1PERLXS(1) Perl Programmers Reference Guide PERLXS(1)
2
6 perlxs - XS language reference manual
7
9 Introduction
10 XS is an interface description file format used to create an extension
11 interface between Perl and C code (or a C library) which one wishes to
12 use with Perl. The XS interface is combined with the library to create
13 a new library which can then be either dynamically loaded or statically
14 linked into perl. The XS interface description is written in the XS
15 language and is the core component of the Perl extension interface.
16 Before writing XS, read the "CAVEATS" section below.
18 An XSUB forms the basic unit of the XS interface. After compilation by
20 the xsubpp compiler, each XSUB amounts to a C function definition which
21 will provide the glue between Perl calling conventions and C calling
22 conventions.
23 The glue code pulls the arguments from the Perl stack, converts these
25 Perl values to the formats expected by a C function, call this C
26 function, transfers the return values of the C function back to Perl.
27 Return values here may be a conventional C return value or any C
28 function arguments that may serve as output parameters. These return
29 values may be passed back to Perl either by putting them on the Perl
30 stack, or by modifying the arguments supplied from the Perl side.
31 The above is a somewhat simplified view of what really happens. Since
33 Perl allows more flexible calling conventions than C, XSUBs may do much
34 more in practice, such as checking input parameters for validity,
35 throwing exceptions (or returning undef/empty list) if the return value
36 from the C function indicates failure, calling different C functions
37 based on numbers and types of the arguments, providing an object-
38 oriented interface, etc.
39 Of course, one could write such glue code directly in C. However, this
41 would be a tedious task, especially if one needs to write glue for
42 multiple C functions, and/or one is not familiar enough with the Perl
43 stack discipline and other such arcana. XS comes to the rescue here:
44 instead of writing this glue C code in long-hand, one can write a more
45 concise short-hand description of what should be done by the glue, and
46 let the XS compiler xsubpp handle the rest.
47 The XS language allows one to describe the mapping between how the C
49 routine is used, and how the corresponding Perl routine is used. It
50 also allows creation of Perl routines which are directly translated to
51 C code and which are not related to a pre-existing C function. In
52 cases when the C interface coincides with the Perl interface, the XSUB
53 declaration is almost identical to a declaration of a C function (in
54 K&R style). In such circumstances, there is another tool called "h2xs"
55 that is able to translate an entire C header file into a corresponding
56 XS file that will provide glue to the functions/macros described in the
57 header file.
58 The XS compiler is called xsubpp. This compiler creates the constructs
60 necessary to let an XSUB manipulate Perl values, and creates the glue
61 necessary to let Perl call the XSUB. The compiler uses typemaps to
62 determine how to map C function parameters and output values to Perl
63 values and back. The default typemap (which comes with Perl) handles
64 many common C types. A supplementary typemap may also be needed to
65 handle any special structures and types for the library being linked.
66 For more information on typemaps, see perlxstypemap.
67 A file in XS format starts with a C language section which goes until
69 the first "MODULE =" directive. Other XS directives and XSUB
70 definitions may follow this line. The "language" used in this part of
71 the file is usually referred to as the XS language. xsubpp recognizes
72 and skips POD (see perlpod) in both the C and XS language sections,
73 which allows the XS file to contain embedded documentation.
74 See perlxstut for a tutorial on the whole extension creation process.
76 Note: For some extensions, Dave Beazley's SWIG system may provide a
78 significantly more convenient mechanism for creating the extension glue
79 code. See <http://www.swig.org/> for more information.
80 On The Road
82 Many of the examples which follow will concentrate on creating an
83 interface between Perl and the ONC+ RPC bind library functions. The
84 rpcb_gettime() function is used to demonstrate many features of the XS
85 language. This function has two parameters; the first is an input
86 parameter and the second is an output parameter. The function also
87 returns a status value.
88 bool_t rpcb_gettime(const char *host, time_t *timep);
90 From C this function will be called with the following statements.
92 #include <rpc/rpc.h>
94 bool_t status;
95 time_t timep;
96 status = rpcb_gettime( "localhost", &timep );
97 If an XSUB is created to offer a direct translation between this
99 function and Perl, then this XSUB will be used from Perl with the
100 following code. The $status and $timep variables will contain the
101 output of the function.
102 use RPC;
104 $status = rpcb_gettime( "localhost", $timep );
105 The following XS file shows an XS subroutine, or XSUB, which
107 demonstrates one possible interface to the rpcb_gettime() function.
108 This XSUB represents a direct translation between C and Perl and so
109 preserves the interface even from Perl. This XSUB will be invoked from
110 Perl with the usage shown above. Note that the first three #include
111 statements, for "EXTERN.h", "perl.h", and "XSUB.h", will always be
112 present at the beginning of an XS file. This approach and others will
113 be expanded later in this document. A #define for
114 "PERL_NO_GET_CONTEXT" should be present to fetch the interpreter
115 context more efficiently, see perlguts for details.
116 #define PERL_NO_GET_CONTEXT
118 #include "EXTERN.h"
119 #include "perl.h"
120 #include "XSUB.h"
121 #include <rpc/rpc.h>
122 MODULE = RPC PACKAGE = RPC
124 bool_t
126 rpcb_gettime(host,timep)
127 char *host
128 time_t &timep
129 OUTPUT:
130 timep
131 Any extension to Perl, including those containing XSUBs, should have a
133 Perl module to serve as the bootstrap which pulls the extension into
134 Perl. This module will export the extension's functions and variables
135 to the Perl program and will cause the extension's XSUBs to be linked
136 into Perl. The following module will be used for most of the examples
137 in this document and should be used from Perl with the "use" command as
138 shown earlier. Perl modules are explained in more detail later in this
139 document.
140 package RPC;
142 require Exporter;
144 require DynaLoader;
145 @ISA = qw(Exporter DynaLoader);
146 @EXPORT = qw( rpcb_gettime );
147 bootstrap RPC;
149 1;
150 Throughout this document a variety of interfaces to the rpcb_gettime()
152 XSUB will be explored. The XSUBs will take their parameters in
153 different orders or will take different numbers of parameters. In each
154 case the XSUB is an abstraction between Perl and the real C
155 rpcb_gettime() function, and the XSUB must always ensure that the real
156 rpcb_gettime() function is called with the correct parameters. This
157 abstraction will allow the programmer to create a more Perl-like
158 interface to the C function.
159 The Anatomy of an XSUB
161 The simplest XSUBs consist of 3 parts: a description of the return
162 value, the name of the XSUB routine and the names of its arguments, and
163 a description of types or formats of the arguments.
164 The following XSUB allows a Perl program to access a C library function
166 called sin(). The XSUB will imitate the C function which takes a
167 single argument and returns a single value.
168 double
170 sin(x)
171 double x
172 Optionally, one can merge the description of types and the list of
174 argument names, rewriting this as
175 double
177 sin(double x)
178 This makes this XSUB look similar to an ANSI C declaration. An
180 optional semicolon is allowed after the argument list, as in
181 double
183 sin(double x);
184 Parameters with C pointer types can have different semantic: C
186 functions with similar declarations
187 bool string_looks_as_a_number(char *s);
189 bool make_char_uppercase(char *c);
190 are used in absolutely incompatible manner. Parameters to these
192 functions could be described xsubpp like this:
193 char * s
195 char &c
196 Both these XS declarations correspond to the "char*" C type, but they
198 have different semantics, see "The & Unary Operator".
199 It is convenient to think that the indirection operator "*" should be
201 considered as a part of the type and the address operator "&" should be
202 considered part of the variable. See perlxstypemap for more info about
203 handling qualifiers and unary operators in C types.
204 The function name and the return type must be placed on separate lines
206 and should be flush left-adjusted.
207 INCORRECT CORRECT
209 double sin(x) double
211 double x sin(x)
212 double x
213 The rest of the function description may be indented or left-adjusted.
215 The following example shows a function with its body left-adjusted.
216 Most examples in this document will indent the body for better
217 readability.
218 CORRECT
220 double
222 sin(x)
223 double x
224 More complicated XSUBs may contain many other sections. Each section
226 of an XSUB starts with the corresponding keyword, such as INIT: or
227 CLEANUP:. However, the first two lines of an XSUB always contain the
228 same data: descriptions of the return type and the names of the
229 function and its parameters. Whatever immediately follows these is
230 considered to be an INPUT: section unless explicitly marked with
231 another keyword. (See "The INPUT: Keyword".)
232 An XSUB section continues until another section-start keyword is found.
234 The Argument Stack
236 The Perl argument stack is used to store the values which are sent as
237 parameters to the XSUB and to store the XSUB's return value(s). In
238 reality all Perl functions (including non-XSUB ones) keep their values
239 on this stack all the same time, each limited to its own range of
240 positions on the stack. In this document the first position on that
241 stack which belongs to the active function will be referred to as
242 position 0 for that function.
243 XSUBs refer to their stack arguments with the macro ST(x), where x
245 refers to a position in this XSUB's part of the stack. Position 0 for
246 that function would be known to the XSUB as ST(0). The XSUB's incoming
247 parameters and outgoing return values always begin at ST(0). For many
248 simple cases the xsubpp compiler will generate the code necessary to
249 handle the argument stack by embedding code fragments found in the
250 typemaps. In more complex cases the programmer must supply the code.
251 The RETVAL Variable
253 The RETVAL variable is a special C variable that is declared
254 automatically for you. The C type of RETVAL matches the return type of
255 the C library function. The xsubpp compiler will declare this variable
256 in each XSUB with non-"void" return type. By default the generated C
257 function will use RETVAL to hold the return value of the C library
258 function being called. In simple cases the value of RETVAL will be
259 placed in ST(0) of the argument stack where it can be received by Perl
260 as the return value of the XSUB.
261 If the XSUB has a return type of "void" then the compiler will not
263 declare a RETVAL variable for that function. When using a PPCODE:
264 section no manipulation of the RETVAL variable is required, the section
265 may use direct stack manipulation to place output values on the stack.
266 If PPCODE: directive is not used, "void" return value should be used
268 only for subroutines which do not return a value, even if CODE:
269 directive is used which sets ST(0) explicitly.
270 Older versions of this document recommended to use "void" return value
272 in such cases. It was discovered that this could lead to segfaults in
273 cases when XSUB was truly "void". This practice is now deprecated, and
274 may be not supported at some future version. Use the return value "SV
275 *" in such cases. (Currently "xsubpp" contains some heuristic code
276 which tries to disambiguate between "truly-void" and "old-practice-
277 declared-as-void" functions. Hence your code is at mercy of this
278 heuristics unless you use "SV *" as return value.)
279 Returning SVs, AVs and HVs through RETVAL
281 When you're using RETVAL to return an "SV *", there's some magic going
282 on behind the scenes that should be mentioned. When you're manipulating
283 the argument stack using the ST(x) macro, for example, you usually have
284 to pay special attention to reference counts. (For more about reference
285 counts, see perlguts.) To make your life easier, the typemap file
286 automatically makes "RETVAL" mortal when you're returning an "SV *".
287 Thus, the following two XSUBs are more or less equivalent:
288 void
290 alpha()
291 PPCODE:
292 ST(0) = newSVpv("Hello World",0);
293 sv_2mortal(ST(0));
294 XSRETURN(1);
295 SV *
297 beta()
298 CODE:
299 RETVAL = newSVpv("Hello World",0);
300 OUTPUT:
301 RETVAL
302 This is quite useful as it usually improves readability. While this
304 works fine for an "SV *", it's unfortunately not as easy to have "AV *"
305 or "HV *" as a return value. You should be able to write:
306 AV *
308 array()
309 CODE:
310 RETVAL = newAV();
311 /* do something with RETVAL */
312 OUTPUT:
313 RETVAL
314 But due to an unfixable bug (fixing it would break lots of existing
316 CPAN modules) in the typemap file, the reference count of the "AV *" is
317 not properly decremented. Thus, the above XSUB would leak memory
318 whenever it is being called. The same problem exists for "HV *", "CV
319 *", and "SVREF" (which indicates a scalar reference, not a general "SV
320 *"). In XS code on perls starting with perl 5.16, you can override the
321 typemaps for any of these types with a version that has proper handling
322 of refcounts. In your "TYPEMAP" section, do
323 AV* T_AVREF_REFCOUNT_FIXED
325 to get the repaired variant. For backward compatibility with older
327 versions of perl, you can instead decrement the reference count
328 manually when you're returning one of the aforementioned types using
329 "sv_2mortal":
330 AV *
332 array()
333 CODE:
334 RETVAL = newAV();
335 sv_2mortal((SV*)RETVAL);
336 /* do something with RETVAL */
337 OUTPUT:
338 RETVAL
339 Remember that you don't have to do this for an "SV *". The reference
341 documentation for all core typemaps can be found in perlxstypemap.
342 The MODULE Keyword
344 The MODULE keyword is used to start the XS code and to specify the
345 package of the functions which are being defined. All text preceding
346 the first MODULE keyword is considered C code and is passed through to
347 the output with POD stripped, but otherwise untouched. Every XS module
348 will have a bootstrap function which is used to hook the XSUBs into
349 Perl. The package name of this bootstrap function will match the value
350 of the last MODULE statement in the XS source files. The value of
351 MODULE should always remain constant within the same XS file, though
352 this is not required.
353 The following example will start the XS code and will place all
355 functions in a package named RPC.
356 MODULE = RPC
358 The PACKAGE Keyword
360 When functions within an XS source file must be separated into packages
361 the PACKAGE keyword should be used. This keyword is used with the
362 MODULE keyword and must follow immediately after it when used.
363 MODULE = RPC PACKAGE = RPC
365 [ XS code in package RPC ]
367 MODULE = RPC PACKAGE = RPCB
369 [ XS code in package RPCB ]
371 MODULE = RPC PACKAGE = RPC
373 [ XS code in package RPC ]
375 The same package name can be used more than once, allowing for non-
377 contiguous code. This is useful if you have a stronger ordering
378 principle than package names.
379 Although this keyword is optional and in some cases provides redundant
381 information it should always be used. This keyword will ensure that
382 the XSUBs appear in the desired package.
383 The PREFIX Keyword
385 The PREFIX keyword designates prefixes which should be removed from the
386 Perl function names. If the C function is "rpcb_gettime()" and the
387 PREFIX value is "rpcb_" then Perl will see this function as
388 "gettime()".
389 This keyword should follow the PACKAGE keyword when used. If PACKAGE
391 is not used then PREFIX should follow the MODULE keyword.
392 MODULE = RPC PREFIX = rpc_
394 MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_
396 The OUTPUT: Keyword
398 The OUTPUT: keyword indicates that certain function parameters should
399 be updated (new values made visible to Perl) when the XSUB terminates
400 or that certain values should be returned to the calling Perl function.
401 For simple functions which have no CODE: or PPCODE: section, such as
402 the sin() function above, the RETVAL variable is automatically
403 designated as an output value. For more complex functions the xsubpp
404 compiler will need help to determine which variables are output
405 variables.
406 This keyword will normally be used to complement the CODE: keyword.
408 The RETVAL variable is not recognized as an output variable when the
409 CODE: keyword is present. The OUTPUT: keyword is used in this
410 situation to tell the compiler that RETVAL really is an output
411 variable.
412 The OUTPUT: keyword can also be used to indicate that function
414 parameters are output variables. This may be necessary when a
415 parameter has been modified within the function and the programmer
416 would like the update to be seen by Perl.
417 bool_t
419 rpcb_gettime(host,timep)
420 char *host
421 time_t &timep
422 OUTPUT:
423 timep
424 The OUTPUT: keyword will also allow an output parameter to be mapped to
426 a matching piece of code rather than to a typemap.
427 bool_t
429 rpcb_gettime(host,timep)
430 char *host
431 time_t &timep
432 OUTPUT:
433 timep sv_setnv(ST(1), (double)timep);
434 xsubpp emits an automatic "SvSETMAGIC()" for all parameters in the
436 OUTPUT section of the XSUB, except RETVAL. This is the usually desired
437 behavior, as it takes care of properly invoking 'set' magic on output
438 parameters (needed for hash or array element parameters that must be
439 created if they didn't exist). If for some reason, this behavior is
440 not desired, the OUTPUT section may contain a "SETMAGIC: DISABLE" line
441 to disable it for the remainder of the parameters in the OUTPUT
442 section. Likewise, "SETMAGIC: ENABLE" can be used to reenable it for
443 the remainder of the OUTPUT section. See perlguts for more details
444 about 'set' magic.
445 The NO_OUTPUT Keyword
447 The NO_OUTPUT can be placed as the first token of the XSUB. This
448 keyword indicates that while the C subroutine we provide an interface
449 to has a non-"void" return type, the return value of this C subroutine
450 should not be returned from the generated Perl subroutine.
451 With this keyword present "The RETVAL Variable" is created, and in the
453 generated call to the subroutine this variable is assigned to, but the
454 value of this variable is not going to be used in the auto-generated
455 code.
456 This keyword makes sense only if "RETVAL" is going to be accessed by
458 the user-supplied code. It is especially useful to make a function
459 interface more Perl-like, especially when the C return value is just an
460 error condition indicator. For example,
461 NO_OUTPUT int
463 delete_file(char *name)
464 POSTCALL:
465 if (RETVAL != 0)
466 croak("Error %d while deleting file '%s'", RETVAL, name);
467 Here the generated XS function returns nothing on success, and will
469 die() with a meaningful error message on error.
470 The CODE: Keyword
472 This keyword is used in more complicated XSUBs which require special
473 handling for the C function. The RETVAL variable is still declared,
474 but it will not be returned unless it is specified in the OUTPUT:
475 section.
476 The following XSUB is for a C function which requires special handling
478 of its parameters. The Perl usage is given first.
479 $status = rpcb_gettime( "localhost", $timep );
481 The XSUB follows.
483 bool_t
485 rpcb_gettime(host,timep)
486 char *host
487 time_t timep
488 CODE:
489 RETVAL = rpcb_gettime( host, &timep );
490 OUTPUT:
491 timep
492 RETVAL
493 The INIT: Keyword
495 The INIT: keyword allows initialization to be inserted into the XSUB
496 before the compiler generates the call to the C function. Unlike the
497 CODE: keyword above, this keyword does not affect the way the compiler
498 handles RETVAL.
499 bool_t
501 rpcb_gettime(host,timep)
502 char *host
503 time_t &timep
504 INIT:
505 printf("# Host is %s\n", host );
506 OUTPUT:
507 timep
508 Another use for the INIT: section is to check for preconditions before
510 making a call to the C function:
511 long long
513 lldiv(a,b)
514 long long a
515 long long b
516 INIT:
517 if (a == 0 && b == 0)
518 XSRETURN_UNDEF;
519 if (b == 0)
520 croak("lldiv: cannot divide by 0");
521 The NO_INIT Keyword
523 The NO_INIT keyword is used to indicate that a function parameter is
524 being used only as an output value. The xsubpp compiler will normally
525 generate code to read the values of all function parameters from the
526 argument stack and assign them to C variables upon entry to the
527 function. NO_INIT will tell the compiler that some parameters will be
528 used for output rather than for input and that they will be handled
529 before the function terminates.
530 The following example shows a variation of the rpcb_gettime() function.
532 This function uses the timep variable only as an output variable and
533 does not care about its initial contents.
534 bool_t
536 rpcb_gettime(host,timep)
537 char *host
538 time_t &timep = NO_INIT
539 OUTPUT:
540 timep
541 The TYPEMAP: Keyword
543 Starting with Perl 5.16, you can embed typemaps into your XS code
544 instead of or in addition to typemaps in a separate file. Multiple
545 such embedded typemaps will be processed in order of appearance in the
546 XS code and like local typemap files take precedence over the default
547 typemap, the embedded typemaps may overwrite previous definitions of
548 TYPEMAP, INPUT, and OUTPUT stanzas. The syntax for embedded typemaps
549 is
550 TYPEMAP: <<HERE
552 ... your typemap code here ...
553 HERE
554 where the "TYPEMAP" keyword must appear in the first column of a new
556 line.
557 Refer to perlxstypemap for details on writing typemaps.
559 Initializing Function Parameters
561 C function parameters are normally initialized with their values from
562 the argument stack (which in turn contains the parameters that were
563 passed to the XSUB from Perl). The typemaps contain the code segments
564 which are used to translate the Perl values to the C parameters. The
565 programmer, however, is allowed to override the typemaps and supply
566 alternate (or additional) initialization code. Initialization code
567 starts with the first "=", ";" or "+" on a line in the INPUT: section.
568 The only exception happens if this ";" terminates the line, then this
569 ";" is quietly ignored.
570 The following code demonstrates how to supply initialization code for
572 function parameters. The initialization code is eval'ed within double
573 quotes by the compiler before it is added to the output so anything
574 which should be interpreted literally [mainly "$", "@", or "\\"] must
575 be protected with backslashes. The variables $var, $arg, and $type can
576 be used as in typemaps.
577 bool_t
579 rpcb_gettime(host,timep)
580 char *host = (char *)SvPV_nolen($arg);
581 time_t &timep = 0;
582 OUTPUT:
583 timep
584 This should not be used to supply default values for parameters. One
586 would normally use this when a function parameter must be processed by
587 another library function before it can be used. Default parameters are
588 covered in the next section.
589 If the initialization begins with "=", then it is output in the
591 declaration for the input variable, replacing the initialization
592 supplied by the typemap. If the initialization begins with ";" or "+",
593 then it is performed after all of the input variables have been
594 declared. In the ";" case the initialization normally supplied by the
595 typemap is not performed. For the "+" case, the declaration for the
596 variable will include the initialization from the typemap. A global
597 variable, %v, is available for the truly rare case where information
598 from one initialization is needed in another initialization.
599 Here's a truly obscure example:
601 bool_t
603 rpcb_gettime(host,timep)
604 time_t &timep; /* \$v{timep}=@{[$v{timep}=$arg]} */
605 char *host + SvOK($v{timep}) ? SvPV_nolen($arg) : NULL;
606 OUTPUT:
607 timep
608 The construct "\$v{timep}=@{[$v{timep}=$arg]}" used in the above
610 example has a two-fold purpose: first, when this line is processed by
611 xsubpp, the Perl snippet "$v{timep}=$arg" is evaluated. Second, the
612 text of the evaluated snippet is output into the generated C file
613 (inside a C comment)! During the processing of "char *host" line, $arg
614 will evaluate to ST(0), and $v{timep} will evaluate to ST(1).
615 Default Parameter Values
617 Default values for XSUB arguments can be specified by placing an
618 assignment statement in the parameter list. The default value may be a
619 number, a string or the special string "NO_INIT". Defaults should
620 always be used on the right-most parameters only.
621 To allow the XSUB for rpcb_gettime() to have a default host value the
623 parameters to the XSUB could be rearranged. The XSUB will then call
624 the real rpcb_gettime() function with the parameters in the correct
625 order. This XSUB can be called from Perl with either of the following
626 statements:
627 $status = rpcb_gettime( $timep, $host );
629 $status = rpcb_gettime( $timep );
631 The XSUB will look like the code which follows. A CODE: block is
633 used to call the real rpcb_gettime() function with the parameters in
634 the correct order for that function.
635 bool_t
637 rpcb_gettime(timep,host="localhost")
638 char *host
639 time_t timep = NO_INIT
640 CODE:
641 RETVAL = rpcb_gettime( host, &timep );
642 OUTPUT:
643 timep
644 RETVAL
645 The PREINIT: Keyword
647 The PREINIT: keyword allows extra variables to be declared immediately
648 before or after the declarations of the parameters from the INPUT:
649 section are emitted.
650 If a variable is declared inside a CODE: section it will follow any
652 typemap code that is emitted for the input parameters. This may result
653 in the declaration ending up after C code, which is C syntax error.
654 Similar errors may happen with an explicit ";"-type or "+"-type
655 initialization of parameters is used (see "Initializing Function
656 Parameters"). Declaring these variables in an INIT: section will not
657 help.
658 In such cases, to force an additional variable to be declared together
660 with declarations of other variables, place the declaration into a
661 PREINIT: section. The PREINIT: keyword may be used one or more times
662 within an XSUB.
663 The following examples are equivalent, but if the code is using complex
665 typemaps then the first example is safer.
666 bool_t
668 rpcb_gettime(timep)
669 time_t timep = NO_INIT
670 PREINIT:
671 char *host = "localhost";
672 CODE:
673 RETVAL = rpcb_gettime( host, &timep );
674 OUTPUT:
675 timep
676 RETVAL
677 For this particular case an INIT: keyword would generate the same C
679 code as the PREINIT: keyword. Another correct, but error-prone
680 example:
681 bool_t
683 rpcb_gettime(timep)
684 time_t timep = NO_INIT
685 CODE:
686 char *host = "localhost";
687 RETVAL = rpcb_gettime( host, &timep );
688 OUTPUT:
689 timep
690 RETVAL
691 Another way to declare "host" is to use a C block in the CODE: section:
693 bool_t
695 rpcb_gettime(timep)
696 time_t timep = NO_INIT
697 CODE:
698 {
699 char *host = "localhost";
700 RETVAL = rpcb_gettime( host, &timep );
701 }
702 OUTPUT:
703 timep
704 RETVAL
705 The ability to put additional declarations before the typemap entries
707 are processed is very handy in the cases when typemap conversions
708 manipulate some global state:
709 MyObject
711 mutate(o)
712 PREINIT:
713 MyState st = global_state;
714 INPUT:
715 MyObject o;
716 CLEANUP:
717 reset_to(global_state, st);
718 Here we suppose that conversion to "MyObject" in the INPUT: section and
720 from MyObject when processing RETVAL will modify a global variable
721 "global_state". After these conversions are performed, we restore the
722 old value of "global_state" (to avoid memory leaks, for example).
723 There is another way to trade clarity for compactness: INPUT sections
725 allow declaration of C variables which do not appear in the parameter
726 list of a subroutine. Thus the above code for mutate() can be
727 rewritten as
728 MyObject
730 mutate(o)
731 MyState st = global_state;
732 MyObject o;
733 CLEANUP:
734 reset_to(global_state, st);
735 and the code for rpcb_gettime() can be rewritten as
737 bool_t
739 rpcb_gettime(timep)
740 time_t timep = NO_INIT
741 char *host = "localhost";
742 C_ARGS:
743 host, &timep
744 OUTPUT:
745 timep
746 RETVAL
747 The SCOPE: Keyword
749 The SCOPE: keyword allows scoping to be enabled for a particular XSUB.
750 If enabled, the XSUB will invoke ENTER and LEAVE automatically.
751 To support potentially complex type mappings, if a typemap entry used
753 by an XSUB contains a comment like "/*scope*/" then scoping will be
754 automatically enabled for that XSUB.
755 To enable scoping:
757 SCOPE: ENABLE
759 To disable scoping:
761 SCOPE: DISABLE
763 The INPUT: Keyword
765 The XSUB's parameters are usually evaluated immediately after entering
766 the XSUB. The INPUT: keyword can be used to force those parameters to
767 be evaluated a little later. The INPUT: keyword can be used multiple
768 times within an XSUB and can be used to list one or more input
769 variables. This keyword is used with the PREINIT: keyword.
770 The following example shows how the input parameter "timep" can be
772 evaluated late, after a PREINIT.
773 bool_t
775 rpcb_gettime(host,timep)
776 char *host
777 PREINIT:
778 time_t tt;
779 INPUT:
780 time_t timep
781 CODE:
782 RETVAL = rpcb_gettime( host, &tt );
783 timep = tt;
784 OUTPUT:
785 timep
786 RETVAL
787 The next example shows each input parameter evaluated late.
789 bool_t
791 rpcb_gettime(host,timep)
792 PREINIT:
793 time_t tt;
794 INPUT:
795 char *host
796 PREINIT:
797 char *h;
798 INPUT:
799 time_t timep
800 CODE:
801 h = host;
802 RETVAL = rpcb_gettime( h, &tt );
803 timep = tt;
804 OUTPUT:
805 timep
806 RETVAL
807 Since INPUT sections allow declaration of C variables which do not
809 appear in the parameter list of a subroutine, this may be shortened to:
810 bool_t
812 rpcb_gettime(host,timep)
813 time_t tt;
814 char *host;
815 char *h = host;
816 time_t timep;
817 CODE:
818 RETVAL = rpcb_gettime( h, &tt );
819 timep = tt;
820 OUTPUT:
821 timep
822 RETVAL
823 (We used our knowledge that input conversion for "char *" is a "simple"
825 one, thus "host" is initialized on the declaration line, and our
826 assignment "h = host" is not performed too early. Otherwise one would
827 need to have the assignment "h = host" in a CODE: or INIT: section.)
828 The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
830 In the list of parameters for an XSUB, one can precede parameter names
831 by the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords. "IN"
832 keyword is the default, the other keywords indicate how the Perl
833 interface should differ from the C interface.
834 Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords
836 are considered to be used by the C subroutine via pointers.
837 "OUTLIST"/"OUT" keywords indicate that the C subroutine does not
838 inspect the memory pointed by this parameter, but will write through
839 this pointer to provide additional return values.
840 Parameters preceded by "OUTLIST" keyword do not appear in the usage
842 signature of the generated Perl function.
843 Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do appear as
845 parameters to the Perl function. With the exception of
846 "OUT"-parameters, these parameters are converted to the corresponding C
847 type, then pointers to these data are given as arguments to the C
848 function. It is expected that the C function will write through these
849 pointers.
850 The return list of the generated Perl function consists of the C return
852 value from the function (unless the XSUB is of "void" return type or
853 "The NO_OUTPUT Keyword" was used) followed by all the "OUTLIST" and
854 "IN_OUTLIST" parameters (in the order of appearance). On the return
855 from the XSUB the "IN_OUT"/"OUT" Perl parameter will be modified to
856 have the values written by the C function.
857 For example, an XSUB
859 void
861 day_month(OUTLIST day, IN unix_time, OUTLIST month)
862 int day
863 int unix_time
864 int month
865 should be used from Perl as
867 my ($day, $month) = day_month(time);
869 The C signature of the corresponding function should be
871 void day_month(int *day, int unix_time, int *month);
873 The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords can be mixed
875 with ANSI-style declarations, as in
876 void
878 day_month(OUTLIST int day, int unix_time, OUTLIST int month)
879 (here the optional "IN" keyword is omitted).
881 The "IN_OUT" parameters are identical with parameters introduced with
883 "The & Unary Operator" and put into the "OUTPUT:" section (see "The
884 OUTPUT: Keyword"). The "IN_OUTLIST" parameters are very similar, the
885 only difference being that the value C function writes through the
886 pointer would not modify the Perl parameter, but is put in the output
887 list.
888 The "OUTLIST"/"OUT" parameter differ from "IN_OUTLIST"/"IN_OUT"
890 parameters only by the initial value of the Perl parameter not being
891 read (and not being given to the C function - which gets some garbage
892 instead). For example, the same C function as above can be interfaced
893 with as
894 void day_month(OUT int day, int unix_time, OUT int month);
896 or
898 void
900 day_month(day, unix_time, month)
901 int &day = NO_INIT
902 int unix_time
903 int &month = NO_INIT
904 OUTPUT:
905 day
906 month
907 However, the generated Perl function is called in very C-ish style:
909 my ($day, $month);
911 day_month($day, time, $month);
912 The "length(NAME)" Keyword
914 If one of the input arguments to the C function is the length of a
915 string argument "NAME", one can substitute the name of the length-
916 argument by "length(NAME)" in the XSUB declaration. This argument must
917 be omitted when the generated Perl function is called. E.g.,
918 void
920 dump_chars(char *s, short l)
921 {
922 short n = 0;
923 while (n < l) {
924 printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
925 n++;
926 }
927 }
928 MODULE = x PACKAGE = x
930 void dump_chars(char *s, short length(s))
932 should be called as "dump_chars($string)".
934 This directive is supported with ANSI-type function declarations only.
936 Variable-length Parameter Lists
938 XSUBs can have variable-length parameter lists by specifying an
939 ellipsis "(...)" in the parameter list. This use of the ellipsis is
940 similar to that found in ANSI C. The programmer is able to determine
941 the number of arguments passed to the XSUB by examining the "items"
942 variable which the xsubpp compiler supplies for all XSUBs. By using
943 this mechanism one can create an XSUB which accepts a list of
944 parameters of unknown length.
945 The host parameter for the rpcb_gettime() XSUB can be optional so the
947 ellipsis can be used to indicate that the XSUB will take a variable
948 number of parameters. Perl should be able to call this XSUB with
949 either of the following statements.
950 $status = rpcb_gettime( $timep, $host );
952 $status = rpcb_gettime( $timep );
954 The XS code, with ellipsis, follows.
956 bool_t
958 rpcb_gettime(timep, ...)
959 time_t timep = NO_INIT
960 PREINIT:
961 char *host = "localhost";
962 CODE:
963 if( items > 1 )
964 host = (char *)SvPV_nolen(ST(1));
965 RETVAL = rpcb_gettime( host, &timep );
966 OUTPUT:
967 timep
968 RETVAL
969 The C_ARGS: Keyword
971 The C_ARGS: keyword allows creating of XSUBS which have different
972 calling sequence from Perl than from C, without a need to write CODE:
973 or PPCODE: section. The contents of the C_ARGS: paragraph is put as
974 the argument to the called C function without any change.
975 For example, suppose that a C function is declared as
977 symbolic nth_derivative(int n, symbolic function, int flags);
979 and that the default flags are kept in a global C variable
981 "default_flags". Suppose that you want to create an interface which is
982 called as
983 $second_deriv = $function->nth_derivative(2);
985 To do this, declare the XSUB as
987 symbolic
989 nth_derivative(function, n)
990 symbolic function
991 int n
992 C_ARGS:
993 n, function, default_flags
994 The PPCODE: Keyword
996 The PPCODE: keyword is an alternate form of the CODE: keyword and is
997 used to tell the xsubpp compiler that the programmer is supplying the
998 code to control the argument stack for the XSUBs return values.
999 Occasionally one will want an XSUB to return a list of values rather
1000 than a single value. In these cases one must use PPCODE: and then
1001 explicitly push the list of values on the stack. The PPCODE: and CODE:
1002 keywords should not be used together within the same XSUB.
1003 The actual difference between PPCODE: and CODE: sections is in the
1005 initialization of "SP" macro (which stands for the current Perl stack
1006 pointer), and in the handling of data on the stack when returning from
1007 an XSUB. In CODE: sections SP preserves the value which was on entry
1008 to the XSUB: SP is on the function pointer (which follows the last
1009 parameter). In PPCODE: sections SP is moved backward to the beginning
1010 of the parameter list, which allows "PUSH*()" macros to place output
1011 values in the place Perl expects them to be when the XSUB returns back
1012 to Perl.
1013 The generated trailer for a CODE: section ensures that the number of
1015 return values Perl will see is either 0 or 1 (depending on the
1016 "void"ness of the return value of the C function, and heuristics
1017 mentioned in "The RETVAL Variable"). The trailer generated for a
1018 PPCODE: section is based on the number of return values and on the
1019 number of times "SP" was updated by "[X]PUSH*()" macros.
1020 Note that macros ST(i), "XST_m*()" and "XSRETURN*()" work equally well
1022 in CODE: sections and PPCODE: sections.
1023 The following XSUB will call the C rpcb_gettime() function and will
1025 return its two output values, timep and status, to Perl as a single
1026 list.
1027 void
1029 rpcb_gettime(host)
1030 char *host
1031 PREINIT:
1032 time_t timep;
1033 bool_t status;
1034 PPCODE:
1035 status = rpcb_gettime( host, &timep );
1036 EXTEND(SP, 2);
1037 PUSHs(sv_2mortal(newSViv(status)));
1038 PUSHs(sv_2mortal(newSViv(timep)));
1039 Notice that the programmer must supply the C code necessary to have the
1041 real rpcb_gettime() function called and to have the return values
1042 properly placed on the argument stack.
1043 The "void" return type for this function tells the xsubpp compiler that
1045 the RETVAL variable is not needed or used and that it should not be
1046 created. In most scenarios the void return type should be used with
1047 the PPCODE: directive.
1048 The EXTEND() macro is used to make room on the argument stack for 2
1050 return values. The PPCODE: directive causes the xsubpp compiler to
1051 create a stack pointer available as "SP", and it is this pointer which
1052 is being used in the EXTEND() macro. The values are then pushed onto
1053 the stack with the PUSHs() macro.
1054 Now the rpcb_gettime() function can be used from Perl with the
1056 following statement.
1057 ($status, $timep) = rpcb_gettime("localhost");
1059 When handling output parameters with a PPCODE section, be sure to
1061 handle 'set' magic properly. See perlguts for details about 'set'
1062 magic.
1063 Returning Undef And Empty Lists
1065 Occasionally the programmer will want to return simply "undef" or an
1066 empty list if a function fails rather than a separate status value.
1067 The rpcb_gettime() function offers just this situation. If the
1068 function succeeds we would like to have it return the time and if it
1069 fails we would like to have undef returned. In the following Perl code
1070 the value of $timep will either be undef or it will be a valid time.
1071 $timep = rpcb_gettime( "localhost" );
1073 The following XSUB uses the "SV *" return type as a mnemonic only, and
1075 uses a CODE: block to indicate to the compiler that the programmer has
1076 supplied all the necessary code. The sv_newmortal() call will
1077 initialize the return value to undef, making that the default return
1078 value.
1079 SV *
1081 rpcb_gettime(host)
1082 char * host
1083 PREINIT:
1084 time_t timep;
1085 bool_t x;
1086 CODE:
1087 ST(0) = sv_newmortal();
1088 if( rpcb_gettime( host, &timep ) )
1089 sv_setnv( ST(0), (double)timep);
1090 The next example demonstrates how one would place an explicit undef in
1092 the return value, should the need arise.
1093 SV *
1095 rpcb_gettime(host)
1096 char * host
1097 PREINIT:
1098 time_t timep;
1099 bool_t x;
1100 CODE:
1101 if( rpcb_gettime( host, &timep ) ){
1102 ST(0) = sv_newmortal();
1103 sv_setnv( ST(0), (double)timep);
1104 }
1105 else{
1106 ST(0) = &PL_sv_undef;
1107 }
1108 To return an empty list one must use a PPCODE: block and then not push
1110 return values on the stack.
1111 void
1113 rpcb_gettime(host)
1114 char *host
1115 PREINIT:
1116 time_t timep;
1117 PPCODE:
1118 if( rpcb_gettime( host, &timep ) )
1119 PUSHs(sv_2mortal(newSViv(timep)));
1120 else{
1121 /* Nothing pushed on stack, so an empty
1122 * list is implicitly returned. */
1123 }
1124 Some people may be inclined to include an explicit "return" in the
1126 above XSUB, rather than letting control fall through to the end. In
1127 those situations "XSRETURN_EMPTY" should be used, instead. This will
1128 ensure that the XSUB stack is properly adjusted. Consult perlapi for
1129 other "XSRETURN" macros.
1130 Since "XSRETURN_*" macros can be used with CODE blocks as well, one can
1132 rewrite this example as:
1133 int
1135 rpcb_gettime(host)
1136 char *host
1137 PREINIT:
1138 time_t timep;
1139 CODE:
1140 RETVAL = rpcb_gettime( host, &timep );
1141 if (RETVAL == 0)
1142 XSRETURN_UNDEF;
1143 OUTPUT:
1144 RETVAL
1145 In fact, one can put this check into a POSTCALL: section as well.
1147 Together with PREINIT: simplifications, this leads to:
1148 int
1150 rpcb_gettime(host)
1151 char *host
1152 time_t timep;
1153 POSTCALL:
1154 if (RETVAL == 0)
1155 XSRETURN_UNDEF;
1156 The REQUIRE: Keyword
1158 The REQUIRE: keyword is used to indicate the minimum version of the
1159 xsubpp compiler needed to compile the XS module. An XS module which
1160 contains the following statement will compile with only xsubpp version
1161 1.922 or greater:
1162 REQUIRE: 1.922
1164 The CLEANUP: Keyword
1166 This keyword can be used when an XSUB requires special cleanup
1167 procedures before it terminates. When the CLEANUP: keyword is used it
1168 must follow any CODE:, or OUTPUT: blocks which are present in the XSUB.
1169 The code specified for the cleanup block will be added as the last
1170 statements in the XSUB.
1171 The POSTCALL: Keyword
1173 This keyword can be used when an XSUB requires special procedures
1174 executed after the C subroutine call is performed. When the POSTCALL:
1175 keyword is used it must precede OUTPUT: and CLEANUP: blocks which are
1176 present in the XSUB.
1177 See examples in "The NO_OUTPUT Keyword" and "Returning Undef And Empty
1179 Lists".
1180 The POSTCALL: block does not make a lot of sense when the C subroutine
1182 call is supplied by user by providing either CODE: or PPCODE: section.
1183 The BOOT: Keyword
1185 The BOOT: keyword is used to add code to the extension's bootstrap
1186 function. The bootstrap function is generated by the xsubpp compiler
1187 and normally holds the statements necessary to register any XSUBs with
1188 Perl. With the BOOT: keyword the programmer can tell the compiler to
1189 add extra statements to the bootstrap function.
1190 This keyword may be used any time after the first MODULE keyword and
1192 should appear on a line by itself. The first blank line after the
1193 keyword will terminate the code block.
1194 BOOT:
1196 # The following message will be printed when the
1197 # bootstrap function executes.
1198 printf("Hello from the bootstrap!\n");
1199 The VERSIONCHECK: Keyword
1201 The VERSIONCHECK: keyword corresponds to xsubpp's "-versioncheck" and
1202 "-noversioncheck" options. This keyword overrides the command line
1203 options. Version checking is enabled by default. When version
1204 checking is enabled the XS module will attempt to verify that its
1205 version matches the version of the PM module.
1206 To enable version checking:
1208 VERSIONCHECK: ENABLE
1210 To disable version checking:
1212 VERSIONCHECK: DISABLE
1214 Note that if the version of the PM module is an NV (a floating point
1216 number), it will be stringified with a possible loss of precision
1217 (currently chopping to nine decimal places) so that it may not match
1218 the version of the XS module anymore. Quoting the $VERSION declaration
1219 to make it a string is recommended if long version numbers are used.
1220 The PROTOTYPES: Keyword
1222 The PROTOTYPES: keyword corresponds to xsubpp's "-prototypes" and
1223 "-noprototypes" options. This keyword overrides the command line
1224 options. Prototypes are disabled by default. When prototypes are
1225 enabled, XSUBs will be given Perl prototypes. This keyword may be used
1226 multiple times in an XS module to enable and disable prototypes for
1227 different parts of the module. Note that xsubpp will nag you if you
1228 don't explicitly enable or disable prototypes, with:
1229 Please specify prototyping behavior for Foo.xs (see perlxs manual)
1231 To enable prototypes:
1233 PROTOTYPES: ENABLE
1235 To disable prototypes:
1237 PROTOTYPES: DISABLE
1239 The PROTOTYPE: Keyword
1241 This keyword is similar to the PROTOTYPES: keyword above but can be
1242 used to force xsubpp to use a specific prototype for the XSUB. This
1243 keyword overrides all other prototype options and keywords but affects
1244 only the current XSUB. Consult "Prototypes" in perlsub for information
1245 about Perl prototypes.
1246 bool_t
1248 rpcb_gettime(timep, ...)
1249 time_t timep = NO_INIT
1250 PROTOTYPE: $;$
1251 PREINIT:
1252 char *host = "localhost";
1253 CODE:
1254 if( items > 1 )
1255 host = (char *)SvPV_nolen(ST(1));
1256 RETVAL = rpcb_gettime( host, &timep );
1257 OUTPUT:
1258 timep
1259 RETVAL
1260 If the prototypes are enabled, you can disable it locally for a given
1262 XSUB as in the following example:
1263 void
1265 rpcb_gettime_noproto()
1266 PROTOTYPE: DISABLE
1267 ...
1268 The ALIAS: Keyword
1270 The ALIAS: keyword allows an XSUB to have two or more unique Perl names
1271 and to know which of those names was used when it was invoked. The
1272 Perl names may be fully-qualified with package names. Each alias is
1273 given an index. The compiler will setup a variable called "ix" which
1274 contain the index of the alias which was used. When the XSUB is called
1275 with its declared name "ix" will be 0.
1276 The following example will create aliases "FOO::gettime()" and
1278 "BAR::getit()" for this function.
1279 bool_t
1281 rpcb_gettime(host,timep)
1282 char *host
1283 time_t &timep
1284 ALIAS:
1285 FOO::gettime = 1
1286 BAR::getit = 2
1287 INIT:
1288 printf("# ix = %d\n", ix );
1289 OUTPUT:
1290 timep
1291 The OVERLOAD: Keyword
1293 Instead of writing an overloaded interface using pure Perl, you can
1294 also use the OVERLOAD keyword to define additional Perl names for your
1295 functions (like the ALIAS: keyword above). However, the overloaded
1296 functions must be defined with three parameters (except for the
1297 nomethod() function which needs four parameters). If any function has
1298 the OVERLOAD: keyword, several additional lines will be defined in the
1299 c file generated by xsubpp in order to register with the overload
1300 magic.
1301 Since blessed objects are actually stored as RV's, it is useful to use
1303 the typemap features to preprocess parameters and extract the actual SV
1304 stored within the blessed RV. See the sample for T_PTROBJ_SPECIAL
1305 below.
1306 To use the OVERLOAD: keyword, create an XS function which takes three
1308 input parameters ( or use the c style '...' definition) like this:
1309 SV *
1311 cmp (lobj, robj, swap)
1312 My_Module_obj lobj
1313 My_Module_obj robj
1314 IV swap
1315 OVERLOAD: cmp <=>
1316 { /* function defined here */}
1317 In this case, the function will overload both of the three way
1319 comparison operators. For all overload operations using non-alpha
1320 characters, you must type the parameter without quoting, separating
1321 multiple overloads with whitespace. Note that "" (the stringify
1322 overload) should be entered as \"\" (i.e. escaped).
1323 The FALLBACK: Keyword
1325 In addition to the OVERLOAD keyword, if you need to control how Perl
1326 autogenerates missing overloaded operators, you can set the FALLBACK
1327 keyword in the module header section, like this:
1328 MODULE = RPC PACKAGE = RPC
1330 FALLBACK: TRUE
1332 ...
1333 where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF.
1335 If you do not set any FALLBACK value when using OVERLOAD, it defaults
1336 to UNDEF. FALLBACK is not used except when one or more functions using
1337 OVERLOAD have been defined. Please see "fallback" in overload for more
1338 details.
1339 The INTERFACE: Keyword
1341 This keyword declares the current XSUB as a keeper of the given calling
1342 signature. If some text follows this keyword, it is considered as a
1343 list of functions which have this signature, and should be attached to
1344 the current XSUB.
1345 For example, if you have 4 C functions multiply(), divide(), add(),
1347 subtract() all having the signature:
1348 symbolic f(symbolic, symbolic);
1350 you can make them all to use the same XSUB using this:
1352 symbolic
1354 interface_s_ss(arg1, arg2)
1355 symbolic arg1
1356 symbolic arg2
1357 INTERFACE:
1358 multiply divide
1359 add subtract
1360 (This is the complete XSUB code for 4 Perl functions!) Four generated
1362 Perl function share names with corresponding C functions.
1363 The advantage of this approach comparing to ALIAS: keyword is that
1365 there is no need to code a switch statement, each Perl function (which
1366 shares the same XSUB) knows which C function it should call.
1367 Additionally, one can attach an extra function remainder() at runtime
1368 by using
1369 CV *mycv = newXSproto("Symbolic::remainder",
1371 XS_Symbolic_interface_s_ss, __FILE__, "$$");
1372 XSINTERFACE_FUNC_SET(mycv, remainder);
1373 say, from another XSUB. (This example supposes that there was no
1375 INTERFACE_MACRO: section, otherwise one needs to use something else
1376 instead of "XSINTERFACE_FUNC_SET", see the next section.)
1377 The INTERFACE_MACRO: Keyword
1379 This keyword allows one to define an INTERFACE using a different way to
1380 extract a function pointer from an XSUB. The text which follows this
1381 keyword should give the name of macros which would extract/set a
1382 function pointer. The extractor macro is given return type, "CV*", and
1383 "XSANY.any_dptr" for this "CV*". The setter macro is given cv, and the
1384 function pointer.
1385 The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET". An
1387 INTERFACE keyword with an empty list of functions can be omitted if
1388 INTERFACE_MACRO keyword is used.
1389 Suppose that in the previous example functions pointers for multiply(),
1391 divide(), add(), subtract() are kept in a global C array "fp[]" with
1392 offsets being "multiply_off", "divide_off", "add_off", "subtract_off".
1393 Then one can use
1394 #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
1396 ((XSINTERFACE_CVT_ANON(ret))fp[CvXSUBANY(cv).any_i32])
1397 #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
1398 CvXSUBANY(cv).any_i32 = CAT2( f, _off )
1399 in C section,
1401 symbolic
1403 interface_s_ss(arg1, arg2)
1404 symbolic arg1
1405 symbolic arg2
1406 INTERFACE_MACRO:
1407 XSINTERFACE_FUNC_BYOFFSET
1408 XSINTERFACE_FUNC_BYOFFSET_set
1409 INTERFACE:
1410 multiply divide
1411 add subtract
1412 in XSUB section.
1414 The INCLUDE: Keyword
1416 This keyword can be used to pull other files into the XS module. The
1417 other files may have XS code. INCLUDE: can also be used to run a
1418 command to generate the XS code to be pulled into the module.
1419 The file Rpcb1.xsh contains our "rpcb_gettime()" function:
1421 bool_t
1423 rpcb_gettime(host,timep)
1424 char *host
1425 time_t &timep
1426 OUTPUT:
1427 timep
1428 The XS module can use INCLUDE: to pull that file into it.
1430 INCLUDE: Rpcb1.xsh
1432 If the parameters to the INCLUDE: keyword are followed by a pipe ("|")
1434 then the compiler will interpret the parameters as a command. This
1435 feature is mildly deprecated in favour of the "INCLUDE_COMMAND:"
1436 directive, as documented below.
1437 INCLUDE: cat Rpcb1.xsh |
1439 Do not use this to run perl: "INCLUDE: perl |" will run the perl that
1441 happens to be the first in your path and not necessarily the same perl
1442 that is used to run "xsubpp". See "The INCLUDE_COMMAND: Keyword".
1443 The INCLUDE_COMMAND: Keyword
1445 Runs the supplied command and includes its output into the current XS
1446 document. "INCLUDE_COMMAND" assigns special meaning to the $^X token in
1447 that it runs the same perl interpreter that is running "xsubpp":
1448 INCLUDE_COMMAND: cat Rpcb1.xsh
1450 INCLUDE_COMMAND: $^X -e ...
1452 The CASE: Keyword
1454 The CASE: keyword allows an XSUB to have multiple distinct parts with
1455 each part acting as a virtual XSUB. CASE: is greedy and if it is used
1456 then all other XS keywords must be contained within a CASE:. This
1457 means nothing may precede the first CASE: in the XSUB and anything
1458 following the last CASE: is included in that case.
1459 A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS:
1461 variable (see "The ALIAS: Keyword"), or maybe via the "items" variable
1462 (see "Variable-length Parameter Lists"). The last CASE: becomes the
1463 default case if it is not associated with a conditional. The following
1464 example shows CASE switched via "ix" with a function "rpcb_gettime()"
1465 having an alias "x_gettime()". When the function is called as
1466 "rpcb_gettime()" its parameters are the usual "(char *host, time_t
1467 *timep)", but when the function is called as "x_gettime()" its
1468 parameters are reversed, "(time_t *timep, char *host)".
1469 long
1471 rpcb_gettime(a,b)
1472 CASE: ix == 1
1473 ALIAS:
1474 x_gettime = 1
1475 INPUT:
1476 # 'a' is timep, 'b' is host
1477 char *b
1478 time_t a = NO_INIT
1479 CODE:
1480 RETVAL = rpcb_gettime( b, &a );
1481 OUTPUT:
1482 a
1483 RETVAL
1484 CASE:
1485 # 'a' is host, 'b' is timep
1486 char *a
1487 time_t &b = NO_INIT
1488 OUTPUT:
1489 b
1490 RETVAL
1491 That function can be called with either of the following statements.
1493 Note the different argument lists.
1494 $status = rpcb_gettime( $host, $timep );
1496 $status = x_gettime( $timep, $host );
1498 The EXPORT_XSUB_SYMBOLS: Keyword
1500 The EXPORT_XSUB_SYMBOLS: keyword is likely something you will never
1501 need. In perl versions earlier than 5.16.0, this keyword does nothing.
1502 Starting with 5.16, XSUB symbols are no longer exported by default.
1503 That is, they are "static" functions. If you include
1504 EXPORT_XSUB_SYMBOLS: ENABLE
1506 in your XS code, the XSUBs following this line will not be declared
1508 "static". You can later disable this with
1509 EXPORT_XSUB_SYMBOLS: DISABLE
1511 which, again, is the default that you should probably never change.
1513 You cannot use this keyword on versions of perl before 5.16 to make
1514 XSUBs "static".
1515 The & Unary Operator
1517 The "&" unary operator in the INPUT: section is used to tell xsubpp
1518 that it should convert a Perl value to/from C using the C type to the
1519 left of "&", but provide a pointer to this value when the C function is
1520 called.
1521 This is useful to avoid a CODE: block for a C function which takes a
1523 parameter by reference. Typically, the parameter should be not a
1524 pointer type (an "int" or "long" but not an "int*" or "long*").
1525 The following XSUB will generate incorrect C code. The xsubpp compiler
1527 will turn this into code which calls "rpcb_gettime()" with parameters
1528 "(char *host, time_t timep)", but the real "rpcb_gettime()" wants the
1529 "timep" parameter to be of type "time_t*" rather than "time_t".
1530 bool_t
1532 rpcb_gettime(host,timep)
1533 char *host
1534 time_t timep
1535 OUTPUT:
1536 timep
1537 That problem is corrected by using the "&" operator. The xsubpp
1539 compiler will now turn this into code which calls "rpcb_gettime()"
1540 correctly with parameters "(char *host, time_t *timep)". It does this
1541 by carrying the "&" through, so the function call looks like
1542 "rpcb_gettime(host, &timep)".
1543 bool_t
1545 rpcb_gettime(host,timep)
1546 char *host
1547 time_t &timep
1548 OUTPUT:
1549 timep
1550 Inserting POD, Comments and C Preprocessor Directives
1552 C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
1553 CODE:, PPCODE:, POSTCALL:, and CLEANUP: blocks, as well as outside the
1554 functions. Comments are allowed anywhere after the MODULE keyword.
1555 The compiler will pass the preprocessor directives through untouched
1556 and will remove the commented lines. POD documentation is allowed at
1557 any point, both in the C and XS language sections. POD must be
1558 terminated with a "=cut" command; "xsubpp" will exit with an error if
1559 it does not. It is very unlikely that human generated C code will be
1560 mistaken for POD, as most indenting styles result in whitespace in
1561 front of any line starting with "=". Machine generated XS files may
1562 fall into this trap unless care is taken to ensure that a space breaks
1563 the sequence "\n=".
1564 Comments can be added to XSUBs by placing a "#" as the first non-
1566 whitespace of a line. Care should be taken to avoid making the comment
1567 look like a C preprocessor directive, lest it be interpreted as such.
1568 The simplest way to prevent this is to put whitespace in front of the
1569 "#".
1570 If you use preprocessor directives to choose one of two versions of a
1572 function, use
1573 #if ... version1
1575 #else /* ... version2 */
1576 #endif
1577 and not
1579 #if ... version1
1581 #endif
1582 #if ... version2
1583 #endif
1584 because otherwise xsubpp will believe that you made a duplicate
1586 definition of the function. Also, put a blank line before the
1587 #else/#endif so it will not be seen as part of the function body.
1588 Using XS With C++
1590 If an XSUB name contains "::", it is considered to be a C++ method.
1591 The generated Perl function will assume that its first argument is an
1592 object pointer. The object pointer will be stored in a variable called
1593 THIS. The object should have been created by C++ with the new()
1594 function and should be blessed by Perl with the sv_setref_pv() macro.
1595 The blessing of the object by Perl can be handled by a typemap. An
1596 example typemap is shown at the end of this section.
1597 If the return type of the XSUB includes "static", the method is
1599 considered to be a static method. It will call the C++ function using
1600 the class::method() syntax. If the method is not static the function
1601 will be called using the THIS->method() syntax.
1602 The next examples will use the following C++ class.
1604 class color {
1606 public:
1607 color();
1608 ~color();
1609 int blue();
1610 void set_blue( int );
1611 private:
1613 int c_blue;
1614 };
1615 The XSUBs for the blue() and set_blue() methods are defined with the
1617 class name but the parameter for the object (THIS, or "self") is
1618 implicit and is not listed.
1619 int
1621 color::blue()
1622 void
1624 color::set_blue( val )
1625 int val
1626 Both Perl functions will expect an object as the first parameter. In
1628 the generated C++ code the object is called "THIS", and the method call
1629 will be performed on this object. So in the C++ code the blue() and
1630 set_blue() methods will be called as this:
1631 RETVAL = THIS->blue();
1633 THIS->set_blue( val );
1635 You could also write a single get/set method using an optional
1637 argument:
1638 int
1640 color::blue( val = NO_INIT )
1641 int val
1642 PROTOTYPE $;$
1643 CODE:
1644 if (items > 1)
1645 THIS->set_blue( val );
1646 RETVAL = THIS->blue();
1647 OUTPUT:
1648 RETVAL
1649 If the function's name is DESTROY then the C++ "delete" function will
1651 be called and "THIS" will be given as its parameter. The generated C++
1652 code for
1653 void
1655 color::DESTROY()
1656 will look like this:
1658 color *THIS = ...; // Initialized as in typemap
1660 delete THIS;
1662 If the function's name is new then the C++ "new" function will be
1664 called to create a dynamic C++ object. The XSUB will expect the class
1665 name, which will be kept in a variable called "CLASS", to be given as
1666 the first argument.
1667 color *
1669 color::new()
1670 The generated C++ code will call "new".
1672 RETVAL = new color();
1674 The following is an example of a typemap that could be used for this
1676 C++ example.
1677 TYPEMAP
1679 color * O_OBJECT
1680 OUTPUT
1682 # The Perl object is blessed into 'CLASS', which should be a
1683 # char* having the name of the package for the blessing.
1684 O_OBJECT
1685 sv_setref_pv( $arg, CLASS, (void*)$var );
1686 INPUT
1688 O_OBJECT
1689 if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
1690 $var = ($type)SvIV((SV*)SvRV( $arg ));
1691 else{
1692 warn("${Package}::$func_name() -- " .
1693 "$var is not a blessed SV reference");
1694 XSRETURN_UNDEF;
1695 }
1696 Interface Strategy
1698 When designing an interface between Perl and a C library a straight
1699 translation from C to XS (such as created by "h2xs -x") is often
1700 sufficient. However, sometimes the interface will look very C-like and
1701 occasionally nonintuitive, especially when the C function modifies one
1702 of its parameters, or returns failure inband (as in "negative return
1703 values mean failure"). In cases where the programmer wishes to create
1704 a more Perl-like interface the following strategy may help to identify
1705 the more critical parts of the interface.
1706 Identify the C functions with input/output or output parameters. The
1708 XSUBs for these functions may be able to return lists to Perl.
1709 Identify the C functions which use some inband info as an indication of
1711 failure. They may be candidates to return undef or an empty list in
1712 case of failure. If the failure may be detected without a call to the
1713 C function, you may want to use an INIT: section to report the failure.
1714 For failures detectable after the C function returns one may want to
1715 use a POSTCALL: section to process the failure. In more complicated
1716 cases use CODE: or PPCODE: sections.
1717 If many functions use the same failure indication based on the return
1719 value, you may want to create a special typedef to handle this
1720 situation. Put
1721 typedef int negative_is_failure;
1723 near the beginning of XS file, and create an OUTPUT typemap entry for
1725 "negative_is_failure" which converts negative values to "undef", or
1726 maybe croak()s. After this the return value of type
1727 "negative_is_failure" will create more Perl-like interface.
1728 Identify which values are used by only the C and XSUB functions
1730 themselves, say, when a parameter to a function should be a contents of
1731 a global variable. If Perl does not need to access the contents of the
1732 value then it may not be necessary to provide a translation for that
1733 value from C to Perl.
1734 Identify the pointers in the C function parameter lists and return
1736 values. Some pointers may be used to implement input/output or output
1737 parameters, they can be handled in XS with the "&" unary operator, and,
1738 possibly, using the NO_INIT keyword. Some others will require handling
1739 of types like "int *", and one needs to decide what a useful Perl
1740 translation will do in such a case. When the semantic is clear, it is
1741 advisable to put the translation into a typemap file.
1742 Identify the structures used by the C functions. In many cases it may
1744 be helpful to use the T_PTROBJ typemap for these structures so they can
1745 be manipulated by Perl as blessed objects. (This is handled
1746 automatically by "h2xs -x".)
1747 If the same C type is used in several different contexts which require
1749 different translations, "typedef" several new types mapped to this C
1750 type, and create separate typemap entries for these new types. Use
1751 these types in declarations of return type and parameters to XSUBs.
1752 Perl Objects And C Structures
1754 When dealing with C structures one should select either T_PTROBJ or
1755 T_PTRREF for the XS type. Both types are designed to handle pointers
1756 to complex objects. The T_PTRREF type will allow the Perl object to be
1757 unblessed while the T_PTROBJ type requires that the object be blessed.
1758 By using T_PTROBJ one can achieve a form of type-checking because the
1759 XSUB will attempt to verify that the Perl object is of the expected
1760 type.
1761 The following XS code shows the getnetconfigent() function which is
1763 used with ONC+ TIRPC. The getnetconfigent() function will return a
1764 pointer to a C structure and has the C prototype shown below. The
1765 example will demonstrate how the C pointer will become a Perl
1766 reference. Perl will consider this reference to be a pointer to a
1767 blessed object and will attempt to call a destructor for the object. A
1768 destructor will be provided in the XS source to free the memory used by
1769 getnetconfigent(). Destructors in XS can be created by specifying an
1770 XSUB function whose name ends with the word DESTROY. XS destructors
1771 can be used to free memory which may have been malloc'd by another
1772 XSUB.
1773 struct netconfig *getnetconfigent(const char *netid);
1775 A "typedef" will be created for "struct netconfig". The Perl object
1777 will be blessed in a class matching the name of the C type, with the
1778 tag "Ptr" appended, and the name should not have embedded spaces if it
1779 will be a Perl package name. The destructor will be placed in a class
1780 corresponding to the class of the object and the PREFIX keyword will be
1781 used to trim the name to the word DESTROY as Perl will expect.
1782 typedef struct netconfig Netconfig;
1784 MODULE = RPC PACKAGE = RPC
1786 Netconfig *
1788 getnetconfigent(netid)
1789 char *netid
1790 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1792 void
1794 rpcb_DESTROY(netconf)
1795 Netconfig *netconf
1796 CODE:
1797 printf("Now in NetconfigPtr::DESTROY\n");
1798 free( netconf );
1799 This example requires the following typemap entry. Consult
1801 perlxstypemap for more information about adding new typemaps for an
1802 extension.
1803 TYPEMAP
1805 Netconfig * T_PTROBJ
1806 This example will be used with the following Perl statements.
1808 use RPC;
1810 $netconf = getnetconfigent("udp");
1811 When Perl destroys the object referenced by $netconf it will send the
1813 object to the supplied XSUB DESTROY function. Perl cannot determine,
1814 and does not care, that this object is a C struct and not a Perl
1815 object. In this sense, there is no difference between the object
1816 created by the getnetconfigent() XSUB and an object created by a normal
1817 Perl subroutine.
1818 Safely Storing Static Data in XS
1820 Starting with Perl 5.8, a macro framework has been defined to allow
1821 static data to be safely stored in XS modules that will be accessed
1822 from a multi-threaded Perl.
1823 Although primarily designed for use with multi-threaded Perl, the
1825 macros have been designed so that they will work with non-threaded Perl
1826 as well.
1827 It is therefore strongly recommended that these macros be used by all
1829 XS modules that make use of static data.
1830 The easiest way to get a template set of macros to use is by specifying
1832 the "-g" ("--global") option with h2xs (see h2xs).
1833 Below is an example module that makes use of the macros.
1835 #define PERL_NO_GET_CONTEXT
1837 #include "EXTERN.h"
1838 #include "perl.h"
1839 #include "XSUB.h"
1840 /* Global Data */
1842 #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION
1844 typedef struct {
1846 int count;
1847 char name[3][100];
1848 } my_cxt_t;
1849 START_MY_CXT
1851 MODULE = BlindMice PACKAGE = BlindMice
1853 BOOT:
1855 {
1856 MY_CXT_INIT;
1857 MY_CXT.count = 0;
1858 strcpy(MY_CXT.name[0], "None");
1859 strcpy(MY_CXT.name[1], "None");
1860 strcpy(MY_CXT.name[2], "None");
1861 }
1862 int
1864 newMouse(char * name)
1865 PREINIT:
1866 dMY_CXT;
1867 CODE:
1868 if (MY_CXT.count >= 3) {
1869 warn("Already have 3 blind mice");
1870 RETVAL = 0;
1871 }
1872 else {
1873 RETVAL = ++ MY_CXT.count;
1874 strcpy(MY_CXT.name[MY_CXT.count - 1], name);
1875 }
1876 OUTPUT:
1877 RETVAL
1878 char *
1880 get_mouse_name(index)
1881 int index
1882 PREINIT:
1883 dMY_CXT;
1884 CODE:
1885 if (index > MY_CXT.count)
1886 croak("There are only 3 blind mice.");
1887 else
1888 RETVAL = MY_CXT.name[index - 1];
1889 OUTPUT:
1890 RETVAL
1891 void
1893 CLONE(...)
1894 CODE:
1895 MY_CXT_CLONE;
1896 MY_CXT REFERENCE
1898 MY_CXT_KEY
1900 This macro is used to define a unique key to refer to the static
1901 data for an XS module. The suggested naming scheme, as used by
1902 h2xs, is to use a string that consists of the module name, the
1903 string "::_guts" and the module version number.
1904 #define MY_CXT_KEY "MyModule::_guts" XS_VERSION
1906 typedef my_cxt_t
1908 This struct typedef must always be called "my_cxt_t". The other
1909 "CXT*" macros assume the existence of the "my_cxt_t" typedef name.
1910 Declare a typedef named "my_cxt_t" that is a structure that
1912 contains all the data that needs to be interpreter-local.
1913 typedef struct {
1915 int some_value;
1916 } my_cxt_t;
1917 START_MY_CXT
1919 Always place the START_MY_CXT macro directly after the declaration
1920 of "my_cxt_t".
1921 MY_CXT_INIT
1923 The MY_CXT_INIT macro initializes storage for the "my_cxt_t"
1924 struct.
1925 It must be called exactly once, typically in a BOOT: section. If
1927 you are maintaining multiple interpreters, it should be called
1928 once in each interpreter instance, except for interpreters cloned
1929 from existing ones. (But see "MY_CXT_CLONE" below.)
1930 dMY_CXT
1932 Use the dMY_CXT macro (a declaration) in all the functions that
1933 access MY_CXT.
1934 MY_CXT
1936 Use the MY_CXT macro to access members of the "my_cxt_t" struct.
1937 For example, if "my_cxt_t" is
1938 typedef struct {
1940 int index;
1941 } my_cxt_t;
1942 then use this to access the "index" member
1944 dMY_CXT;
1946 MY_CXT.index = 2;
1947 aMY_CXT/pMY_CXT
1949 "dMY_CXT" may be quite expensive to calculate, and to avoid the
1950 overhead of invoking it in each function it is possible to pass
1951 the declaration onto other functions using the "aMY_CXT"/"pMY_CXT"
1952 macros, eg
1953 void sub1() {
1955 dMY_CXT;
1956 MY_CXT.index = 1;
1957 sub2(aMY_CXT);
1958 }
1959 void sub2(pMY_CXT) {
1961 MY_CXT.index = 2;
1962 }
1963 Analogously to "pTHX", there are equivalent forms for when the
1965 macro is the first or last in multiple arguments, where an
1966 underscore represents a comma, i.e. "_aMY_CXT", "aMY_CXT_",
1967 "_pMY_CXT" and "pMY_CXT_".
1968 MY_CXT_CLONE
1970 By default, when a new interpreter is created as a copy of an
1971 existing one (eg via "threads->create()"), both interpreters share
1972 the same physical my_cxt_t structure. Calling "MY_CXT_CLONE"
1973 (typically via the package's "CLONE()" function), causes a byte-
1974 for-byte copy of the structure to be taken, and any future dMY_CXT
1975 will cause the copy to be accessed instead.
1976 MY_CXT_INIT_INTERP(my_perl)
1978 dMY_CXT_INTERP(my_perl)
1979 These are versions of the macros which take an explicit
1980 interpreter as an argument.
1981 Note that these macros will only work together within the same source
1983 file; that is, a dMY_CTX in one source file will access a different
1984 structure than a dMY_CTX in another source file.
1985 Thread-aware system interfaces
1987 Starting from Perl 5.8, in C/C++ level Perl knows how to wrap
1988 system/library interfaces that have thread-aware versions (e.g.
1989 getpwent_r()) into frontend macros (e.g. getpwent()) that correctly
1990 handle the multithreaded interaction with the Perl interpreter. This
1991 will happen transparently, the only thing you need to do is to
1992 instantiate a Perl interpreter.
1993 This wrapping happens always when compiling Perl core source (PERL_CORE
1995 is defined) or the Perl core extensions (PERL_EXT is defined). When
1996 compiling XS code outside of Perl core the wrapping does not take
1997 place. Note, however, that intermixing the _r-forms (as Perl compiled
1998 for multithreaded operation will do) and the _r-less forms is neither
1999 well-defined (inconsistent results, data corruption, or even crashes
2000 become more likely), nor is it very portable.
2001
2003 File "RPC.xs": Interface to some ONC+ RPC bind library functions.
2004 #define PERL_NO_GET_CONTEXT
2006 #include "EXTERN.h"
2007 #include "perl.h"
2008 #include "XSUB.h"
2009 #include <rpc/rpc.h>
2011 typedef struct netconfig Netconfig;
2013 MODULE = RPC PACKAGE = RPC
2015 SV *
2017 rpcb_gettime(host="localhost")
2018 char *host
2019 PREINIT:
2020 time_t timep;
2021 CODE:
2022 ST(0) = sv_newmortal();
2023 if( rpcb_gettime( host, &timep ) )
2024 sv_setnv( ST(0), (double)timep );
2025 Netconfig *
2027 getnetconfigent(netid="udp")
2028 char *netid
2029 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
2031 void
2033 rpcb_DESTROY(netconf)
2034 Netconfig *netconf
2035 CODE:
2036 printf("NetconfigPtr::DESTROY\n");
2037 free( netconf );
2038 File "typemap": Custom typemap for RPC.xs. (cf. perlxstypemap)
2040 TYPEMAP
2042 Netconfig * T_PTROBJ
2043 File "RPC.pm": Perl module for the RPC extension.
2045 package RPC;
2047 require Exporter;
2049 require DynaLoader;
2050 @ISA = qw(Exporter DynaLoader);
2051 @EXPORT = qw(rpcb_gettime getnetconfigent);
2052 bootstrap RPC;
2054 1;
2055 File "rpctest.pl": Perl test program for the RPC extension.
2057 use RPC;
2059 $netconf = getnetconfigent();
2061 $a = rpcb_gettime();
2062 print "time = $a\n";
2063 print "netconf = $netconf\n";
2064 $netconf = getnetconfigent("tcp");
2066 $a = rpcb_gettime("poplar");
2067 print "time = $a\n";
2068 print "netconf = $netconf\n";
2069
2071 XS code has full access to system calls including C library functions.
2072 It thus has the capability of interfering with things that the Perl
2073 core or other modules have set up, such as signal handlers or file
2074 handles. It could mess with the memory, or any number of harmful
2075 things. Don't.
2076 Some modules have an event loop, waiting for user-input. It is highly
2078 unlikely that two such modules would work adequately together in a
2079 single Perl application.
2080 In general, the perl interpreter views itself as the center of the
2082 universe as far as the Perl program goes. XS code is viewed as a help-
2083 mate, to accomplish things that perl doesn't do, or doesn't do fast
2084 enough, but always subservient to perl. The closer XS code adheres to
2085 this model, the less likely conflicts will occur.
2086 One area where there has been conflict is in regards to C locales.
2088 (See perllocale.) perl, with one exception and unless told otherwise,
2089 sets up the underlying locale the program is running in to the locale
2090 passed into it from the environment. This is an important difference
2091 from a generic C language program, where the underlying locale is the
2092 "C" locale unless the program changes it. As of v5.20, this underlying
2093 locale is completely hidden from pure perl code outside the lexical
2094 scope of "use locale" except for a couple of function calls in the
2095 POSIX module which of necessity use it. But the underlying locale,
2096 with that one exception is exposed to XS code, affecting all C library
2097 routines whose behavior is locale-dependent. Your XS code better not
2098 assume that the underlying locale is "C". The exception is the
2099 "LC_NUMERIC" locale category, and the reason it is an exception is that
2100 experience has shown that it can be problematic for XS code, whereas we
2101 have not had reports of problems with the other locale categories. And
2102 the reason for this one category being problematic is that the
2103 character used as a decimal point can vary. Many European languages
2104 use a comma, whereas English, and hence Perl are expecting a dot
2105 (U+002E: FULL STOP). Many modules can handle only the radix character
2106 being a dot, and so perl attempts to make it so. Up through Perl
2107 v5.20, the attempt was merely to set "LC_NUMERIC" upon startup to the
2108 "C" locale. Any setlocale() otherwise would change it; this caused
2109 some failures. Therefore, starting in v5.22, perl tries to keep
2110 "LC_NUMERIC" always set to "C" for XS code.
2111 To summarize, here's what to expect and how to handle locales in XS
2113 code:
2114 Non-locale-aware XS code
2116 Keep in mind that even if you think your code is not locale-aware,
2117 it may call a C library function that is. Hopefully the man page
2118 for such a function will indicate that dependency, but the
2119 documentation is imperfect.
2120 The current locale is exposed to XS code except possibly
2122 "LC_NUMERIC" (explained in the next paragraph). There have not
2123 been reports of problems with the other categories. Perl
2124 initializes things on start-up so that the current locale is the
2125 one which is indicated by the user's environment in effect at that
2126 time. See "ENVIRONMENT" in perllocale.
2127 However, up through v5.20, Perl initialized things on start-up so
2129 that "LC_NUMERIC" was set to the "C" locale. But if any code
2130 anywhere changed it, it would stay changed. This means that your
2131 module can't count on "LC_NUMERIC" being something in particular,
2132 and you can't expect floating point numbers (including version
2133 strings) to have dots in them. If you don't allow for a non-dot,
2134 your code could break if anyone anywhere changed the locale. For
2135 this reason, v5.22 changed the behavior so that Perl tries to keep
2136 "LC_NUMERIC" in the "C" locale except around the operations
2137 internally where it should be something else. Misbehaving XS code
2138 will always be able to change the locale anyway, but the most
2139 common instance of this is checked for and handled.
2140 Locale-aware XS code
2142 If the locale from the user's environment is desired, there should
2143 be no need for XS code to set the locale except for "LC_NUMERIC",
2144 as perl has already set it up. XS code should avoid changing the
2145 locale, as it can adversely affect other, unrelated, code and may
2146 not be thread safe. However, some alien libraries that may be
2147 called do set it, such as "Gtk". This can cause problems for the
2148 perl core and other modules. Starting in v5.20.1, calling the
2149 function sync_locale() from XS should be sufficient to avoid most
2150 of these problems. Prior to this, you need a pure Perl statement
2151 that does this:
2152 POSIX::setlocale(LC_ALL, POSIX::setlocale(LC_ALL));
2154 In the event that your XS code may need the underlying "LC_NUMERIC"
2156 locale, there are macros available to access this; see "Locale-
2157 related functions and macros" in perlapi.
2158
2160 This document covers features supported by "ExtUtils::ParseXS" (also
2161 known as "xsubpp") 3.13_01.
2162
2164 Originally written by Dean Roehrich <roehrich@cray.com>.
2165 Maintained since 1996 by The Perl Porters <perlbug@perl.org>.
2167perl v5.26.3 2018-03-01 PERLXS(1)