1perlxs(3) User Contributed Perl Documentation perlxs(3)
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 used
633 to call the real rpcb_gettime() function with the parameters in the
634 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 in such a way as to accept the number of
1297 parameters supplied by perl's overload system. For most overload
1298 methods, it will be three parameters; for the "nomethod" function it
1299 will be four. However, the bitwise operators "&", "|", "^", and "~"
1300 may be called with three or five arguments (see overload).
1301 If any function has the OVERLOAD: keyword, several additional lines
1303 will be defined in the c file generated by xsubpp in order to register
1304 with the overload magic.
1305 Since blessed objects are actually stored as RV's, it is useful to use
1307 the typemap features to preprocess parameters and extract the actual SV
1308 stored within the blessed RV. See the sample for T_PTROBJ_SPECIAL
1309 below.
1310 To use the OVERLOAD: keyword, create an XS function which takes three
1312 input parameters (or use the C-style '...' definition) like this:
1313 SV *
1315 cmp (lobj, robj, swap)
1316 My_Module_obj lobj
1317 My_Module_obj robj
1318 IV swap
1319 OVERLOAD: cmp <=>
1320 { /* function defined here */}
1321 In this case, the function will overload both of the three way
1323 comparison operators. For all overload operations using non-alpha
1324 characters, you must type the parameter without quoting, separating
1325 multiple overloads with whitespace. Note that "" (the stringify
1326 overload) should be entered as \"\" (i.e. escaped).
1327 Since, as mentioned above, bitwise operators may take extra arguments,
1329 you may want to use something like "(lobj, robj, swap, ...)" (with
1330 literal "...") as your parameter list.
1331 The FALLBACK: Keyword
1333 In addition to the OVERLOAD keyword, if you need to control how Perl
1334 autogenerates missing overloaded operators, you can set the FALLBACK
1335 keyword in the module header section, like this:
1336 MODULE = RPC PACKAGE = RPC
1338 FALLBACK: TRUE
1340 ...
1341 where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF.
1343 If you do not set any FALLBACK value when using OVERLOAD, it defaults
1344 to UNDEF. FALLBACK is not used except when one or more functions using
1345 OVERLOAD have been defined. Please see "fallback" in overload for more
1346 details.
1347 The INTERFACE: Keyword
1349 This keyword declares the current XSUB as a keeper of the given calling
1350 signature. If some text follows this keyword, it is considered as a
1351 list of functions which have this signature, and should be attached to
1352 the current XSUB.
1353 For example, if you have 4 C functions multiply(), divide(), add(),
1355 subtract() all having the signature:
1356 symbolic f(symbolic, symbolic);
1358 you can make them all to use the same XSUB using this:
1360 symbolic
1362 interface_s_ss(arg1, arg2)
1363 symbolic arg1
1364 symbolic arg2
1365 INTERFACE:
1366 multiply divide
1367 add subtract
1368 (This is the complete XSUB code for 4 Perl functions!) Four generated
1370 Perl function share names with corresponding C functions.
1371 The advantage of this approach comparing to ALIAS: keyword is that
1373 there is no need to code a switch statement, each Perl function (which
1374 shares the same XSUB) knows which C function it should call.
1375 Additionally, one can attach an extra function remainder() at runtime
1376 by using
1377 CV *mycv = newXSproto("Symbolic::remainder",
1379 XS_Symbolic_interface_s_ss, __FILE__, "$$");
1380 XSINTERFACE_FUNC_SET(mycv, remainder);
1381 say, from another XSUB. (This example supposes that there was no
1383 INTERFACE_MACRO: section, otherwise one needs to use something else
1384 instead of "XSINTERFACE_FUNC_SET", see the next section.)
1385 The INTERFACE_MACRO: Keyword
1387 This keyword allows one to define an INTERFACE using a different way to
1388 extract a function pointer from an XSUB. The text which follows this
1389 keyword should give the name of macros which would extract/set a
1390 function pointer. The extractor macro is given return type, "CV*", and
1391 "XSANY.any_dptr" for this "CV*". The setter macro is given cv, and the
1392 function pointer.
1393 The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET". An
1395 INTERFACE keyword with an empty list of functions can be omitted if
1396 INTERFACE_MACRO keyword is used.
1397 Suppose that in the previous example functions pointers for multiply(),
1399 divide(), add(), subtract() are kept in a global C array "fp[]" with
1400 offsets being "multiply_off", "divide_off", "add_off", "subtract_off".
1401 Then one can use
1402 #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
1404 ((XSINTERFACE_CVT_ANON(ret))fp[CvXSUBANY(cv).any_i32])
1405 #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
1406 CvXSUBANY(cv).any_i32 = CAT2( f, _off )
1407 in C section,
1409 symbolic
1411 interface_s_ss(arg1, arg2)
1412 symbolic arg1
1413 symbolic arg2
1414 INTERFACE_MACRO:
1415 XSINTERFACE_FUNC_BYOFFSET
1416 XSINTERFACE_FUNC_BYOFFSET_set
1417 INTERFACE:
1418 multiply divide
1419 add subtract
1420 in XSUB section.
1422 The INCLUDE: Keyword
1424 This keyword can be used to pull other files into the XS module. The
1425 other files may have XS code. INCLUDE: can also be used to run a
1426 command to generate the XS code to be pulled into the module.
1427 The file Rpcb1.xsh contains our "rpcb_gettime()" function:
1429 bool_t
1431 rpcb_gettime(host,timep)
1432 char *host
1433 time_t &timep
1434 OUTPUT:
1435 timep
1436 The XS module can use INCLUDE: to pull that file into it.
1438 INCLUDE: Rpcb1.xsh
1440 If the parameters to the INCLUDE: keyword are followed by a pipe ("|")
1442 then the compiler will interpret the parameters as a command. This
1443 feature is mildly deprecated in favour of the "INCLUDE_COMMAND:"
1444 directive, as documented below.
1445 INCLUDE: cat Rpcb1.xsh |
1447 Do not use this to run perl: "INCLUDE: perl |" will run the perl that
1449 happens to be the first in your path and not necessarily the same perl
1450 that is used to run "xsubpp". See "The INCLUDE_COMMAND: Keyword".
1451 The INCLUDE_COMMAND: Keyword
1453 Runs the supplied command and includes its output into the current XS
1454 document. "INCLUDE_COMMAND" assigns special meaning to the $^X token in
1455 that it runs the same perl interpreter that is running "xsubpp":
1456 INCLUDE_COMMAND: cat Rpcb1.xsh
1458 INCLUDE_COMMAND: $^X -e ...
1460 The CASE: Keyword
1462 The CASE: keyword allows an XSUB to have multiple distinct parts with
1463 each part acting as a virtual XSUB. CASE: is greedy and if it is used
1464 then all other XS keywords must be contained within a CASE:. This
1465 means nothing may precede the first CASE: in the XSUB and anything
1466 following the last CASE: is included in that case.
1467 A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS:
1469 variable (see "The ALIAS: Keyword"), or maybe via the "items" variable
1470 (see "Variable-length Parameter Lists"). The last CASE: becomes the
1471 default case if it is not associated with a conditional. The following
1472 example shows CASE switched via "ix" with a function "rpcb_gettime()"
1473 having an alias "x_gettime()". When the function is called as
1474 "rpcb_gettime()" its parameters are the usual "(char *host, time_t
1475 *timep)", but when the function is called as "x_gettime()" its
1476 parameters are reversed, "(time_t *timep, char *host)".
1477 long
1479 rpcb_gettime(a,b)
1480 CASE: ix == 1
1481 ALIAS:
1482 x_gettime = 1
1483 INPUT:
1484 # 'a' is timep, 'b' is host
1485 char *b
1486 time_t a = NO_INIT
1487 CODE:
1488 RETVAL = rpcb_gettime( b, &a );
1489 OUTPUT:
1490 a
1491 RETVAL
1492 CASE:
1493 # 'a' is host, 'b' is timep
1494 char *a
1495 time_t &b = NO_INIT
1496 OUTPUT:
1497 b
1498 RETVAL
1499 That function can be called with either of the following statements.
1501 Note the different argument lists.
1502 $status = rpcb_gettime( $host, $timep );
1504 $status = x_gettime( $timep, $host );
1506 The EXPORT_XSUB_SYMBOLS: Keyword
1508 The EXPORT_XSUB_SYMBOLS: keyword is likely something you will never
1509 need. In perl versions earlier than 5.16.0, this keyword does nothing.
1510 Starting with 5.16, XSUB symbols are no longer exported by default.
1511 That is, they are "static" functions. If you include
1512 EXPORT_XSUB_SYMBOLS: ENABLE
1514 in your XS code, the XSUBs following this line will not be declared
1516 "static". You can later disable this with
1517 EXPORT_XSUB_SYMBOLS: DISABLE
1519 which, again, is the default that you should probably never change.
1521 You cannot use this keyword on versions of perl before 5.16 to make
1522 XSUBs "static".
1523 The & Unary Operator
1525 The "&" unary operator in the INPUT: section is used to tell xsubpp
1526 that it should convert a Perl value to/from C using the C type to the
1527 left of "&", but provide a pointer to this value when the C function is
1528 called.
1529 This is useful to avoid a CODE: block for a C function which takes a
1531 parameter by reference. Typically, the parameter should be not a
1532 pointer type (an "int" or "long" but not an "int*" or "long*").
1533 The following XSUB will generate incorrect C code. The xsubpp compiler
1535 will turn this into code which calls "rpcb_gettime()" with parameters
1536 "(char *host, time_t timep)", but the real "rpcb_gettime()" wants the
1537 "timep" parameter to be of type "time_t*" rather than "time_t".
1538 bool_t
1540 rpcb_gettime(host,timep)
1541 char *host
1542 time_t timep
1543 OUTPUT:
1544 timep
1545 That problem is corrected by using the "&" operator. The xsubpp
1547 compiler will now turn this into code which calls "rpcb_gettime()"
1548 correctly with parameters "(char *host, time_t *timep)". It does this
1549 by carrying the "&" through, so the function call looks like
1550 "rpcb_gettime(host, &timep)".
1551 bool_t
1553 rpcb_gettime(host,timep)
1554 char *host
1555 time_t &timep
1556 OUTPUT:
1557 timep
1558 Inserting POD, Comments and C Preprocessor Directives
1560 C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
1561 CODE:, PPCODE:, POSTCALL:, and CLEANUP: blocks, as well as outside the
1562 functions. Comments are allowed anywhere after the MODULE keyword.
1563 The compiler will pass the preprocessor directives through untouched
1564 and will remove the commented lines. POD documentation is allowed at
1565 any point, both in the C and XS language sections. POD must be
1566 terminated with a "=cut" command; "xsubpp" will exit with an error if
1567 it does not. It is very unlikely that human generated C code will be
1568 mistaken for POD, as most indenting styles result in whitespace in
1569 front of any line starting with "=". Machine generated XS files may
1570 fall into this trap unless care is taken to ensure that a space breaks
1571 the sequence "\n=".
1572 Comments can be added to XSUBs by placing a "#" as the first non-
1574 whitespace of a line. Care should be taken to avoid making the comment
1575 look like a C preprocessor directive, lest it be interpreted as such.
1576 The simplest way to prevent this is to put whitespace in front of the
1577 "#".
1578 If you use preprocessor directives to choose one of two versions of a
1580 function, use
1581 #if ... version1
1583 #else /* ... version2 */
1584 #endif
1585 and not
1587 #if ... version1
1589 #endif
1590 #if ... version2
1591 #endif
1592 because otherwise xsubpp will believe that you made a duplicate
1594 definition of the function. Also, put a blank line before the
1595 #else/#endif so it will not be seen as part of the function body.
1596 Using XS With C++
1598 If an XSUB name contains "::", it is considered to be a C++ method.
1599 The generated Perl function will assume that its first argument is an
1600 object pointer. The object pointer will be stored in a variable called
1601 THIS. The object should have been created by C++ with the new()
1602 function and should be blessed by Perl with the sv_setref_pv() macro.
1603 The blessing of the object by Perl can be handled by a typemap. An
1604 example typemap is shown at the end of this section.
1605 If the return type of the XSUB includes "static", the method is
1607 considered to be a static method. It will call the C++ function using
1608 the class::method() syntax. If the method is not static the function
1609 will be called using the THIS->method() syntax.
1610 The next examples will use the following C++ class.
1612 class color {
1614 public:
1615 color();
1616 ~color();
1617 int blue();
1618 void set_blue( int );
1619 private:
1621 int c_blue;
1622 };
1623 The XSUBs for the blue() and set_blue() methods are defined with the
1625 class name but the parameter for the object (THIS, or "self") is
1626 implicit and is not listed.
1627 int
1629 color::blue()
1630 void
1632 color::set_blue( val )
1633 int val
1634 Both Perl functions will expect an object as the first parameter. In
1636 the generated C++ code the object is called "THIS", and the method call
1637 will be performed on this object. So in the C++ code the blue() and
1638 set_blue() methods will be called as this:
1639 RETVAL = THIS->blue();
1641 THIS->set_blue( val );
1643 You could also write a single get/set method using an optional
1645 argument:
1646 int
1648 color::blue( val = NO_INIT )
1649 int val
1650 PROTOTYPE $;$
1651 CODE:
1652 if (items > 1)
1653 THIS->set_blue( val );
1654 RETVAL = THIS->blue();
1655 OUTPUT:
1656 RETVAL
1657 If the function's name is DESTROY then the C++ "delete" function will
1659 be called and "THIS" will be given as its parameter. The generated C++
1660 code for
1661 void
1663 color::DESTROY()
1664 will look like this:
1666 color *THIS = ...; // Initialized as in typemap
1668 delete THIS;
1670 If the function's name is new then the C++ "new" function will be
1672 called to create a dynamic C++ object. The XSUB will expect the class
1673 name, which will be kept in a variable called "CLASS", to be given as
1674 the first argument.
1675 color *
1677 color::new()
1678 The generated C++ code will call "new".
1680 RETVAL = new color();
1682 The following is an example of a typemap that could be used for this
1684 C++ example.
1685 TYPEMAP
1687 color * O_OBJECT
1688 OUTPUT
1690 # The Perl object is blessed into 'CLASS', which should be a
1691 # char* having the name of the package for the blessing.
1692 O_OBJECT
1693 sv_setref_pv( $arg, CLASS, (void*)$var );
1694 INPUT
1696 O_OBJECT
1697 if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
1698 $var = ($type)SvIV((SV*)SvRV( $arg ));
1699 else{
1700 warn("${Package}::$func_name() -- " .
1701 "$var is not a blessed SV reference");
1702 XSRETURN_UNDEF;
1703 }
1704 Interface Strategy
1706 When designing an interface between Perl and a C library a straight
1707 translation from C to XS (such as created by "h2xs -x") is often
1708 sufficient. However, sometimes the interface will look very C-like and
1709 occasionally nonintuitive, especially when the C function modifies one
1710 of its parameters, or returns failure inband (as in "negative return
1711 values mean failure"). In cases where the programmer wishes to create
1712 a more Perl-like interface the following strategy may help to identify
1713 the more critical parts of the interface.
1714 Identify the C functions with input/output or output parameters. The
1716 XSUBs for these functions may be able to return lists to Perl.
1717 Identify the C functions which use some inband info as an indication of
1719 failure. They may be candidates to return undef or an empty list in
1720 case of failure. If the failure may be detected without a call to the
1721 C function, you may want to use an INIT: section to report the failure.
1722 For failures detectable after the C function returns one may want to
1723 use a POSTCALL: section to process the failure. In more complicated
1724 cases use CODE: or PPCODE: sections.
1725 If many functions use the same failure indication based on the return
1727 value, you may want to create a special typedef to handle this
1728 situation. Put
1729 typedef int negative_is_failure;
1731 near the beginning of XS file, and create an OUTPUT typemap entry for
1733 "negative_is_failure" which converts negative values to "undef", or
1734 maybe croak()s. After this the return value of type
1735 "negative_is_failure" will create more Perl-like interface.
1736 Identify which values are used by only the C and XSUB functions
1738 themselves, say, when a parameter to a function should be a contents of
1739 a global variable. If Perl does not need to access the contents of the
1740 value then it may not be necessary to provide a translation for that
1741 value from C to Perl.
1742 Identify the pointers in the C function parameter lists and return
1744 values. Some pointers may be used to implement input/output or output
1745 parameters, they can be handled in XS with the "&" unary operator, and,
1746 possibly, using the NO_INIT keyword. Some others will require handling
1747 of types like "int *", and one needs to decide what a useful Perl
1748 translation will do in such a case. When the semantic is clear, it is
1749 advisable to put the translation into a typemap file.
1750 Identify the structures used by the C functions. In many cases it may
1752 be helpful to use the T_PTROBJ typemap for these structures so they can
1753 be manipulated by Perl as blessed objects. (This is handled
1754 automatically by "h2xs -x".)
1755 If the same C type is used in several different contexts which require
1757 different translations, "typedef" several new types mapped to this C
1758 type, and create separate typemap entries for these new types. Use
1759 these types in declarations of return type and parameters to XSUBs.
1760 Perl Objects And C Structures
1762 When dealing with C structures one should select either T_PTROBJ or
1763 T_PTRREF for the XS type. Both types are designed to handle pointers
1764 to complex objects. The T_PTRREF type will allow the Perl object to be
1765 unblessed while the T_PTROBJ type requires that the object be blessed.
1766 By using T_PTROBJ one can achieve a form of type-checking because the
1767 XSUB will attempt to verify that the Perl object is of the expected
1768 type.
1769 The following XS code shows the getnetconfigent() function which is
1771 used with ONC+ TIRPC. The getnetconfigent() function will return a
1772 pointer to a C structure and has the C prototype shown below. The
1773 example will demonstrate how the C pointer will become a Perl
1774 reference. Perl will consider this reference to be a pointer to a
1775 blessed object and will attempt to call a destructor for the object. A
1776 destructor will be provided in the XS source to free the memory used by
1777 getnetconfigent(). Destructors in XS can be created by specifying an
1778 XSUB function whose name ends with the word DESTROY. XS destructors
1779 can be used to free memory which may have been malloc'd by another
1780 XSUB.
1781 struct netconfig *getnetconfigent(const char *netid);
1783 A "typedef" will be created for "struct netconfig". The Perl object
1785 will be blessed in a class matching the name of the C type, with the
1786 tag "Ptr" appended, and the name should not have embedded spaces if it
1787 will be a Perl package name. The destructor will be placed in a class
1788 corresponding to the class of the object and the PREFIX keyword will be
1789 used to trim the name to the word DESTROY as Perl will expect.
1790 typedef struct netconfig Netconfig;
1792 MODULE = RPC PACKAGE = RPC
1794 Netconfig *
1796 getnetconfigent(netid)
1797 char *netid
1798 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1800 void
1802 rpcb_DESTROY(netconf)
1803 Netconfig *netconf
1804 CODE:
1805 printf("Now in NetconfigPtr::DESTROY\n");
1806 free( netconf );
1807 This example requires the following typemap entry. Consult
1809 perlxstypemap for more information about adding new typemaps for an
1810 extension.
1811 TYPEMAP
1813 Netconfig * T_PTROBJ
1814 This example will be used with the following Perl statements.
1816 use RPC;
1818 $netconf = getnetconfigent("udp");
1819 When Perl destroys the object referenced by $netconf it will send the
1821 object to the supplied XSUB DESTROY function. Perl cannot determine,
1822 and does not care, that this object is a C struct and not a Perl
1823 object. In this sense, there is no difference between the object
1824 created by the getnetconfigent() XSUB and an object created by a normal
1825 Perl subroutine.
1826 Safely Storing Static Data in XS
1828 Starting with Perl 5.8, a macro framework has been defined to allow
1829 static data to be safely stored in XS modules that will be accessed
1830 from a multi-threaded Perl.
1831 Although primarily designed for use with multi-threaded Perl, the
1833 macros have been designed so that they will work with non-threaded Perl
1834 as well.
1835 It is therefore strongly recommended that these macros be used by all
1837 XS modules that make use of static data.
1838 The easiest way to get a template set of macros to use is by specifying
1840 the "-g" ("--global") option with h2xs (see h2xs).
1841 Below is an example module that makes use of the macros.
1843 #define PERL_NO_GET_CONTEXT
1845 #include "EXTERN.h"
1846 #include "perl.h"
1847 #include "XSUB.h"
1848 /* Global Data */
1850 #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION
1852 typedef struct {
1854 int count;
1855 char name[3][100];
1856 } my_cxt_t;
1857 START_MY_CXT
1859 MODULE = BlindMice PACKAGE = BlindMice
1861 BOOT:
1863 {
1864 MY_CXT_INIT;
1865 MY_CXT.count = 0;
1866 strcpy(MY_CXT.name[0], "None");
1867 strcpy(MY_CXT.name[1], "None");
1868 strcpy(MY_CXT.name[2], "None");
1869 }
1870 int
1872 newMouse(char * name)
1873 PREINIT:
1874 dMY_CXT;
1875 CODE:
1876 if (MY_CXT.count >= 3) {
1877 warn("Already have 3 blind mice");
1878 RETVAL = 0;
1879 }
1880 else {
1881 RETVAL = ++ MY_CXT.count;
1882 strcpy(MY_CXT.name[MY_CXT.count - 1], name);
1883 }
1884 OUTPUT:
1885 RETVAL
1886 char *
1888 get_mouse_name(index)
1889 int index
1890 PREINIT:
1891 dMY_CXT;
1892 CODE:
1893 if (index > MY_CXT.count)
1894 croak("There are only 3 blind mice.");
1895 else
1896 RETVAL = MY_CXT.name[index - 1];
1897 OUTPUT:
1898 RETVAL
1899 void
1901 CLONE(...)
1902 CODE:
1903 MY_CXT_CLONE;
1904 MY_CXT REFERENCE
1906 MY_CXT_KEY
1908 This macro is used to define a unique key to refer to the static
1909 data for an XS module. The suggested naming scheme, as used by
1910 h2xs, is to use a string that consists of the module name, the
1911 string "::_guts" and the module version number.
1912 #define MY_CXT_KEY "MyModule::_guts" XS_VERSION
1914 typedef my_cxt_t
1916 This struct typedef must always be called "my_cxt_t". The other
1917 "CXT*" macros assume the existence of the "my_cxt_t" typedef name.
1918 Declare a typedef named "my_cxt_t" that is a structure that
1920 contains all the data that needs to be interpreter-local.
1921 typedef struct {
1923 int some_value;
1924 } my_cxt_t;
1925 START_MY_CXT
1927 Always place the START_MY_CXT macro directly after the declaration
1928 of "my_cxt_t".
1929 MY_CXT_INIT
1931 The MY_CXT_INIT macro initializes storage for the "my_cxt_t"
1932 struct.
1933 It must be called exactly once, typically in a BOOT: section. If
1935 you are maintaining multiple interpreters, it should be called
1936 once in each interpreter instance, except for interpreters cloned
1937 from existing ones. (But see "MY_CXT_CLONE" below.)
1938 dMY_CXT
1940 Use the dMY_CXT macro (a declaration) in all the functions that
1941 access MY_CXT.
1942 MY_CXT
1944 Use the MY_CXT macro to access members of the "my_cxt_t" struct.
1945 For example, if "my_cxt_t" is
1946 typedef struct {
1948 int index;
1949 } my_cxt_t;
1950 then use this to access the "index" member
1952 dMY_CXT;
1954 MY_CXT.index = 2;
1955 aMY_CXT/pMY_CXT
1957 "dMY_CXT" may be quite expensive to calculate, and to avoid the
1958 overhead of invoking it in each function it is possible to pass
1959 the declaration onto other functions using the "aMY_CXT"/"pMY_CXT"
1960 macros, eg
1961 void sub1() {
1963 dMY_CXT;
1964 MY_CXT.index = 1;
1965 sub2(aMY_CXT);
1966 }
1967 void sub2(pMY_CXT) {
1969 MY_CXT.index = 2;
1970 }
1971 Analogously to "pTHX", there are equivalent forms for when the
1973 macro is the first or last in multiple arguments, where an
1974 underscore represents a comma, i.e. "_aMY_CXT", "aMY_CXT_",
1975 "_pMY_CXT" and "pMY_CXT_".
1976 MY_CXT_CLONE
1978 By default, when a new interpreter is created as a copy of an
1979 existing one (eg via "threads->create()"), both interpreters share
1980 the same physical my_cxt_t structure. Calling "MY_CXT_CLONE"
1981 (typically via the package's "CLONE()" function), causes a byte-
1982 for-byte copy of the structure to be taken, and any future dMY_CXT
1983 will cause the copy to be accessed instead.
1984 MY_CXT_INIT_INTERP(my_perl)
1986 dMY_CXT_INTERP(my_perl)
1987 These are versions of the macros which take an explicit
1988 interpreter as an argument.
1989 Note that these macros will only work together within the same source
1991 file; that is, a dMY_CTX in one source file will access a different
1992 structure than a dMY_CTX in another source file.
1993 Thread-aware system interfaces
1995 Starting from Perl 5.8, in C/C++ level Perl knows how to wrap
1996 system/library interfaces that have thread-aware versions (e.g.
1997 getpwent_r()) into frontend macros (e.g. getpwent()) that correctly
1998 handle the multithreaded interaction with the Perl interpreter. This
1999 will happen transparently, the only thing you need to do is to
2000 instantiate a Perl interpreter.
2001 This wrapping happens always when compiling Perl core source (PERL_CORE
2003 is defined) or the Perl core extensions (PERL_EXT is defined). When
2004 compiling XS code outside of the Perl core, the wrapping does not take
2005 place before Perl 5.28. Starting in that release you can
2006 #define PERL_REENTRANT
2008 in your code to enable the wrapping. It is advisable to do so if you
2010 are using such functions, as intermixing the "_r"-forms (as Perl
2011 compiled for multithreaded operation will do) and the "_r"-less forms
2012 is neither well-defined (inconsistent results, data corruption, or even
2013 crashes become more likely), nor is it very portable. Unfortunately,
2014 not all systems have all the "_r" forms, but using this "#define" gives
2015 you whatever protection that Perl is aware is available on each system.
2016
2018 File "RPC.xs": Interface to some ONC+ RPC bind library functions.
2019 #define PERL_NO_GET_CONTEXT
2021 #include "EXTERN.h"
2022 #include "perl.h"
2023 #include "XSUB.h"
2024 #include <rpc/rpc.h>
2026 typedef struct netconfig Netconfig;
2028 MODULE = RPC PACKAGE = RPC
2030 SV *
2032 rpcb_gettime(host="localhost")
2033 char *host
2034 PREINIT:
2035 time_t timep;
2036 CODE:
2037 ST(0) = sv_newmortal();
2038 if( rpcb_gettime( host, &timep ) )
2039 sv_setnv( ST(0), (double)timep );
2040 Netconfig *
2042 getnetconfigent(netid="udp")
2043 char *netid
2044 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
2046 void
2048 rpcb_DESTROY(netconf)
2049 Netconfig *netconf
2050 CODE:
2051 printf("NetconfigPtr::DESTROY\n");
2052 free( netconf );
2053 File "typemap": Custom typemap for RPC.xs. (cf. perlxstypemap)
2055 TYPEMAP
2057 Netconfig * T_PTROBJ
2058 File "RPC.pm": Perl module for the RPC extension.
2060 package RPC;
2062 require Exporter;
2064 require DynaLoader;
2065 @ISA = qw(Exporter DynaLoader);
2066 @EXPORT = qw(rpcb_gettime getnetconfigent);
2067 bootstrap RPC;
2069 1;
2070 File "rpctest.pl": Perl test program for the RPC extension.
2072 use RPC;
2074 $netconf = getnetconfigent();
2076 $a = rpcb_gettime();
2077 print "time = $a\n";
2078 print "netconf = $netconf\n";
2079 $netconf = getnetconfigent("tcp");
2081 $a = rpcb_gettime("poplar");
2082 print "time = $a\n";
2083 print "netconf = $netconf\n";
2084
2086 XS code has full access to system calls including C library functions.
2087 It thus has the capability of interfering with things that the Perl
2088 core or other modules have set up, such as signal handlers or file
2089 handles. It could mess with the memory, or any number of harmful
2090 things. Don't.
2091 Some modules have an event loop, waiting for user-input. It is highly
2093 unlikely that two such modules would work adequately together in a
2094 single Perl application.
2095 In general, the perl interpreter views itself as the center of the
2097 universe as far as the Perl program goes. XS code is viewed as a help-
2098 mate, to accomplish things that perl doesn't do, or doesn't do fast
2099 enough, but always subservient to perl. The closer XS code adheres to
2100 this model, the less likely conflicts will occur.
2101 One area where there has been conflict is in regards to C locales.
2103 (See perllocale.) perl, with one exception and unless told otherwise,
2104 sets up the underlying locale the program is running in to the locale
2105 passed into it from the environment. This is an important difference
2106 from a generic C language program, where the underlying locale is the
2107 "C" locale unless the program changes it. As of v5.20, this underlying
2108 locale is completely hidden from pure Perl code outside the lexical
2109 scope of "use locale" except for a couple of function calls in the
2110 POSIX module which of necessity use it. But the underlying locale,
2111 with that one exception is exposed to XS code, affecting all C library
2112 routines whose behavior is locale-dependent. Your XS code better not
2113 assume that the underlying locale is "C". The exception is the
2114 "LC_NUMERIC" locale category, and the reason it is an exception is that
2115 experience has shown that it can be problematic for XS code, whereas we
2116 have not had reports of problems with the other locale categories. And
2117 the reason for this one category being problematic is that the
2118 character used as a decimal point can vary. Many European languages
2119 use a comma, whereas English, and hence Perl are expecting a dot
2120 (U+002E: FULL STOP). Many modules can handle only the radix character
2121 being a dot, and so perl attempts to make it so. Up through Perl
2122 v5.20, the attempt was merely to set "LC_NUMERIC" upon startup to the
2123 "C" locale. Any setlocale() otherwise would change it; this caused
2124 some failures. Therefore, starting in v5.22, perl tries to keep
2125 "LC_NUMERIC" always set to "C" for XS code.
2126 To summarize, here's what to expect and how to handle locales in XS
2128 code:
2129 Non-locale-aware XS code
2131 Keep in mind that even if you think your code is not locale-aware,
2132 it may call a library function that is. Hopefully the man page for
2133 such a function will indicate that dependency, but the
2134 documentation is imperfect.
2135 The current locale is exposed to XS code except possibly
2137 "LC_NUMERIC" (explained in the next paragraph). There have not
2138 been reports of problems with the other categories. Perl
2139 initializes things on start-up so that the current locale is the
2140 one which is indicated by the user's environment in effect at that
2141 time. See "ENVIRONMENT" in perllocale.
2142 However, up through v5.20, Perl initialized things on start-up so
2144 that "LC_NUMERIC" was set to the "C" locale. But if any code
2145 anywhere changed it, it would stay changed. This means that your
2146 module can't count on "LC_NUMERIC" being something in particular,
2147 and you can't expect floating point numbers (including version
2148 strings) to have dots in them. If you don't allow for a non-dot,
2149 your code could break if anyone anywhere changed the locale. For
2150 this reason, v5.22 changed the behavior so that Perl tries to keep
2151 "LC_NUMERIC" in the "C" locale except around the operations
2152 internally where it should be something else. Misbehaving XS code
2153 will always be able to change the locale anyway, but the most
2154 common instance of this is checked for and handled.
2155 Locale-aware XS code
2157 If the locale from the user's environment is desired, there should
2158 be no need for XS code to set the locale except for "LC_NUMERIC",
2159 as perl has already set the others up. XS code should avoid
2160 changing the locale, as it can adversely affect other, unrelated,
2161 code and may not be thread-safe. To minimize problems, the macros
2162 "STORE_LC_NUMERIC_SET_TO_NEEDED" in perlapi,
2163 "STORE_LC_NUMERIC_FORCE_TO_UNDERLYING" in perlapi, and
2164 "RESTORE_LC_NUMERIC" in perlapi should be used to affect any needed
2165 change.
2166 But, starting with Perl v5.28, locales are thread-safe on platforms
2168 that support this functionality. Windows has this starting with
2169 Visual Studio 2005. Many other modern platforms support the
2170 thread-safe POSIX 2008 functions. The C "#define"
2171 "USE_THREAD_SAFE_LOCALE" will be defined iff this build is using
2172 these. From Perl-space, the read-only variable "${SAFE_LOCALES}"
2173 is 1 if either the build is not threaded, or if
2174 "USE_THREAD_SAFE_LOCALE" is defined; otherwise it is 0.
2175 The way this works under-the-hood is that every thread has a choice
2177 of using a locale specific to it (this is the Windows and POSIX
2178 2008 functionality), or the global locale that is accessible to all
2179 threads (this is the functionality that has always been there).
2180 The implementations for Windows and POSIX are completely different.
2181 On Windows, the runtime can be set up so that the standard
2182 setlocale(3) function either only knows about the global locale or
2183 the locale for this thread. On POSIX, "setlocale" always deals
2184 with the global locale, and other functions have been created to
2185 handle per-thread locales. Perl makes this transparent to perl-
2186 space code. It continues to use "POSIX::setlocale()", and the
2187 interpreter translates that into the per-thread functions.
2188 All other locale-senstive functions automatically use the per-
2190 thread locale, if that is turned on, and failing that, the global
2191 locale. Thus calls to "setlocale" are ineffective on POSIX systems
2192 for the current thread if that thread is using a per-thread locale.
2193 If perl is compiled for single-thread operation, it does not use
2194 the per-thread functions, so "setlocale" does work as expected.
2195 If you have loaded the "POSIX" module you can use the methods given
2197 in perlcall to call "POSIX::setlocale" to safely change or query
2198 the locale (on systems where it is safe to do so), or you can use
2199 the new 5.28 function "Perl_setlocale" in perlapi instead, which is
2200 a drop-in replacement for the system setlocale(3), and handles
2201 single-threaded and multi-threaded applications transparently.
2202 There are some locale-related library calls that still aren't
2204 thread-safe because they return data in a buffer global to all
2205 threads. In the past, these didn't matter as locales weren't
2206 thread-safe at all. But now you have to be aware of them in case
2207 your module is called in a multi-threaded application. The known
2208 ones are
2209 asctime()
2211 ctime()
2212 gcvt() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
2213 getdate()
2214 wcrtomb() if its final argument is NULL
2215 wcsrtombs() if its final argument is NULL
2216 wcstombs()
2217 wctomb()
2218 Some of these shouldn't really be called in a Perl application, and
2220 for others there are thread-safe versions of these already
2221 implemented:
2222 asctime_r()
2224 ctime_r()
2225 Perl_langinfo()
2226 The "_r" forms are automatically used, starting in Perl 5.28, if
2228 you compile your code, with
2229 #define PERL_REENTRANT
2231 See also "Perl_langinfo" in perlapi. You can use the methods given
2233 in perlcall, to get the best available locale-safe versions of
2234 these
2235 POSIX::localeconv()
2237 POSIX::wcstombs()
2238 POSIX::wctomb()
2239 And note, that some items returned by "Localeconv" are available
2241 through "Perl_langinfo" in perlapi.
2242 The others shouldn't be used in a threaded application.
2244 Some modules may call a non-perl library that is locale-aware.
2246 This is fine as long as it doesn't try to query or change the
2247 locale using the system "setlocale". But if these do call the
2248 system "setlocale", those calls may be ineffective. Instead,
2249 "Perl_setlocale" works in all circumstances. Plain setlocale is
2250 ineffective on multi-threaded POSIX 2008 systems. It operates only
2251 on the global locale, whereas each thread has its own locale,
2252 paying no attention to the global one. Since converting these non-
2253 Perl libraries to "Perl_setlocale" is out of the question, there is
2254 a new function in v5.28 "switch_to_global_locale" that will switch
2255 the thread it is called from so that any system "setlocale" calls
2256 will have their desired effect. The function "sync_locale" must be
2257 called before returning to perl.
2258 This thread can change the locale all it wants and it won't affect
2260 any other thread, except any that also have been switched to the
2261 global locale. This means that a multi-threaded application can
2262 have a single thread using an alien library without a problem; but
2263 no more than a single thread can be so-occupied. Bad results
2264 likely will happen.
2265 In perls without multi-thread locale support, some alien libraries,
2267 such as "Gtk" change locales. This can cause problems for the Perl
2268 core and other modules. For these, before control is returned to
2269 perl, starting in v5.20.1, calling the function sync_locale() from
2270 XS should be sufficient to avoid most of these problems. Prior to
2271 this, you need a pure Perl statement that does this:
2272 POSIX::setlocale(LC_ALL, POSIX::setlocale(LC_ALL));
2274 or use the methods given in perlcall.
2276
2278 This document covers features supported by "ExtUtils::ParseXS" (also
2279 known as "xsubpp") 3.13_01.
2280
2282 Originally written by Dean Roehrich <roehrich@cray.com>.
2283 Maintained since 1996 by The Perl Porters <perlbug@perl.org>.
2285perl v5.30.1 2020-01-30 perlxs(3)