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1 | =head1 NAME |
2 | |
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3 | perlxs - XS language reference manual |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Introduction |
8 | |
9 | XS is a language used to create an extension interface |
10 | between Perl and some C library which one wishes to use with |
11 | Perl. The XS interface is combined with the library to |
12 | create a new library which can be linked to Perl. An B<XSUB> |
13 | is a function in the XS language and is the core component |
14 | of the Perl application interface. |
15 | |
16 | The XS compiler is called B<xsubpp>. This compiler will embed |
17 | the constructs necessary to let an XSUB, which is really a C |
18 | function in disguise, manipulate Perl values and creates the |
19 | glue necessary to let Perl access the XSUB. The compiler |
20 | uses B<typemaps> to determine how to map C function parameters |
21 | and variables to Perl values. The default typemap handles |
22 | many common C types. A supplement typemap must be created |
23 | to handle special structures and types for the library being |
24 | linked. |
25 | |
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26 | See L<perlxstut> for a tutorial on the whole extension creation process. |
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27 | |
28 | =head2 On The Road |
29 | |
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30 | Many of the examples which follow will concentrate on creating an |
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31 | interface between Perl and the ONC+ RPC bind library functions. |
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32 | Specifically, the rpcb_gettime() function will be used to demonstrate many |
33 | features of the XS language. This function has two parameters; the first |
34 | is an input parameter and the second is an output parameter. The function |
35 | also returns a status value. |
36 | |
37 | bool_t rpcb_gettime(const char *host, time_t *timep); |
38 | |
39 | From C this function will be called with the following |
40 | statements. |
41 | |
42 | #include <rpc/rpc.h> |
43 | bool_t status; |
44 | time_t timep; |
45 | status = rpcb_gettime( "localhost", &timep ); |
46 | |
47 | If an XSUB is created to offer a direct translation between this function |
48 | and Perl, then this XSUB will be used from Perl with the following code. |
49 | The $status and $timep variables will contain the output of the function. |
50 | |
51 | use RPC; |
52 | $status = rpcb_gettime( "localhost", $timep ); |
53 | |
54 | The following XS file shows an XS subroutine, or XSUB, which |
55 | demonstrates one possible interface to the rpcb_gettime() |
56 | function. This XSUB represents a direct translation between |
57 | C and Perl and so preserves the interface even from Perl. |
58 | This XSUB will be invoked from Perl with the usage shown |
59 | above. Note that the first three #include statements, for |
60 | C<EXTERN.h>, C<perl.h>, and C<XSUB.h>, will always be present at the |
61 | beginning of an XS file. This approach and others will be |
62 | expanded later in this document. |
63 | |
64 | #include "EXTERN.h" |
65 | #include "perl.h" |
66 | #include "XSUB.h" |
67 | #include <rpc/rpc.h> |
68 | |
69 | MODULE = RPC PACKAGE = RPC |
70 | |
71 | bool_t |
72 | rpcb_gettime(host,timep) |
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73 | char *host |
74 | time_t &timep |
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75 | OUTPUT: |
76 | timep |
77 | |
78 | Any extension to Perl, including those containing XSUBs, |
79 | should have a Perl module to serve as the bootstrap which |
80 | pulls the extension into Perl. This module will export the |
81 | extension's functions and variables to the Perl program and |
82 | will cause the extension's XSUBs to be linked into Perl. |
83 | The following module will be used for most of the examples |
84 | in this document and should be used from Perl with the C<use> |
85 | command as shown earlier. Perl modules are explained in |
86 | more detail later in this document. |
87 | |
88 | package RPC; |
89 | |
90 | require Exporter; |
91 | require DynaLoader; |
92 | @ISA = qw(Exporter DynaLoader); |
93 | @EXPORT = qw( rpcb_gettime ); |
94 | |
95 | bootstrap RPC; |
96 | 1; |
97 | |
98 | Throughout this document a variety of interfaces to the rpcb_gettime() |
99 | XSUB will be explored. The XSUBs will take their parameters in different |
100 | orders or will take different numbers of parameters. In each case the |
101 | XSUB is an abstraction between Perl and the real C rpcb_gettime() |
102 | function, and the XSUB must always ensure that the real rpcb_gettime() |
103 | function is called with the correct parameters. This abstraction will |
104 | allow the programmer to create a more Perl-like interface to the C |
105 | function. |
106 | |
107 | =head2 The Anatomy of an XSUB |
108 | |
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109 | The following XSUB allows a Perl program to access a C library function |
110 | called sin(). The XSUB will imitate the C function which takes a single |
111 | argument and returns a single value. |
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112 | |
113 | double |
114 | sin(x) |
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115 | double x |
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116 | |
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117 | When using C pointers the indirection operator C<*> should be considered |
118 | part of the type and the address operator C<&> should be considered part of |
119 | the variable, as is demonstrated in the rpcb_gettime() function above. See |
120 | the section on typemaps for more about handling qualifiers and unary |
121 | operators in C types. |
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122 | |
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123 | The function name and the return type must be placed on |
124 | separate lines. |
125 | |
126 | INCORRECT CORRECT |
127 | |
128 | double sin(x) double |
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129 | double x sin(x) |
130 | double x |
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131 | |
132 | =head2 The Argument Stack |
133 | |
134 | The argument stack is used to store the values which are |
135 | sent as parameters to the XSUB and to store the XSUB's |
136 | return value. In reality all Perl functions keep their |
137 | values on this stack at the same time, each limited to its |
138 | own range of positions on the stack. In this document the |
139 | first position on that stack which belongs to the active |
140 | function will be referred to as position 0 for that function. |
141 | |
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142 | XSUBs refer to their stack arguments with the macro B<ST(x)>, where I<x> |
143 | refers to a position in this XSUB's part of the stack. Position 0 for that |
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144 | function would be known to the XSUB as ST(0). The XSUB's incoming |
145 | parameters and outgoing return values always begin at ST(0). For many |
146 | simple cases the B<xsubpp> compiler will generate the code necessary to |
147 | handle the argument stack by embedding code fragments found in the |
148 | typemaps. In more complex cases the programmer must supply the code. |
149 | |
150 | =head2 The RETVAL Variable |
151 | |
152 | The RETVAL variable is a magic variable which always matches |
153 | the return type of the C library function. The B<xsubpp> compiler will |
154 | supply this variable in each XSUB and by default will use it to hold the |
155 | return value of the C library function being called. In simple cases the |
156 | value of RETVAL will be placed in ST(0) of the argument stack where it can |
157 | be received by Perl as the return value of the XSUB. |
158 | |
159 | If the XSUB has a return type of C<void> then the compiler will |
160 | not supply a RETVAL variable for that function. When using |
161 | the PPCODE: directive the RETVAL variable may not be needed. |
162 | |
163 | =head2 The MODULE Keyword |
164 | |
165 | The MODULE keyword is used to start the XS code and to |
166 | specify the package of the functions which are being |
167 | defined. All text preceding the first MODULE keyword is |
168 | considered C code and is passed through to the output |
169 | untouched. Every XS module will have a bootstrap function |
170 | which is used to hook the XSUBs into Perl. The package name |
171 | of this bootstrap function will match the value of the last |
172 | MODULE statement in the XS source files. The value of |
173 | MODULE should always remain constant within the same XS |
174 | file, though this is not required. |
175 | |
176 | The following example will start the XS code and will place |
177 | all functions in a package named RPC. |
178 | |
179 | MODULE = RPC |
180 | |
181 | =head2 The PACKAGE Keyword |
182 | |
183 | When functions within an XS source file must be separated into packages |
184 | the PACKAGE keyword should be used. This keyword is used with the MODULE |
185 | keyword and must follow immediately after it when used. |
186 | |
187 | MODULE = RPC PACKAGE = RPC |
188 | |
189 | [ XS code in package RPC ] |
190 | |
191 | MODULE = RPC PACKAGE = RPCB |
192 | |
193 | [ XS code in package RPCB ] |
194 | |
195 | MODULE = RPC PACKAGE = RPC |
196 | |
197 | [ XS code in package RPC ] |
198 | |
199 | Although this keyword is optional and in some cases provides redundant |
200 | information it should always be used. This keyword will ensure that the |
201 | XSUBs appear in the desired package. |
202 | |
203 | =head2 The PREFIX Keyword |
204 | |
205 | The PREFIX keyword designates prefixes which should be |
206 | removed from the Perl function names. If the C function is |
207 | C<rpcb_gettime()> and the PREFIX value is C<rpcb_> then Perl will |
208 | see this function as C<gettime()>. |
209 | |
210 | This keyword should follow the PACKAGE keyword when used. |
211 | If PACKAGE is not used then PREFIX should follow the MODULE |
212 | keyword. |
213 | |
214 | MODULE = RPC PREFIX = rpc_ |
215 | |
216 | MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_ |
217 | |
218 | =head2 The OUTPUT: Keyword |
219 | |
220 | The OUTPUT: keyword indicates that certain function parameters should be |
221 | updated (new values made visible to Perl) when the XSUB terminates or that |
222 | certain values should be returned to the calling Perl function. For |
223 | simple functions, such as the sin() function above, the RETVAL variable is |
224 | automatically designated as an output value. In more complex functions |
225 | the B<xsubpp> compiler will need help to determine which variables are output |
226 | variables. |
227 | |
228 | This keyword will normally be used to complement the CODE: keyword. |
229 | The RETVAL variable is not recognized as an output variable when the |
230 | CODE: keyword is present. The OUTPUT: keyword is used in this |
231 | situation to tell the compiler that RETVAL really is an output |
232 | variable. |
233 | |
234 | The OUTPUT: keyword can also be used to indicate that function parameters |
235 | are output variables. This may be necessary when a parameter has been |
236 | modified within the function and the programmer would like the update to |
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237 | be seen by Perl. |
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238 | |
239 | bool_t |
240 | rpcb_gettime(host,timep) |
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241 | char *host |
242 | time_t &timep |
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243 | OUTPUT: |
244 | timep |
245 | |
246 | The OUTPUT: keyword will also allow an output parameter to |
247 | be mapped to a matching piece of code rather than to a |
248 | typemap. |
249 | |
250 | bool_t |
251 | rpcb_gettime(host,timep) |
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252 | char *host |
253 | time_t &timep |
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254 | OUTPUT: |
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255 | timep sv_setnv(ST(1), (double)timep); |
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256 | |
257 | =head2 The CODE: Keyword |
258 | |
259 | This keyword is used in more complicated XSUBs which require |
260 | special handling for the C function. The RETVAL variable is |
261 | available but will not be returned unless it is specified |
262 | under the OUTPUT: keyword. |
263 | |
264 | The following XSUB is for a C function which requires special handling of |
265 | its parameters. The Perl usage is given first. |
266 | |
267 | $status = rpcb_gettime( "localhost", $timep ); |
268 | |
269 | The XSUB follows. |
270 | |
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271 | bool_t |
272 | rpcb_gettime(host,timep) |
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273 | char *host |
274 | time_t timep |
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275 | CODE: |
276 | RETVAL = rpcb_gettime( host, &timep ); |
277 | OUTPUT: |
278 | timep |
279 | RETVAL |
280 | |
281 | In many of the examples shown here the CODE: block (and |
282 | other blocks) will often be contained within braces ( C<{> and |
283 | C<}> ). This protects the CODE: block from complex INPUT |
284 | typemaps and ensures the resulting C code is legal. |
285 | |
286 | =head2 The NO_INIT Keyword |
287 | |
288 | The NO_INIT keyword is used to indicate that a function |
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289 | parameter is being used as only an output value. The B<xsubpp> |
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290 | compiler will normally generate code to read the values of |
291 | all function parameters from the argument stack and assign |
292 | them to C variables upon entry to the function. NO_INIT |
293 | will tell the compiler that some parameters will be used for |
294 | output rather than for input and that they will be handled |
295 | before the function terminates. |
296 | |
297 | The following example shows a variation of the rpcb_gettime() function. |
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298 | This function uses the timep variable as only an output variable and does |
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299 | not care about its initial contents. |
300 | |
301 | bool_t |
302 | rpcb_gettime(host,timep) |
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303 | char *host |
304 | time_t &timep = NO_INIT |
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305 | OUTPUT: |
306 | timep |
307 | |
308 | =head2 Initializing Function Parameters |
309 | |
310 | Function parameters are normally initialized with their |
311 | values from the argument stack. The typemaps contain the |
312 | code segments which are used to transfer the Perl values to |
313 | the C parameters. The programmer, however, is allowed to |
314 | override the typemaps and supply alternate initialization |
315 | code. |
316 | |
317 | The following code demonstrates how to supply initialization code for |
318 | function parameters. The initialization code is eval'd by the compiler |
319 | before it is added to the output so anything which should be interpreted |
320 | literally, such as double quotes, must be protected with backslashes. |
321 | |
322 | bool_t |
323 | rpcb_gettime(host,timep) |
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324 | char *host = (char *)SvPV(ST(0),na); |
325 | time_t &timep = 0; |
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326 | OUTPUT: |
327 | timep |
328 | |
329 | This should not be used to supply default values for parameters. One |
330 | would normally use this when a function parameter must be processed by |
331 | another library function before it can be used. Default parameters are |
332 | covered in the next section. |
333 | |
334 | =head2 Default Parameter Values |
335 | |
336 | Default values can be specified for function parameters by |
337 | placing an assignment statement in the parameter list. The |
338 | default value may be a number or a string. Defaults should |
339 | always be used on the right-most parameters only. |
340 | |
341 | To allow the XSUB for rpcb_gettime() to have a default host |
342 | value the parameters to the XSUB could be rearranged. The |
343 | XSUB will then call the real rpcb_gettime() function with |
344 | the parameters in the correct order. Perl will call this |
345 | XSUB with either of the following statements. |
346 | |
347 | $status = rpcb_gettime( $timep, $host ); |
348 | |
349 | $status = rpcb_gettime( $timep ); |
350 | |
351 | The XSUB will look like the code which follows. A CODE: |
352 | block is used to call the real rpcb_gettime() function with |
353 | the parameters in the correct order for that function. |
354 | |
355 | bool_t |
356 | rpcb_gettime(timep,host="localhost") |
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357 | char *host |
358 | time_t timep = NO_INIT |
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359 | CODE: |
360 | RETVAL = rpcb_gettime( host, &timep ); |
361 | OUTPUT: |
362 | timep |
363 | RETVAL |
364 | |
365 | =head2 Variable-length Parameter Lists |
366 | |
367 | XSUBs can have variable-length parameter lists by specifying an ellipsis |
368 | C<(...)> in the parameter list. This use of the ellipsis is similar to that |
369 | found in ANSI C. The programmer is able to determine the number of |
370 | arguments passed to the XSUB by examining the C<items> variable which the |
371 | B<xsubpp> compiler supplies for all XSUBs. By using this mechanism one can |
372 | create an XSUB which accepts a list of parameters of unknown length. |
373 | |
374 | The I<host> parameter for the rpcb_gettime() XSUB can be |
375 | optional so the ellipsis can be used to indicate that the |
376 | XSUB will take a variable number of parameters. Perl should |
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377 | be able to call this XSUB with either of the following statements. |
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378 | |
379 | $status = rpcb_gettime( $timep, $host ); |
380 | |
381 | $status = rpcb_gettime( $timep ); |
382 | |
383 | The XS code, with ellipsis, follows. |
384 | |
385 | bool_t |
386 | rpcb_gettime(timep, ...) |
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387 | time_t timep = NO_INIT |
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388 | CODE: |
389 | { |
390 | char *host = "localhost"; |
391 | |
392 | if( items > 1 ) |
393 | host = (char *)SvPV(ST(1), na); |
394 | RETVAL = rpcb_gettime( host, &timep ); |
395 | } |
396 | OUTPUT: |
397 | timep |
398 | RETVAL |
399 | |
400 | =head2 The PPCODE: Keyword |
401 | |
402 | The PPCODE: keyword is an alternate form of the CODE: keyword and is used |
403 | to tell the B<xsubpp> compiler that the programmer is supplying the code to |
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404 | control the argument stack for the XSUBs return values. Occasionally one |
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405 | will want an XSUB to return a list of values rather than a single value. |
406 | In these cases one must use PPCODE: and then explicitly push the list of |
407 | values on the stack. The PPCODE: and CODE: keywords are not used |
408 | together within the same XSUB. |
409 | |
410 | The following XSUB will call the C rpcb_gettime() function |
411 | and will return its two output values, timep and status, to |
412 | Perl as a single list. |
413 | |
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414 | void |
415 | rpcb_gettime(host) |
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416 | char *host |
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417 | PPCODE: |
418 | { |
419 | time_t timep; |
420 | bool_t status; |
421 | status = rpcb_gettime( host, &timep ); |
422 | EXTEND(sp, 2); |
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423 | PUSHs(sv_2mortal(newSViv(status))); |
424 | PUSHs(sv_2mortal(newSViv(timep))); |
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425 | } |
426 | |
427 | Notice that the programmer must supply the C code necessary |
428 | to have the real rpcb_gettime() function called and to have |
429 | the return values properly placed on the argument stack. |
430 | |
431 | The C<void> return type for this function tells the B<xsubpp> compiler that |
432 | the RETVAL variable is not needed or used and that it should not be created. |
433 | In most scenarios the void return type should be used with the PPCODE: |
434 | directive. |
435 | |
436 | The EXTEND() macro is used to make room on the argument |
437 | stack for 2 return values. The PPCODE: directive causes the |
438 | B<xsubpp> compiler to create a stack pointer called C<sp>, and it |
439 | is this pointer which is being used in the EXTEND() macro. |
440 | The values are then pushed onto the stack with the PUSHs() |
441 | macro. |
442 | |
443 | Now the rpcb_gettime() function can be used from Perl with |
444 | the following statement. |
445 | |
446 | ($status, $timep) = rpcb_gettime("localhost"); |
447 | |
448 | =head2 Returning Undef And Empty Lists |
449 | |
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450 | Occasionally the programmer will want to simply return |
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451 | C<undef> or an empty list if a function fails rather than a |
452 | separate status value. The rpcb_gettime() function offers |
453 | just this situation. If the function succeeds we would like |
454 | to have it return the time and if it fails we would like to |
455 | have undef returned. In the following Perl code the value |
456 | of $timep will either be undef or it will be a valid time. |
457 | |
458 | $timep = rpcb_gettime( "localhost" ); |
459 | |
460 | The following XSUB uses the C<void> return type to disable the generation of |
461 | the RETVAL variable and uses a CODE: block to indicate to the compiler |
462 | that the programmer has supplied all the necessary code. The |
463 | sv_newmortal() call will initialize the return value to undef, making that |
464 | the default return value. |
465 | |
466 | void |
467 | rpcb_gettime(host) |
468 | char * host |
469 | CODE: |
470 | { |
471 | time_t timep; |
472 | bool_t x; |
473 | ST(0) = sv_newmortal(); |
474 | if( rpcb_gettime( host, &timep ) ) |
475 | sv_setnv( ST(0), (double)timep); |
476 | } |
477 | |
478 | The next example demonstrates how one would place an explicit undef in the |
479 | return value, should the need arise. |
480 | |
481 | void |
482 | rpcb_gettime(host) |
483 | char * host |
484 | CODE: |
485 | { |
486 | time_t timep; |
487 | bool_t x; |
488 | ST(0) = sv_newmortal(); |
489 | if( rpcb_gettime( host, &timep ) ){ |
490 | sv_setnv( ST(0), (double)timep); |
491 | } |
492 | else{ |
493 | ST(0) = &sv_undef; |
494 | } |
495 | } |
496 | |
497 | To return an empty list one must use a PPCODE: block and |
498 | then not push return values on the stack. |
499 | |
500 | void |
501 | rpcb_gettime(host) |
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502 | char *host |
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503 | PPCODE: |
504 | { |
505 | time_t timep; |
506 | if( rpcb_gettime( host, &timep ) ) |
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507 | PUSHs(sv_2mortal(newSViv(timep))); |
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508 | else{ |
509 | /* Nothing pushed on stack, so an empty */ |
510 | /* list is implicitly returned. */ |
511 | } |
512 | } |
513 | |
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514 | =head2 The REQUIRE: Keyword |
515 | |
516 | The REQUIRE: keyword is used to indicate the minimum version of the |
517 | B<xsubpp> compiler needed to compile the XS module. An XS module which |
518 | contains the following statement will only compile with B<xsubpp> version |
519 | 1.922 or greater: |
520 | |
521 | REQUIRE: 1.922 |
522 | |
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523 | =head2 The CLEANUP: Keyword |
524 | |
525 | This keyword can be used when an XSUB requires special cleanup procedures |
526 | before it terminates. When the CLEANUP: keyword is used it must follow |
527 | any CODE:, PPCODE:, or OUTPUT: blocks which are present in the XSUB. The |
528 | code specified for the cleanup block will be added as the last statements |
529 | in the XSUB. |
530 | |
531 | =head2 The BOOT: Keyword |
532 | |
533 | The BOOT: keyword is used to add code to the extension's bootstrap |
534 | function. The bootstrap function is generated by the B<xsubpp> compiler and |
535 | normally holds the statements necessary to register any XSUBs with Perl. |
536 | With the BOOT: keyword the programmer can tell the compiler to add extra |
537 | statements to the bootstrap function. |
538 | |
539 | This keyword may be used any time after the first MODULE keyword and should |
540 | appear on a line by itself. The first blank line after the keyword will |
541 | terminate the code block. |
542 | |
543 | BOOT: |
544 | # The following message will be printed when the |
545 | # bootstrap function executes. |
546 | printf("Hello from the bootstrap!\n"); |
547 | |
548 | =head2 Inserting Comments and C Preprocessor Directives |
549 | |
550 | Comments and C preprocessor directives are allowed within |
551 | CODE:, PPCODE:, BOOT:, and CLEANUP: blocks. The compiler |
552 | will pass the preprocessor directives through untouched and |
553 | will remove the commented lines. Comments can be added to |
554 | XSUBs by placing a C<#> at the beginning of the line. Care |
555 | should be taken to avoid making the comment look like a C |
556 | preprocessor directive, lest it be interpreted as such. |
557 | |
558 | =head2 Using XS With C++ |
559 | |
560 | If a function is defined as a C++ method then it will assume |
561 | its first argument is an object pointer. The object pointer |
562 | will be stored in a variable called THIS. The object should |
563 | have been created by C++ with the new() function and should |
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564 | be blessed by Perl with the sv_setref_pv() macro. The |
565 | blessing of the object by Perl can be handled by a typemap. An example |
566 | typemap is shown at the end of this section. |
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567 | |
568 | If the method is defined as static it will call the C++ |
569 | function using the class::method() syntax. If the method is not static |
570 | the function will be called using the THIS->method() syntax. |
571 | |
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572 | The next examples will use the following C++ class. |
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573 | |
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574 | class colors { |
575 | public: |
576 | colors(); |
577 | ~colors(); |
578 | int blue(); |
579 | void set_blue( int ); |
580 | |
581 | private: |
582 | int c_blue; |
583 | }; |
584 | |
585 | The XSUBs for the blue() and set_blue() methods are defined with the class |
586 | name but the parameter for the object (THIS, or "self") is implicit and is |
587 | not listed. |
588 | |
589 | int |
590 | color::blue() |
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591 | |
592 | void |
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593 | color::set_blue( val ) |
594 | int val |
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595 | |
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596 | Both functions will expect an object as the first parameter. The xsubpp |
597 | compiler will call that object C<THIS> and will use it to call the specified |
598 | method. So in the C++ code the blue() and set_blue() methods will be called |
599 | in the following manner. |
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600 | |
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601 | RETVAL = THIS->blue(); |
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602 | |
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603 | THIS->set_blue( val ); |
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604 | |
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605 | If the function's name is B<DESTROY> then the C++ C<delete> function will be |
606 | called and C<THIS> will be given as its parameter. |
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607 | |
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608 | void |
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609 | color::DESTROY() |
610 | |
611 | The C++ code will call C<delete>. |
612 | |
613 | delete THIS; |
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614 | |
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615 | If the function's name is B<new> then the C++ C<new> function will be called |
616 | to create a dynamic C++ object. The XSUB will expect the class name, which |
617 | will be kept in a variable called C<CLASS>, to be given as the first |
618 | argument. |
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619 | |
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620 | color * |
621 | color::new() |
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622 | |
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623 | The C++ code will call C<new>. |
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624 | |
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625 | RETVAL = new color(); |
626 | |
627 | The following is an example of a typemap that could be used for this C++ |
628 | example. |
629 | |
630 | TYPEMAP |
631 | color * O_OBJECT |
632 | |
633 | OUTPUT |
634 | # The Perl object is blessed into 'CLASS', which should be a |
635 | # char* having the name of the package for the blessing. |
636 | O_OBJECT |
637 | sv_setref_pv( $arg, CLASS, (void*)$var ); |
638 | |
639 | INPUT |
640 | O_OBJECT |
641 | if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) ) |
642 | $var = ($type)SvIV((SV*)SvRV( $arg )); |
643 | else{ |
644 | warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" ); |
645 | XSRETURN_UNDEF; |
646 | } |
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647 | |
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648 | =head2 Interface Strategy |
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649 | |
650 | When designing an interface between Perl and a C library a straight |
651 | translation from C to XS is often sufficient. The interface will often be |
652 | very C-like and occasionally nonintuitive, especially when the C function |
653 | modifies one of its parameters. In cases where the programmer wishes to |
654 | create a more Perl-like interface the following strategy may help to |
655 | identify the more critical parts of the interface. |
656 | |
657 | Identify the C functions which modify their parameters. The XSUBs for |
658 | these functions may be able to return lists to Perl, or may be |
659 | candidates to return undef or an empty list in case of failure. |
660 | |
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661 | Identify which values are used by only the C and XSUB functions |
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662 | themselves. If Perl does not need to access the contents of the value |
663 | then it may not be necessary to provide a translation for that value |
664 | from C to Perl. |
665 | |
666 | Identify the pointers in the C function parameter lists and return |
667 | values. Some pointers can be handled in XS with the & unary operator on |
668 | the variable name while others will require the use of the * operator on |
669 | the type name. In general it is easier to work with the & operator. |
670 | |
671 | Identify the structures used by the C functions. In many |
672 | cases it may be helpful to use the T_PTROBJ typemap for |
673 | these structures so they can be manipulated by Perl as |
674 | blessed objects. |
675 | |
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676 | =head2 Perl Objects And C Structures |
677 | |
678 | When dealing with C structures one should select either |
679 | B<T_PTROBJ> or B<T_PTRREF> for the XS type. Both types are |
680 | designed to handle pointers to complex objects. The |
681 | T_PTRREF type will allow the Perl object to be unblessed |
682 | while the T_PTROBJ type requires that the object be blessed. |
683 | By using T_PTROBJ one can achieve a form of type-checking |
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684 | because the XSUB will attempt to verify that the Perl object |
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685 | is of the expected type. |
686 | |
687 | The following XS code shows the getnetconfigent() function which is used |
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688 | with ONC+ TIRPC. The getnetconfigent() function will return a pointer to a |
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689 | C structure and has the C prototype shown below. The example will |
690 | demonstrate how the C pointer will become a Perl reference. Perl will |
691 | consider this reference to be a pointer to a blessed object and will |
692 | attempt to call a destructor for the object. A destructor will be |
693 | provided in the XS source to free the memory used by getnetconfigent(). |
694 | Destructors in XS can be created by specifying an XSUB function whose name |
695 | ends with the word B<DESTROY>. XS destructors can be used to free memory |
696 | which may have been malloc'd by another XSUB. |
697 | |
698 | struct netconfig *getnetconfigent(const char *netid); |
699 | |
700 | A C<typedef> will be created for C<struct netconfig>. The Perl |
701 | object will be blessed in a class matching the name of the C |
702 | type, with the tag C<Ptr> appended, and the name should not |
703 | have embedded spaces if it will be a Perl package name. The |
704 | destructor will be placed in a class corresponding to the |
705 | class of the object and the PREFIX keyword will be used to |
706 | trim the name to the word DESTROY as Perl will expect. |
707 | |
708 | typedef struct netconfig Netconfig; |
709 | |
710 | MODULE = RPC PACKAGE = RPC |
711 | |
712 | Netconfig * |
713 | getnetconfigent(netid) |
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714 | char *netid |
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715 | |
716 | MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_ |
717 | |
718 | void |
719 | rpcb_DESTROY(netconf) |
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720 | Netconfig *netconf |
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721 | CODE: |
722 | printf("Now in NetconfigPtr::DESTROY\n"); |
723 | free( netconf ); |
724 | |
725 | This example requires the following typemap entry. Consult the typemap |
726 | section for more information about adding new typemaps for an extension. |
727 | |
728 | TYPEMAP |
729 | Netconfig * T_PTROBJ |
730 | |
731 | This example will be used with the following Perl statements. |
732 | |
733 | use RPC; |
734 | $netconf = getnetconfigent("udp"); |
735 | |
736 | When Perl destroys the object referenced by $netconf it will send the |
737 | object to the supplied XSUB DESTROY function. Perl cannot determine, and |
738 | does not care, that this object is a C struct and not a Perl object. In |
739 | this sense, there is no difference between the object created by the |
740 | getnetconfigent() XSUB and an object created by a normal Perl subroutine. |
741 | |
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742 | =head2 The Typemap |
743 | |
744 | The typemap is a collection of code fragments which are used by the B<xsubpp> |
745 | compiler to map C function parameters and values to Perl values. The |
746 | typemap file may consist of three sections labeled C<TYPEMAP>, C<INPUT>, and |
747 | C<OUTPUT>. The INPUT section tells the compiler how to translate Perl values |
748 | into variables of certain C types. The OUTPUT section tells the compiler |
749 | how to translate the values from certain C types into values Perl can |
750 | understand. The TYPEMAP section tells the compiler which of the INPUT and |
751 | OUTPUT code fragments should be used to map a given C type to a Perl value. |
752 | Each of the sections of the typemap must be preceded by one of the TYPEMAP, |
753 | INPUT, or OUTPUT keywords. |
754 | |
755 | The default typemap in the C<ext> directory of the Perl source contains many |
756 | useful types which can be used by Perl extensions. Some extensions define |
757 | additional typemaps which they keep in their own directory. These |
758 | additional typemaps may reference INPUT and OUTPUT maps in the main |
759 | typemap. The B<xsubpp> compiler will allow the extension's own typemap to |
760 | override any mappings which are in the default typemap. |
761 | |
762 | Most extensions which require a custom typemap will need only the TYPEMAP |
763 | section of the typemap file. The custom typemap used in the |
764 | getnetconfigent() example shown earlier demonstrates what may be the typical |
765 | use of extension typemaps. That typemap is used to equate a C structure |
766 | with the T_PTROBJ typemap. The typemap used by getnetconfigent() is shown |
767 | here. Note that the C type is separated from the XS type with a tab and |
768 | that the C unary operator C<*> is considered to be a part of the C type name. |
769 | |
770 | TYPEMAP |
771 | Netconfig *<tab>T_PTROBJ |
772 | |
773 | =head1 EXAMPLES |
774 | |
775 | File C<RPC.xs>: Interface to some ONC+ RPC bind library functions. |
776 | |
777 | #include "EXTERN.h" |
778 | #include "perl.h" |
779 | #include "XSUB.h" |
780 | |
781 | #include <rpc/rpc.h> |
782 | |
783 | typedef struct netconfig Netconfig; |
784 | |
785 | MODULE = RPC PACKAGE = RPC |
786 | |
787 | void |
788 | rpcb_gettime(host="localhost") |
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789 | char *host |
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790 | CODE: |
791 | { |
792 | time_t timep; |
793 | ST(0) = sv_newmortal(); |
794 | if( rpcb_gettime( host, &timep ) ) |
795 | sv_setnv( ST(0), (double)timep ); |
796 | } |
797 | |
798 | Netconfig * |
799 | getnetconfigent(netid="udp") |
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800 | char *netid |
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801 | |
802 | MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_ |
803 | |
804 | void |
805 | rpcb_DESTROY(netconf) |
8e07c86e |
806 | Netconfig *netconf |
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807 | CODE: |
808 | printf("NetconfigPtr::DESTROY\n"); |
809 | free( netconf ); |
810 | |
811 | File C<typemap>: Custom typemap for RPC.xs. |
812 | |
813 | TYPEMAP |
814 | Netconfig * T_PTROBJ |
815 | |
816 | File C<RPC.pm>: Perl module for the RPC extension. |
817 | |
818 | package RPC; |
819 | |
820 | require Exporter; |
821 | require DynaLoader; |
822 | @ISA = qw(Exporter DynaLoader); |
823 | @EXPORT = qw(rpcb_gettime getnetconfigent); |
824 | |
825 | bootstrap RPC; |
826 | 1; |
827 | |
828 | File C<rpctest.pl>: Perl test program for the RPC extension. |
829 | |
830 | use RPC; |
831 | |
832 | $netconf = getnetconfigent(); |
833 | $a = rpcb_gettime(); |
834 | print "time = $a\n"; |
835 | print "netconf = $netconf\n"; |
836 | |
837 | $netconf = getnetconfigent("tcp"); |
838 | $a = rpcb_gettime("poplar"); |
839 | print "time = $a\n"; |
840 | print "netconf = $netconf\n"; |
841 | |
842 | |
843 | =head1 AUTHOR |
844 | |
d1b91892 |
845 | Dean Roehrich F<E<lt>roehrich@cray.comE<gt>> |
cb1a09d0 |
846 | Dec 10, 1995 |