228 | \& PUSHMARK(SP) ; |
| 1229 | \& call_pv(name, G_DISCARD|G_NOARGS) ; |
| 1230 | .Ve |
| 1231 | .PP |
| 1232 | That is fine as far as it goes. The thing is, the Perl subroutine |
| 1233 | can be specified as only a string. For Perl 4 this was adequate, |
| 1234 | but Perl 5 allows references to subroutines and anonymous subroutines. |
| 1235 | This is where \fIcall_sv\fR is useful. |
| 1236 | .PP |
| 1237 | The code below for \fICallSubSV\fR is identical to \fICallSubPV\fR except |
| 1238 | that the \f(CW\*(C`name\*(C'\fR parameter is now defined as an SV* and we use |
| 1239 | \&\fIcall_sv\fR instead of \fIcall_pv\fR. |
| 1240 | .PP |
| 1241 | .Vb 6 |
| 1242 | \& void |
| 1243 | \& CallSubSV(name) |
| 1244 | \& SV * name |
| 1245 | \& CODE: |
| 1246 | \& PUSHMARK(SP) ; |
| 1247 | \& call_sv(name, G_DISCARD|G_NOARGS) ; |
| 1248 | .Ve |
| 1249 | .PP |
| 1250 | Because we are using an \s-1SV\s0 to call \fIfred\fR the following can all be used |
| 1251 | .PP |
| 1252 | .Vb 5 |
| 1253 | \& CallSubSV("fred") ; |
| 1254 | \& CallSubSV(\e&fred) ; |
| 1255 | \& $ref = \e&fred ; |
| 1256 | \& CallSubSV($ref) ; |
| 1257 | \& CallSubSV( sub { print "Hello there\en" } ) ; |
| 1258 | .Ve |
| 1259 | .PP |
| 1260 | As you can see, \fIcall_sv\fR gives you much greater flexibility in |
| 1261 | how you can specify the Perl subroutine. |
| 1262 | .PP |
| 1263 | You should note that if it is necessary to store the \s-1SV\s0 (\f(CW\*(C`name\*(C'\fR in the |
| 1264 | example above) which corresponds to the Perl subroutine so that it can |
| 1265 | be used later in the program, it not enough just to store a copy of the |
| 1266 | pointer to the \s-1SV\s0. Say the code above had been like this |
| 1267 | .PP |
| 1268 | .Vb 1 |
| 1269 | \& static SV * rememberSub ; |
| 1270 | .Ve |
| 1271 | .PP |
| 1272 | .Vb 5 |
| 1273 | \& void |
| 1274 | \& SaveSub1(name) |
| 1275 | \& SV * name |
| 1276 | \& CODE: |
| 1277 | \& rememberSub = name ; |
| 1278 | .Ve |
| 1279 | .PP |
| 1280 | .Vb 5 |
| 1281 | \& void |
| 1282 | \& CallSavedSub1() |
| 1283 | \& CODE: |
| 1284 | \& PUSHMARK(SP) ; |
| 1285 | \& call_sv(rememberSub, G_DISCARD|G_NOARGS) ; |
| 1286 | .Ve |
| 1287 | .PP |
| 1288 | The reason this is wrong is that by the time you come to use the |
| 1289 | pointer \f(CW\*(C`rememberSub\*(C'\fR in \f(CW\*(C`CallSavedSub1\*(C'\fR, it may or may not still refer |
| 1290 | to the Perl subroutine that was recorded in \f(CW\*(C`SaveSub1\*(C'\fR. This is |
| 1291 | particularly true for these cases |
| 1292 | .PP |
| 1293 | .Vb 2 |
| 1294 | \& SaveSub1(\e&fred) ; |
| 1295 | \& CallSavedSub1() ; |
| 1296 | .Ve |
| 1297 | .PP |
| 1298 | .Vb 2 |
| 1299 | \& SaveSub1( sub { print "Hello there\en" } ) ; |
| 1300 | \& CallSavedSub1() ; |
| 1301 | .Ve |
| 1302 | .PP |
| 1303 | By the time each of the \f(CW\*(C`SaveSub1\*(C'\fR statements above have been executed, |
| 1304 | the SV*s which corresponded to the parameters will no longer exist. |
| 1305 | Expect an error message from Perl of the form |
| 1306 | .PP |
| 1307 | .Vb 1 |
| 1308 | \& Can't use an undefined value as a subroutine reference at ... |
| 1309 | .Ve |
| 1310 | .PP |
| 1311 | for each of the \f(CW\*(C`CallSavedSub1\*(C'\fR lines. |
| 1312 | .PP |
| 1313 | Similarly, with this code |
| 1314 | .PP |
| 1315 | .Vb 4 |
| 1316 | \& $ref = \e&fred ; |
| 1317 | \& SaveSub1($ref) ; |
| 1318 | \& $ref = 47 ; |
| 1319 | \& CallSavedSub1() ; |
| 1320 | .Ve |
| 1321 | .PP |
| 1322 | you can expect one of these messages (which you actually get is dependent on |
| 1323 | the version of Perl you are using) |
| 1324 | .PP |
| 1325 | .Vb 2 |
| 1326 | \& Not a CODE reference at ... |
| 1327 | \& Undefined subroutine &main::47 called ... |
| 1328 | .Ve |
| 1329 | .PP |
| 1330 | The variable \f(CW$ref\fR may have referred to the subroutine \f(CW\*(C`fred\*(C'\fR |
| 1331 | whenever the call to \f(CW\*(C`SaveSub1\*(C'\fR was made but by the time |
| 1332 | \&\f(CW\*(C`CallSavedSub1\*(C'\fR gets called it now holds the number \f(CW47\fR. Because we |
| 1333 | saved only a pointer to the original \s-1SV\s0 in \f(CW\*(C`SaveSub1\*(C'\fR, any changes to |
| 1334 | \&\f(CW$ref\fR will be tracked by the pointer \f(CW\*(C`rememberSub\*(C'\fR. This means that |
| 1335 | whenever \f(CW\*(C`CallSavedSub1\*(C'\fR gets called, it will attempt to execute the |
| 1336 | code which is referenced by the SV* \f(CW\*(C`rememberSub\*(C'\fR. In this case |
| 1337 | though, it now refers to the integer \f(CW47\fR, so expect Perl to complain |
| 1338 | loudly. |
| 1339 | .PP |
| 1340 | A similar but more subtle problem is illustrated with this code |
| 1341 | .PP |
| 1342 | .Vb 4 |
| 1343 | \& $ref = \e&fred ; |
| 1344 | \& SaveSub1($ref) ; |
| 1345 | \& $ref = \e&joe ; |
| 1346 | \& CallSavedSub1() ; |
| 1347 | .Ve |
| 1348 | .PP |
| 1349 | This time whenever \f(CW\*(C`CallSavedSub1\*(C'\fR get called it will execute the Perl |
| 1350 | subroutine \f(CW\*(C`joe\*(C'\fR (assuming it exists) rather than \f(CW\*(C`fred\*(C'\fR as was |
| 1351 | originally requested in the call to \f(CW\*(C`SaveSub1\*(C'\fR. |
| 1352 | .PP |
| 1353 | To get around these problems it is necessary to take a full copy of the |
| 1354 | \&\s-1SV\s0. The code below shows \f(CW\*(C`SaveSub2\*(C'\fR modified to do that |
| 1355 | .PP |
| 1356 | .Vb 1 |
| 1357 | \& static SV * keepSub = (SV*)NULL ; |
| 1358 | .Ve |
| 1359 | .PP |
| 1360 | .Vb 11 |
| 1361 | \& void |
| 1362 | \& SaveSub2(name) |
| 1363 | \& SV * name |
| 1364 | \& CODE: |
| 1365 | \& /* Take a copy of the callback */ |
| 1366 | \& if (keepSub == (SV*)NULL) |
| 1367 | \& /* First time, so create a new SV */ |
| 1368 | \& keepSub = newSVsv(name) ; |
| 1369 | \& else |
| 1370 | \& /* Been here before, so overwrite */ |
| 1371 | \& SvSetSV(keepSub, name) ; |
| 1372 | .Ve |
| 1373 | .PP |
| 1374 | .Vb 5 |
| 1375 | \& void |
| 1376 | \& CallSavedSub2() |
| 1377 | \& CODE: |
| 1378 | \& PUSHMARK(SP) ; |
| 1379 | \& call_sv(keepSub, G_DISCARD|G_NOARGS) ; |
| 1380 | .Ve |
| 1381 | .PP |
| 1382 | To avoid creating a new \s-1SV\s0 every time \f(CW\*(C`SaveSub2\*(C'\fR is called, |
| 1383 | the function first checks to see if it has been called before. If not, |
| 1384 | then space for a new \s-1SV\s0 is allocated and the reference to the Perl |
| 1385 | subroutine, \f(CW\*(C`name\*(C'\fR is copied to the variable \f(CW\*(C`keepSub\*(C'\fR in one |
| 1386 | operation using \f(CW\*(C`newSVsv\*(C'\fR. Thereafter, whenever \f(CW\*(C`SaveSub2\*(C'\fR is called |
| 1387 | the existing \s-1SV\s0, \f(CW\*(C`keepSub\*(C'\fR, is overwritten with the new value using |
| 1388 | \&\f(CW\*(C`SvSetSV\*(C'\fR. |
| 1389 | .Sh "Using call_argv" |
| 1390 | .IX Subsection "Using call_argv" |
| 1391 | Here is a Perl subroutine which prints whatever parameters are passed |
| 1392 | to it. |
| 1393 | .PP |
| 1394 | .Vb 3 |
| 1395 | \& sub PrintList |
| 1396 | \& { |
| 1397 | \& my(@list) = @_ ; |
| 1398 | .Ve |
| 1399 | .PP |
| 1400 | .Vb 2 |
| 1401 | \& foreach (@list) { print "$_\en" } |
| 1402 | \& } |
| 1403 | .Ve |
| 1404 | .PP |
| 1405 | and here is an example of \fIcall_argv\fR which will call |
| 1406 | \&\fIPrintList\fR. |
| 1407 | .PP |
| 1408 | .Vb 1 |
| 1409 | \& static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ; |
| 1410 | .Ve |
| 1411 | .PP |
| 1412 | .Vb 4 |
| 1413 | \& static void |
| 1414 | \& call_PrintList() |
| 1415 | \& { |
| 1416 | \& dSP ; |
| 1417 | .Ve |
| 1418 | .PP |
| 1419 | .Vb 2 |
| 1420 | \& call_argv("PrintList", G_DISCARD, words) ; |
| 1421 | \& } |
| 1422 | .Ve |
| 1423 | .PP |
| 1424 | Note that it is not necessary to call \f(CW\*(C`PUSHMARK\*(C'\fR in this instance. |
| 1425 | This is because \fIcall_argv\fR will do it for you. |
| 1426 | .Sh "Using call_method" |
| 1427 | .IX Subsection "Using call_method" |
| 1428 | Consider the following Perl code |
| 1429 | .PP |
| 1430 | .Vb 2 |
| 1431 | \& { |
| 1432 | \& package Mine ; |
| 1433 | .Ve |
| 1434 | .PP |
| 1435 | .Vb 5 |
| 1436 | \& sub new |
| 1437 | \& { |
| 1438 | \& my($type) = shift ; |
| 1439 | \& bless [@_] |
| 1440 | \& } |
| 1441 | .Ve |
| 1442 | .PP |
| 1443 | .Vb 5 |
| 1444 | \& sub Display |
| 1445 | \& { |
| 1446 | \& my ($self, $index) = @_ ; |
| 1447 | \& print "$index: $$self[$index]\en" ; |
| 1448 | \& } |
| 1449 | .Ve |
| 1450 | .PP |
| 1451 | .Vb 6 |
| 1452 | \& sub PrintID |
| 1453 | \& { |
| 1454 | \& my($class) = @_ ; |
| 1455 | \& print "This is Class $class version 1.0\en" ; |
| 1456 | \& } |
| 1457 | \& } |
| 1458 | .Ve |
| 1459 | .PP |
| 1460 | It implements just a very simple class to manage an array. Apart from |
| 1461 | the constructor, \f(CW\*(C`new\*(C'\fR, it declares methods, one static and one |
| 1462 | virtual. The static method, \f(CW\*(C`PrintID\*(C'\fR, prints out simply the class |
| 1463 | name and a version number. The virtual method, \f(CW\*(C`Display\*(C'\fR, prints out a |
| 1464 | single element of the array. Here is an all Perl example of using it. |
| 1465 | .PP |
| 1466 | .Vb 3 |
| 1467 | \& $a = new Mine ('red', 'green', 'blue') ; |
| 1468 | \& $a->Display(1) ; |
| 1469 | \& PrintID Mine; |
| 1470 | .Ve |
| 1471 | .PP |
| 1472 | will print |
| 1473 | .PP |
| 1474 | .Vb 2 |
| 1475 | \& 1: green |
| 1476 | \& This is Class Mine version 1.0 |
| 1477 | .Ve |
| 1478 | .PP |
| 1479 | Calling a Perl method from C is fairly straightforward. The following |
| 1480 | things are required |
| 1481 | .IP "\(bu" 5 |
| 1482 | a reference to the object for a virtual method or the name of the class |
| 1483 | for a static method. |
| 1484 | .IP "\(bu" 5 |
| 1485 | the name of the method. |
| 1486 | .IP "\(bu" 5 |
| 1487 | any other parameters specific to the method. |
| 1488 | .PP |
| 1489 | Here is a simple \s-1XSUB\s0 which illustrates the mechanics of calling both |
| 1490 | the \f(CW\*(C`PrintID\*(C'\fR and \f(CW\*(C`Display\*(C'\fR methods from C. |
| 1491 | .PP |
| 1492 | .Vb 10 |
| 1493 | \& void |
| 1494 | \& call_Method(ref, method, index) |
| 1495 | \& SV * ref |
| 1496 | \& char * method |
| 1497 | \& int index |
| 1498 | \& CODE: |
| 1499 | \& PUSHMARK(SP); |
| 1500 | \& XPUSHs(ref); |
| 1501 | \& XPUSHs(sv_2mortal(newSViv(index))) ; |
| 1502 | \& PUTBACK; |
| 1503 | .Ve |
| 1504 | .PP |
| 1505 | .Vb 1 |
| 1506 | \& call_method(method, G_DISCARD) ; |
| 1507 | .Ve |
| 1508 | .PP |
| 1509 | .Vb 8 |
| 1510 | \& void |
| 1511 | \& call_PrintID(class, method) |
| 1512 | \& char * class |
| 1513 | \& char * method |
| 1514 | \& CODE: |
| 1515 | \& PUSHMARK(SP); |
| 1516 | \& XPUSHs(sv_2mortal(newSVpv(class, 0))) ; |
| 1517 | \& PUTBACK; |
| 1518 | .Ve |
| 1519 | .PP |
| 1520 | .Vb 1 |
| 1521 | \& call_method(method, G_DISCARD) ; |
| 1522 | .Ve |
| 1523 | .PP |
| 1524 | So the methods \f(CW\*(C`PrintID\*(C'\fR and \f(CW\*(C`Display\*(C'\fR can be invoked like this |
| 1525 | .PP |
| 1526 | .Vb 3 |
| 1527 | \& $a = new Mine ('red', 'green', 'blue') ; |
| 1528 | \& call_Method($a, 'Display', 1) ; |
| 1529 | \& call_PrintID('Mine', 'PrintID') ; |
| 1530 | .Ve |
| 1531 | .PP |
| 1532 | The only thing to note is that in both the static and virtual methods, |
| 1533 | the method name is not passed via the stack\*(--it is used as the first |
| 1534 | parameter to \fIcall_method\fR. |
| 1535 | .Sh "Using \s-1GIMME_V\s0" |
| 1536 | .IX Subsection "Using GIMME_V" |
| 1537 | Here is a trivial \s-1XSUB\s0 which prints the context in which it is |
| 1538 | currently executing. |
| 1539 | .PP |
| 1540 | .Vb 10 |
| 1541 | \& void |
| 1542 | \& PrintContext() |
| 1543 | \& CODE: |
| 1544 | \& I32 gimme = GIMME_V; |
| 1545 | \& if (gimme == G_VOID) |
| 1546 | \& printf ("Context is Void\en") ; |
| 1547 | \& else if (gimme == G_SCALAR) |
| 1548 | \& printf ("Context is Scalar\en") ; |
| 1549 | \& else |
| 1550 | \& printf ("Context is Array\en") ; |
| 1551 | .Ve |
| 1552 | .PP |
| 1553 | and here is some Perl to test it |
| 1554 | .PP |
| 1555 | .Vb 3 |
| 1556 | \& PrintContext ; |
| 1557 | \& $a = PrintContext ; |
| 1558 | \& @a = PrintContext ; |
| 1559 | .Ve |
| 1560 | .PP |
| 1561 | The output from that will be |
| 1562 | .PP |
| 1563 | .Vb 3 |
| 1564 | \& Context is Void |
| 1565 | \& Context is Scalar |
| 1566 | \& Context is Array |
| 1567 | .Ve |
| 1568 | .Sh "Using Perl to dispose of temporaries" |
| 1569 | .IX Subsection "Using Perl to dispose of temporaries" |
| 1570 | In the examples given to date, any temporaries created in the callback |
| 1571 | (i.e., parameters passed on the stack to the \fIcall_*\fR function or |
| 1572 | values returned via the stack) have been freed by one of these methods |
| 1573 | .IP "\(bu" 5 |
| 1574 | specifying the G_DISCARD flag with \fIcall_*\fR. |
| 1575 | .IP "\(bu" 5 |
| 1576 | explicitly disposed of using the \f(CW\*(C`ENTER\*(C'\fR/\f(CW\*(C`SAVETMPS\*(C'\fR \- |
| 1577 | \&\f(CW\*(C`FREETMPS\*(C'\fR/\f(CW\*(C`LEAVE\*(C'\fR pairing. |
| 1578 | .PP |
| 1579 | There is another method which can be used, namely letting Perl do it |
| 1580 | for you automatically whenever it regains control after the callback |
| 1581 | has terminated. This is done by simply not using the |
| 1582 | .PP |
| 1583 | .Vb 5 |
| 1584 | \& ENTER ; |
| 1585 | \& SAVETMPS ; |
| 1586 | \& ... |
| 1587 | \& FREETMPS ; |
| 1588 | \& LEAVE ; |
| 1589 | .Ve |
| 1590 | .PP |
| 1591 | sequence in the callback (and not, of course, specifying the G_DISCARD |
| 1592 | flag). |
| 1593 | .PP |
| 1594 | If you are going to use this method you have to be aware of a possible |
| 1595 | memory leak which can arise under very specific circumstances. To |
| 1596 | explain these circumstances you need to know a bit about the flow of |
| 1597 | control between Perl and the callback routine. |
| 1598 | .PP |
| 1599 | The examples given at the start of the document (an error handler and |
| 1600 | an event driven program) are typical of the two main sorts of flow |
| 1601 | control that you are likely to encounter with callbacks. There is a |
| 1602 | very important distinction between them, so pay attention. |
| 1603 | .PP |
| 1604 | In the first example, an error handler, the flow of control could be as |
| 1605 | follows. You have created an interface to an external library. |
| 1606 | Control can reach the external library like this |
| 1607 | .PP |
| 1608 | .Vb 1 |
| 1609 | \& perl --> XSUB --> external library |
| 1610 | .Ve |
| 1611 | .PP |
| 1612 | Whilst control is in the library, an error condition occurs. You have |
| 1613 | previously set up a Perl callback to handle this situation, so it will |
| 1614 | get executed. Once the callback has finished, control will drop back to |
| 1615 | Perl again. Here is what the flow of control will be like in that |
| 1616 | situation |
| 1617 | .PP |
| 1618 | .Vb 7 |
| 1619 | \& perl --> XSUB --> external library |
| 1620 | \& ... |
| 1621 | \& error occurs |
| 1622 | \& ... |
| 1623 | \& external library --> call_* --> perl |
| 1624 | \& | |
| 1625 | \& perl <-- XSUB <-- external library <-- call_* <----+ |
| 1626 | .Ve |
| 1627 | .PP |
| 1628 | After processing of the error using \fIcall_*\fR is completed, |
| 1629 | control reverts back to Perl more or less immediately. |
| 1630 | .PP |
| 1631 | In the diagram, the further right you go the more deeply nested the |
| 1632 | scope is. It is only when control is back with perl on the extreme |
| 1633 | left of the diagram that you will have dropped back to the enclosing |
| 1634 | scope and any temporaries you have left hanging around will be freed. |
| 1635 | .PP |
| 1636 | In the second example, an event driven program, the flow of control |
| 1637 | will be more like this |
| 1638 | .PP |
| 1639 | .Vb 13 |
| 1640 | \& perl --> XSUB --> event handler |
| 1641 | \& ... |
| 1642 | \& event handler --> call_* --> perl |
| 1643 | \& | |
| 1644 | \& event handler <-- call_* <----+ |
| 1645 | \& ... |
| 1646 | \& event handler --> call_* --> perl |
| 1647 | \& | |
| 1648 | \& event handler <-- call_* <----+ |
| 1649 | \& ... |
| 1650 | \& event handler --> call_* --> perl |
| 1651 | \& | |
| 1652 | \& event handler <-- call_* <----+ |
| 1653 | .Ve |
| 1654 | .PP |
| 1655 | In this case the flow of control can consist of only the repeated |
| 1656 | sequence |
| 1657 | .PP |
| 1658 | .Vb 1 |
| 1659 | \& event handler --> call_* --> perl |
| 1660 | .Ve |
| 1661 | .PP |
| 1662 | for practically the complete duration of the program. This means that |
| 1663 | control may \fInever\fR drop back to the surrounding scope in Perl at the |
| 1664 | extreme left. |
| 1665 | .PP |
| 1666 | So what is the big problem? Well, if you are expecting Perl to tidy up |
| 1667 | those temporaries for you, you might be in for a long wait. For Perl |
| 1668 | to dispose of your temporaries, control must drop back to the |
| 1669 | enclosing scope at some stage. In the event driven scenario that may |
| 1670 | never happen. This means that as time goes on, your program will |
| 1671 | create more and more temporaries, none of which will ever be freed. As |
| 1672 | each of these temporaries consumes some memory your program will |
| 1673 | eventually consume all the available memory in your system\*(--kapow! |
| 1674 | .PP |
| 1675 | So here is the bottom line\*(--if you are sure that control will revert |
| 1676 | back to the enclosing Perl scope fairly quickly after the end of your |
| 1677 | callback, then it isn't absolutely necessary to dispose explicitly of |
| 1678 | any temporaries you may have created. Mind you, if you are at all |
| 1679 | uncertain about what to do, it doesn't do any harm to tidy up anyway. |
| 1680 | .Sh "Strategies for storing Callback Context Information" |
| 1681 | .IX Subsection "Strategies for storing Callback Context Information" |
| 1682 | Potentially one of the trickiest problems to overcome when designing a |
| 1683 | callback interface can be figuring out how to store the mapping between |
| 1684 | the C callback function and the Perl equivalent. |
| 1685 | .PP |
| 1686 | To help understand why this can be a real problem first consider how a |
| 1687 | callback is set up in an all C environment. Typically a C \s-1API\s0 will |
| 1688 | provide a function to register a callback. This will expect a pointer |
| 1689 | to a function as one of its parameters. Below is a call to a |
| 1690 | hypothetical function \f(CW\*(C`register_fatal\*(C'\fR which registers the C function |
| 1691 | to get called when a fatal error occurs. |
| 1692 | .PP |
| 1693 | .Vb 1 |
| 1694 | \& register_fatal(cb1) ; |
| 1695 | .Ve |
| 1696 | .PP |
| 1697 | The single parameter \f(CW\*(C`cb1\*(C'\fR is a pointer to a function, so you must |
| 1698 | have defined \f(CW\*(C`cb1\*(C'\fR in your code, say something like this |
| 1699 | .PP |
| 1700 | .Vb 6 |
| 1701 | \& static void |
| 1702 | \& cb1() |
| 1703 | \& { |
| 1704 | \& printf ("Fatal Error\en") ; |
| 1705 | \& exit(1) ; |
| 1706 | \& } |
| 1707 | .Ve |
| 1708 | .PP |
| 1709 | Now change that to call a Perl subroutine instead |
| 1710 | .PP |
| 1711 | .Vb 1 |
| 1712 | \& static SV * callback = (SV*)NULL; |
| 1713 | .Ve |
| 1714 | .PP |
| 1715 | .Vb 4 |
| 1716 | \& static void |
| 1717 | \& cb1() |
| 1718 | \& { |
| 1719 | \& dSP ; |
| 1720 | .Ve |
| 1721 | .PP |
| 1722 | .Vb 1 |
| 1723 | \& PUSHMARK(SP) ; |
| 1724 | .Ve |
| 1725 | .PP |
| 1726 | .Vb 3 |
| 1727 | \& /* Call the Perl sub to process the callback */ |
| 1728 | \& call_sv(callback, G_DISCARD) ; |
| 1729 | \& } |
| 1730 | .Ve |
| 1731 | .PP |
| 1732 | .Vb 9 |
| 1733 | \& void |
| 1734 | \& register_fatal(fn) |
| 1735 | \& SV * fn |
| 1736 | \& CODE: |
| 1737 | \& /* Remember the Perl sub */ |
| 1738 | \& if (callback == (SV*)NULL) |
| 1739 | \& callback = newSVsv(fn) ; |
| 1740 | \& else |
| 1741 | \& SvSetSV(callback, fn) ; |
| 1742 | .Ve |
| 1743 | .PP |
| 1744 | .Vb 2 |
| 1745 | \& /* register the callback with the external library */ |
| 1746 | \& register_fatal(cb1) ; |
| 1747 | .Ve |
| 1748 | .PP |
| 1749 | where the Perl equivalent of \f(CW\*(C`register_fatal\*(C'\fR and the callback it |
| 1750 | registers, \f(CW\*(C`pcb1\*(C'\fR, might look like this |
| 1751 | .PP |
| 1752 | .Vb 2 |
| 1753 | \& # Register the sub pcb1 |
| 1754 | \& register_fatal(\e&pcb1) ; |
| 1755 | .Ve |
| 1756 | .PP |
| 1757 | .Vb 4 |
| 1758 | \& sub pcb1 |
| 1759 | \& { |
| 1760 | \& die "I'm dying...\en" ; |
| 1761 | \& } |
| 1762 | .Ve |
| 1763 | .PP |
| 1764 | The mapping between the C callback and the Perl equivalent is stored in |
| 1765 | the global variable \f(CW\*(C`callback\*(C'\fR. |
| 1766 | .PP |
| 1767 | This will be adequate if you ever need to have only one callback |
| 1768 | registered at any time. An example could be an error handler like the |
| 1769 | code sketched out above. Remember though, repeated calls to |
| 1770 | \&\f(CW\*(C`register_fatal\*(C'\fR will replace the previously registered callback |
| 1771 | function with the new one. |
| 1772 | .PP |
| 1773 | Say for example you want to interface to a library which allows asynchronous |
| 1774 | file i/o. In this case you may be able to register a callback whenever |
| 1775 | a read operation has completed. To be of any use we want to be able to |
| 1776 | call separate Perl subroutines for each file that is opened. As it |
| 1777 | stands, the error handler example above would not be adequate as it |
| 1778 | allows only a single callback to be defined at any time. What we |
| 1779 | require is a means of storing the mapping between the opened file and |
| 1780 | the Perl subroutine we want to be called for that file. |
| 1781 | .PP |
| 1782 | Say the i/o library has a function \f(CW\*(C`asynch_read\*(C'\fR which associates a C |
| 1783 | function \f(CW\*(C`ProcessRead\*(C'\fR with a file handle \f(CW\*(C`fh\*(C'\fR\-\-this assumes that it |
| 1784 | has also provided some routine to open the file and so obtain the file |
| 1785 | handle. |
| 1786 | .PP |
| 1787 | .Vb 1 |
| 1788 | \& asynch_read(fh, ProcessRead) |
| 1789 | .Ve |
| 1790 | .PP |
| 1791 | This may expect the C \fIProcessRead\fR function of this form |
| 1792 | .PP |
| 1793 | .Vb 7 |
| 1794 | \& void |
| 1795 | \& ProcessRead(fh, buffer) |
| 1796 | \& int fh ; |
| 1797 | \& char * buffer ; |
| 1798 | \& { |
| 1799 | \& ... |
| 1800 | \& } |
| 1801 | .Ve |
| 1802 | .PP |
| 1803 | To provide a Perl interface to this library we need to be able to map |
| 1804 | between the \f(CW\*(C`fh\*(C'\fR parameter and the Perl subroutine we want called. A |
| 1805 | hash is a convenient mechanism for storing this mapping. The code |
| 1806 | below shows a possible implementation |
| 1807 | .PP |
| 1808 | .Vb 1 |
| 1809 | \& static HV * Mapping = (HV*)NULL ; |
| 1810 | .Ve |
| 1811 | .PP |
| 1812 | .Vb 8 |
| 1813 | \& void |
| 1814 | \& asynch_read(fh, callback) |
| 1815 | \& int fh |
| 1816 | \& SV * callback |
| 1817 | \& CODE: |
| 1818 | \& /* If the hash doesn't already exist, create it */ |
| 1819 | \& if (Mapping == (HV*)NULL) |
| 1820 | \& Mapping = newHV() ; |
| 1821 | .Ve |
| 1822 | .PP |
| 1823 | .Vb 2 |
| 1824 | \& /* Save the fh -> callback mapping */ |
| 1825 | \& hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ; |
| 1826 | .Ve |
| 1827 | .PP |
| 1828 | .Vb 2 |
| 1829 | \& /* Register with the C Library */ |
| 1830 | \& asynch_read(fh, asynch_read_if) ; |
| 1831 | .Ve |
| 1832 | .PP |
| 1833 | and \f(CW\*(C`asynch_read_if\*(C'\fR could look like this |
| 1834 | .PP |
| 1835 | .Vb 7 |
| 1836 | \& static void |
| 1837 | \& asynch_read_if(fh, buffer) |
| 1838 | \& int fh ; |
| 1839 | \& char * buffer ; |
| 1840 | \& { |
| 1841 | \& dSP ; |
| 1842 | \& SV ** sv ; |
| 1843 | .Ve |
| 1844 | .PP |
| 1845 | .Vb 4 |
| 1846 | \& /* Get the callback associated with fh */ |
| 1847 | \& sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ; |
| 1848 | \& if (sv == (SV**)NULL) |
| 1849 | \& croak("Internal error...\en") ; |
| 1850 | .Ve |
| 1851 | .PP |
| 1852 | .Vb 4 |
| 1853 | \& PUSHMARK(SP) ; |
| 1854 | \& XPUSHs(sv_2mortal(newSViv(fh))) ; |
| 1855 | \& XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; |
| 1856 | \& PUTBACK ; |
| 1857 | .Ve |
| 1858 | .PP |
| 1859 | .Vb 3 |
| 1860 | \& /* Call the Perl sub */ |
| 1861 | \& call_sv(*sv, G_DISCARD) ; |
| 1862 | \& } |
| 1863 | .Ve |
| 1864 | .PP |
| 1865 | For completeness, here is \f(CW\*(C`asynch_close\*(C'\fR. This shows how to remove |
| 1866 | the entry from the hash \f(CW\*(C`Mapping\*(C'\fR. |
| 1867 | .PP |
| 1868 | .Vb 6 |
| 1869 | \& void |
| 1870 | \& asynch_close(fh) |
| 1871 | \& int fh |
| 1872 | \& CODE: |
| 1873 | \& /* Remove the entry from the hash */ |
| 1874 | \& (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ; |
| 1875 | .Ve |
| 1876 | .PP |
| 1877 | .Vb 2 |
| 1878 | \& /* Now call the real asynch_close */ |
| 1879 | \& asynch_close(fh) ; |
| 1880 | .Ve |
| 1881 | .PP |
| 1882 | So the Perl interface would look like this |
| 1883 | .PP |
| 1884 | .Vb 4 |
| 1885 | \& sub callback1 |
| 1886 | \& { |
| 1887 | \& my($handle, $buffer) = @_ ; |
| 1888 | \& } |
| 1889 | .Ve |
| 1890 | .PP |
| 1891 | .Vb 2 |
| 1892 | \& # Register the Perl callback |
| 1893 | \& asynch_read($fh, \e&callback1) ; |
| 1894 | .Ve |
| 1895 | .PP |
| 1896 | .Vb 1 |
| 1897 | \& asynch_close($fh) ; |
| 1898 | .Ve |
| 1899 | .PP |
| 1900 | The mapping between the C callback and Perl is stored in the global |
| 1901 | hash \f(CW\*(C`Mapping\*(C'\fR this time. Using a hash has the distinct advantage that |
| 1902 | it allows an unlimited number of callbacks to be registered. |
| 1903 | .PP |
| 1904 | What if the interface provided by the C callback doesn't contain a |
| 1905 | parameter which allows the file handle to Perl subroutine mapping? Say |
| 1906 | in the asynchronous i/o package, the callback function gets passed only |
| 1907 | the \f(CW\*(C`buffer\*(C'\fR parameter like this |
| 1908 | .PP |
| 1909 | .Vb 6 |
| 1910 | \& void |
| 1911 | \& ProcessRead(buffer) |
| 1912 | \& char * buffer ; |
| 1913 | \& { |
| 1914 | \& ... |
| 1915 | \& } |
| 1916 | .Ve |
| 1917 | .PP |
| 1918 | Without the file handle there is no straightforward way to map from the |
| 1919 | C callback to the Perl subroutine. |
| 1920 | .PP |
| 1921 | In this case a possible way around this problem is to predefine a |
| 1922 | series of C functions to act as the interface to Perl, thus |
| 1923 | .PP |
| 1924 | .Vb 3 |
| 1925 | \& #define MAX_CB 3 |
| 1926 | \& #define NULL_HANDLE -1 |
| 1927 | \& typedef void (*FnMap)() ; |
| 1928 | .Ve |
| 1929 | .PP |
| 1930 | .Vb 5 |
| 1931 | \& struct MapStruct { |
| 1932 | \& FnMap Function ; |
| 1933 | \& SV * PerlSub ; |
| 1934 | \& int Handle ; |
| 1935 | \& } ; |
| 1936 | .Ve |
| 1937 | .PP |
| 1938 | .Vb 3 |
| 1939 | \& static void fn1() ; |
| 1940 | \& static void fn2() ; |
| 1941 | \& static void fn3() ; |
| 1942 | .Ve |
| 1943 | .PP |
| 1944 | .Vb 6 |
| 1945 | \& static struct MapStruct Map [MAX_CB] = |
| 1946 | \& { |
| 1947 | \& { fn1, NULL, NULL_HANDLE }, |
| 1948 | \& { fn2, NULL, NULL_HANDLE }, |
| 1949 | \& { fn3, NULL, NULL_HANDLE } |
| 1950 | \& } ; |
| 1951 | .Ve |
| 1952 | .PP |
| 1953 | .Vb 6 |
| 1954 | \& static void |
| 1955 | \& Pcb(index, buffer) |
| 1956 | \& int index ; |
| 1957 | \& char * buffer ; |
| 1958 | \& { |
| 1959 | \& dSP ; |
| 1960 | .Ve |
| 1961 | .PP |
| 1962 | .Vb 3 |
| 1963 | \& PUSHMARK(SP) ; |
| 1964 | \& XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ; |
| 1965 | \& PUTBACK ; |
| 1966 | .Ve |
| 1967 | .PP |
| 1968 | .Vb 3 |
| 1969 | \& /* Call the Perl sub */ |
| 1970 | \& call_sv(Map[index].PerlSub, G_DISCARD) ; |
| 1971 | \& } |
| 1972 | .Ve |
| 1973 | .PP |
| 1974 | .Vb 6 |
| 1975 | \& static void |
| 1976 | \& fn1(buffer) |
| 1977 | \& char * buffer ; |
| 1978 | \& { |
| 1979 | \& Pcb(0, buffer) ; |
| 1980 | \& } |
| 1981 | .Ve |
| 1982 | .PP |
| 1983 | .Vb 6 |
| 1984 | \& static void |
| 1985 | \& fn2(buffer) |
| 1986 | \& char * buffer ; |
| 1987 | \& { |
| 1988 | \& Pcb(1, buffer) ; |
| 1989 | \& } |
| 1990 | .Ve |
| 1991 | .PP |
| 1992 | .Vb 6 |
| 1993 | \& static void |
| 1994 | \& fn3(buffer) |
| 1995 | \& char * buffer ; |
| 1996 | \& { |
| 1997 | \& Pcb(2, buffer) ; |
| 1998 | \& } |
| 1999 | .Ve |
| 2000 | .PP |
| 2001 | .Vb 7 |
| 2002 | \& void |
| 2003 | \& array_asynch_read(fh, callback) |
| 2004 | \& int fh |
| 2005 | \& SV * callback |
| 2006 | \& CODE: |
| 2007 | \& int index ; |
| 2008 | \& int null_index = MAX_CB ; |
| 2009 | .Ve |
| 2010 | .PP |
| 2011 | .Vb 5 |
| 2012 | \& /* Find the same handle or an empty entry */ |
| 2013 | \& for (index = 0 ; index < MAX_CB ; ++index) |
| 2014 | \& { |
| 2015 | \& if (Map[index].Handle == fh) |
| 2016 | \& break ; |
| 2017 | .Ve |
| 2018 | .PP |
| 2019 | .Vb 3 |
| 2020 | \& if (Map[index].Handle == NULL_HANDLE) |
| 2021 | \& null_index = index ; |
| 2022 | \& } |
| 2023 | .Ve |
| 2024 | .PP |
| 2025 | .Vb 2 |
| 2026 | \& if (index == MAX_CB && null_index == MAX_CB) |
| 2027 | \& croak ("Too many callback functions registered\en") ; |
| 2028 | .Ve |
| 2029 | .PP |
| 2030 | .Vb 2 |
| 2031 | \& if (index == MAX_CB) |
| 2032 | \& index = null_index ; |
| 2033 | .Ve |
| 2034 | .PP |
| 2035 | .Vb 2 |
| 2036 | \& /* Save the file handle */ |
| 2037 | \& Map[index].Handle = fh ; |
| 2038 | .Ve |
| 2039 | .PP |
| 2040 | .Vb 5 |
| 2041 | \& /* Remember the Perl sub */ |
| 2042 | \& if (Map[index].PerlSub == (SV*)NULL) |
| 2043 | \& Map[index].PerlSub = newSVsv(callback) ; |
| 2044 | \& else |
| 2045 | \& SvSetSV(Map[index].PerlSub, callback) ; |
| 2046 | .Ve |
| 2047 | .PP |
| 2048 | .Vb 1 |
| 2049 | \& asynch_read(fh, Map[index].Function) ; |
| 2050 | .Ve |
| 2051 | .PP |
| 2052 | .Vb 5 |
| 2053 | \& void |
| 2054 | \& array_asynch_close(fh) |
| 2055 | \& int fh |
| 2056 | \& CODE: |
| 2057 | \& int index ; |
| 2058 | .Ve |
| 2059 | .PP |
| 2060 | .Vb 4 |
| 2061 | \& /* Find the file handle */ |
| 2062 | \& for (index = 0; index < MAX_CB ; ++ index) |
| 2063 | \& if (Map[index].Handle == fh) |
| 2064 | \& break ; |
| 2065 | .Ve |
| 2066 | .PP |
| 2067 | .Vb 2 |
| 2068 | \& if (index == MAX_CB) |
| 2069 | \& croak ("could not close fh %d\en", fh) ; |
| 2070 | .Ve |
| 2071 | .PP |
| 2072 | .Vb 3 |
| 2073 | \& Map[index].Handle = NULL_HANDLE ; |
| 2074 | \& SvREFCNT_dec(Map[index].PerlSub) ; |
| 2075 | \& Map[index].PerlSub = (SV*)NULL ; |
| 2076 | .Ve |
| 2077 | .PP |
| 2078 | .Vb 1 |
| 2079 | \& asynch_close(fh) ; |
| 2080 | .Ve |
| 2081 | .PP |
| 2082 | In this case the functions \f(CW\*(C`fn1\*(C'\fR, \f(CW\*(C`fn2\*(C'\fR, and \f(CW\*(C`fn3\*(C'\fR are used to |
| 2083 | remember the Perl subroutine to be called. Each of the functions holds |
| 2084 | a separate hard-wired index which is used in the function \f(CW\*(C`Pcb\*(C'\fR to |
| 2085 | access the \f(CW\*(C`Map\*(C'\fR array and actually call the Perl subroutine. |
| 2086 | .PP |
| 2087 | There are some obvious disadvantages with this technique. |
| 2088 | .PP |
| 2089 | Firstly, the code is considerably more complex than with the previous |
| 2090 | example. |
| 2091 | .PP |
| 2092 | Secondly, there is a hard-wired limit (in this case 3) to the number of |
| 2093 | callbacks that can exist simultaneously. The only way to increase the |
| 2094 | limit is by modifying the code to add more functions and then |
| 2095 | recompiling. None the less, as long as the number of functions is |
| 2096 | chosen with some care, it is still a workable solution and in some |
| 2097 | cases is the only one available. |
| 2098 | .PP |
| 2099 | To summarize, here are a number of possible methods for you to consider |
| 2100 | for storing the mapping between C and the Perl callback |
| 2101 | .IP "1. Ignore the problem \- Allow only 1 callback" 5 |
| 2102 | .IX Item "1. Ignore the problem - Allow only 1 callback" |
| 2103 | For a lot of situations, like interfacing to an error handler, this may |
| 2104 | be a perfectly adequate solution. |
| 2105 | .IP "2. Create a sequence of callbacks \- hard wired limit" 5 |
| 2106 | .IX Item "2. Create a sequence of callbacks - hard wired limit" |
| 2107 | If it is impossible to tell from the parameters passed back from the C |
| 2108 | callback what the context is, then you may need to create a sequence of C |
| 2109 | callback interface functions, and store pointers to each in an array. |
| 2110 | .IP "3. Use a parameter to map to the Perl callback" 5 |
| 2111 | .IX Item "3. Use a parameter to map to the Perl callback" |
| 2112 | A hash is an ideal mechanism to store the mapping between C and Perl. |
| 2113 | .Sh "Alternate Stack Manipulation" |
| 2114 | .IX Subsection "Alternate Stack Manipulation" |
| 2115 | Although I have made use of only the \f(CW\*(C`POP*\*(C'\fR macros to access values |
| 2116 | returned from Perl subroutines, it is also possible to bypass these |
| 2117 | macros and read the stack using the \f(CW\*(C`ST\*(C'\fR macro (See perlxs for a |
| 2118 | full description of the \f(CW\*(C`ST\*(C'\fR macro). |
| 2119 | .PP |
| 2120 | Most of the time the \f(CW\*(C`POP*\*(C'\fR macros should be adequate, the main |
| 2121 | problem with them is that they force you to process the returned values |
| 2122 | in sequence. This may not be the most suitable way to process the |
| 2123 | values in some cases. What we want is to be able to access the stack in |
| 2124 | a random order. The \f(CW\*(C`ST\*(C'\fR macro as used when coding an \s-1XSUB\s0 is ideal |
| 2125 | for this purpose. |
| 2126 | .PP |
| 2127 | The code below is the example given in the section \fIReturning a list |
| 2128 | of values\fR recoded to use \f(CW\*(C`ST\*(C'\fR instead of \f(CW\*(C`POP*\*(C'\fR. |
| 2129 | .PP |
| 2130 | .Vb 8 |
| 2131 | \& static void |
| 2132 | \& call_AddSubtract2(a, b) |
| 2133 | \& int a ; |
| 2134 | \& int b ; |
| 2135 | \& { |
| 2136 | \& dSP ; |
| 2137 | \& I32 ax ; |
| 2138 | \& int count ; |
| 2139 | .Ve |
| 2140 | .PP |
| 2141 | .Vb 2 |
| 2142 | \& ENTER ; |
| 2143 | \& SAVETMPS; |
| 2144 | .Ve |
| 2145 | .PP |
| 2146 | .Vb 4 |
| 2147 | \& PUSHMARK(SP) ; |
| 2148 | \& XPUSHs(sv_2mortal(newSViv(a))); |
| 2149 | \& XPUSHs(sv_2mortal(newSViv(b))); |
| 2150 | \& PUTBACK ; |
| 2151 | .Ve |
| 2152 | .PP |
| 2153 | .Vb 1 |
| 2154 | \& count = call_pv("AddSubtract", G_ARRAY); |
| 2155 | .Ve |
| 2156 | .PP |
| 2157 | .Vb 3 |
| 2158 | \& SPAGAIN ; |
| 2159 | \& SP -= count ; |
| 2160 | \& ax = (SP - PL_stack_base) + 1 ; |
| 2161 | .Ve |
| 2162 | .PP |
| 2163 | .Vb 2 |
| 2164 | \& if (count != 2) |
| 2165 | \& croak("Big trouble\en") ; |
| 2166 | .Ve |
| 2167 | .PP |
| 2168 | .Vb 2 |
| 2169 | \& printf ("%d + %d = %d\en", a, b, SvIV(ST(0))) ; |
| 2170 | \& printf ("%d - %d = %d\en", a, b, SvIV(ST(1))) ; |
| 2171 | .Ve |
| 2172 | .PP |
| 2173 | .Vb 4 |
| 2174 | \& PUTBACK ; |
| 2175 | \& FREETMPS ; |
| 2176 | \& LEAVE ; |
| 2177 | \& } |
| 2178 | .Ve |
| 2179 | .PP |
| 2180 | Notes |
| 2181 | .IP "1." 5 |
| 2182 | Notice that it was necessary to define the variable \f(CW\*(C`ax\*(C'\fR. This is |
| 2183 | because the \f(CW\*(C`ST\*(C'\fR macro expects it to exist. If we were in an \s-1XSUB\s0 it |
| 2184 | would not be necessary to define \f(CW\*(C`ax\*(C'\fR as it is already defined for |
| 2185 | you. |
| 2186 | .IP "2." 5 |
| 2187 | The code |
| 2188 | .Sp |
| 2189 | .Vb 3 |
| 2190 | \& SPAGAIN ; |
| 2191 | \& SP -= count ; |
| 2192 | \& ax = (SP - PL_stack_base) + 1 ; |
| 2193 | .Ve |
| 2194 | .Sp |
| 2195 | sets the stack up so that we can use the \f(CW\*(C`ST\*(C'\fR macro. |
| 2196 | .IP "3." 5 |
| 2197 | Unlike the original coding of this example, the returned |
| 2198 | values are not accessed in reverse order. So \f(CWST(0)\fR refers to the |
| 2199 | first value returned by the Perl subroutine and \f(CW\*(C`ST(count\-1)\*(C'\fR |
| 2200 | refers to the last. |
| 2201 | .Sh "Creating and calling an anonymous subroutine in C" |
| 2202 | .IX Subsection "Creating and calling an anonymous subroutine in C" |
| 2203 | As we've already shown, \f(CW\*(C`call_sv\*(C'\fR can be used to invoke an |
| 2204 | anonymous subroutine. However, our example showed a Perl script |
| 2205 | invoking an \s-1XSUB\s0 to perform this operation. Let's see how it can be |
| 2206 | done inside our C code: |
| 2207 | .PP |
| 2208 | .Vb 1 |
| 2209 | \& ... |
| 2210 | .Ve |
| 2211 | .PP |
| 2212 | .Vb 1 |
| 2213 | \& SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); |
| 2214 | .Ve |
| 2215 | .PP |
| 2216 | .Vb 1 |
| 2217 | \& ... |
| 2218 | .Ve |
| 2219 | .PP |
| 2220 | .Vb 1 |
| 2221 | \& call_sv(cvrv, G_VOID|G_NOARGS); |
| 2222 | .Ve |
| 2223 | .PP |
| 2224 | \&\f(CW\*(C`eval_pv\*(C'\fR is used to compile the anonymous subroutine, which |
| 2225 | will be the return value as well (read more about \f(CW\*(C`eval_pv\*(C'\fR in |
| 2226 | \&\*(L"eval_pv\*(R" in perlapi). Once this code reference is in hand, it |
| 2227 | can be mixed in with all the previous examples we've shown. |
| 2228 | .SH "SEE ALSO" |
| 2229 | .IX Header "SEE ALSO" |
| 2230 | perlxs, perlguts, perlembed |
| 2231 | .SH "AUTHOR" |
| 2232 | .IX Header "AUTHOR" |
| 2233 | Paul Marquess |
| 2234 | .PP |
| 2235 | Special thanks to the following people who assisted in the creation of |
| 2236 | the document. |
| 2237 | .PP |
| 2238 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy |
| 2239 | and Larry Wall. |
| 2240 | .SH "DATE" |
| 2241 | .IX Header "DATE" |
| 2242 | Version 1.3, 14th Apr 1997 |