3 perlxs - XS language reference manual
9 XS is an interface description file format used to create an extension
10 interface between Perl and C code (or a C library) which one wishes
11 to use with Perl. The XS interface is combined with the library to
12 create a new library which can then be either dynamically loaded
13 or statically linked into perl. The XS interface description is
14 written in the XS language and is the core component of the Perl
17 An B<XSUB> forms the basic unit of the XS interface. After compilation
18 by the B<xsubpp> compiler, each XSUB amounts to a C function definition
19 which will provide the glue between Perl calling conventions and C
22 The glue code pulls the arguments from the Perl stack, converts these
23 Perl values to the formats expected by a C function, call this C function,
24 transfers the return values of the C function back to Perl.
25 Return values here may be a conventional C return value or any C
26 function arguments that may serve as output parameters. These return
27 values may be passed back to Perl either by putting them on the
28 Perl stack, or by modifying the arguments supplied from the Perl side.
30 The above is a somewhat simplified view of what really happens. Since
31 Perl allows more flexible calling conventions than C, XSUBs may do much
32 more in practice, such as checking input parameters for validity,
33 throwing exceptions (or returning undef/empty list) if the return value
34 from the C function indicates failure, calling different C functions
35 based on numbers and types of the arguments, providing an object-oriented
38 Of course, one could write such glue code directly in C. However, this
39 would be a tedious task, especially if one needs to write glue for
40 multiple C functions, and/or one is not familiar enough with the Perl
41 stack discipline and other such arcana. XS comes to the rescue here:
42 instead of writing this glue C code in long-hand, one can write
43 a more concise short-hand I<description> of what should be done by
44 the glue, and let the XS compiler B<xsubpp> handle the rest.
46 The XS language allows one to describe the mapping between how the C
47 routine is used, and how the corresponding Perl routine is used. It
48 also allows creation of Perl routines which are directly translated to
49 C code and which are not related to a pre-existing C function. In cases
50 when the C interface coincides with the Perl interface, the XSUB
51 declaration is almost identical to a declaration of a C function (in K&R
52 style). In such circumstances, there is another tool called C<h2xs>
53 that is able to translate an entire C header file into a corresponding
54 XS file that will provide glue to the functions/macros described in
57 The XS compiler is called B<xsubpp>. This compiler creates
58 the constructs necessary to let an XSUB manipulate Perl values, and
59 creates the glue necessary to let Perl call the XSUB. The compiler
60 uses B<typemaps> to determine how to map C function parameters
61 and output values to Perl values and back. The default typemap
62 (which comes with Perl) handles many common C types. A supplementary
63 typemap may also be needed to handle any special structures and types
64 for the library being linked.
66 A file in XS format starts with a C language section which goes until the
67 first C<MODULE =Z<>> directive. Other XS directives and XSUB definitions
68 may follow this line. The "language" used in this part of the file
69 is usually referred to as the XS language. B<xsubpp> recognizes and
70 skips POD (see L<perlpod>) in both the C and XS language sections, which
71 allows the XS file to contain embedded documentation.
73 See L<perlxstut> for a tutorial on the whole extension creation process.
75 Note: For some extensions, Dave Beazley's SWIG system may provide a
76 significantly more convenient mechanism for creating the extension
77 glue code. See http://www.swig.org/ for more information.
81 Many of the examples which follow will concentrate on creating an interface
82 between Perl and the ONC+ RPC bind library functions. The rpcb_gettime()
83 function is used to demonstrate many features of the XS language. This
84 function has two parameters; the first is an input parameter and the second
85 is an output parameter. The function also returns a status value.
87 bool_t rpcb_gettime(const char *host, time_t *timep);
89 From C this function will be called with the following
95 status = rpcb_gettime( "localhost", &timep );
97 If an XSUB is created to offer a direct translation between this function
98 and Perl, then this XSUB will be used from Perl with the following code.
99 The $status and $timep variables will contain the output of the function.
102 $status = rpcb_gettime( "localhost", $timep );
104 The following XS file shows an XS subroutine, or XSUB, which
105 demonstrates one possible interface to the rpcb_gettime()
106 function. This XSUB represents a direct translation between
107 C and Perl and so preserves the interface even from Perl.
108 This XSUB will be invoked from Perl with the usage shown
109 above. Note that the first three #include statements, for
110 C<EXTERN.h>, C<perl.h>, and C<XSUB.h>, will always be present at the
111 beginning of an XS file. This approach and others will be
112 expanded later in this document.
119 MODULE = RPC PACKAGE = RPC
122 rpcb_gettime(host,timep)
128 Any extension to Perl, including those containing XSUBs,
129 should have a Perl module to serve as the bootstrap which
130 pulls the extension into Perl. This module will export the
131 extension's functions and variables to the Perl program and
132 will cause the extension's XSUBs to be linked into Perl.
133 The following module will be used for most of the examples
134 in this document and should be used from Perl with the C<use>
135 command as shown earlier. Perl modules are explained in
136 more detail later in this document.
142 @ISA = qw(Exporter DynaLoader);
143 @EXPORT = qw( rpcb_gettime );
148 Throughout this document a variety of interfaces to the rpcb_gettime()
149 XSUB will be explored. The XSUBs will take their parameters in different
150 orders or will take different numbers of parameters. In each case the
151 XSUB is an abstraction between Perl and the real C rpcb_gettime()
152 function, and the XSUB must always ensure that the real rpcb_gettime()
153 function is called with the correct parameters. This abstraction will
154 allow the programmer to create a more Perl-like interface to the C
157 =head2 The Anatomy of an XSUB
159 The simplest XSUBs consist of 3 parts: a description of the return
160 value, the name of the XSUB routine and the names of its arguments,
161 and a description of types or formats of the arguments.
163 The following XSUB allows a Perl program to access a C library function
164 called sin(). The XSUB will imitate the C function which takes a single
165 argument and returns a single value.
171 Optionally, one can merge the description of types and the list of
172 argument names, rewriting this as
177 This makes this XSUB look similar to an ANSI C declaration. An optional
178 semicolon is allowed after the argument list, as in
183 Parameters with C pointer types can have different semantic: C functions
184 with similar declarations
186 bool string_looks_as_a_number(char *s);
187 bool make_char_uppercase(char *c);
189 are used in absolutely incompatible manner. Parameters to these functions
190 could be described B<xsubpp> like this:
195 Both these XS declarations correspond to the C<char*> C type, but they have
196 different semantics, see L<"The & Unary Operator">.
198 It is convenient to think that the indirection operator
199 C<*> should be considered as a part of the type and the address operator C<&>
200 should be considered part of the variable. See L<"The Typemap">
201 for more info about handling qualifiers and unary operators in C types.
203 The function name and the return type must be placed on
204 separate lines and should be flush left-adjusted.
212 The rest of the function description may be indented or left-adjusted. The
213 following example shows a function with its body left-adjusted. Most
214 examples in this document will indent the body for better readability.
222 More complicated XSUBs may contain many other sections. Each section of
223 an XSUB starts with the corresponding keyword, such as INIT: or CLEANUP:.
224 However, the first two lines of an XSUB always contain the same data:
225 descriptions of the return type and the names of the function and its
226 parameters. Whatever immediately follows these is considered to be
227 an INPUT: section unless explicitly marked with another keyword.
228 (See L<The INPUT: Keyword>.)
230 An XSUB section continues until another section-start keyword is found.
232 =head2 The Argument Stack
234 The Perl argument stack is used to store the values which are
235 sent as parameters to the XSUB and to store the XSUB's
236 return value(s). In reality all Perl functions (including non-XSUB
237 ones) keep their values on this stack all the same time, each limited
238 to its own range of positions on the stack. In this document the
239 first position on that stack which belongs to the active
240 function will be referred to as position 0 for that function.
242 XSUBs refer to their stack arguments with the macro B<ST(x)>, where I<x>
243 refers to a position in this XSUB's part of the stack. Position 0 for that
244 function would be known to the XSUB as ST(0). The XSUB's incoming
245 parameters and outgoing return values always begin at ST(0). For many
246 simple cases the B<xsubpp> compiler will generate the code necessary to
247 handle the argument stack by embedding code fragments found in the
248 typemaps. In more complex cases the programmer must supply the code.
250 =head2 The RETVAL Variable
252 The RETVAL variable is a special C variable that is declared automatically
253 for you. The C type of RETVAL matches the return type of the C library
254 function. The B<xsubpp> compiler will declare this variable in each XSUB
255 with non-C<void> return type. By default the generated C function
256 will use RETVAL to hold the return value of the C library function being
257 called. In simple cases the value of RETVAL will be placed in ST(0) of
258 the argument stack where it can be received by Perl as the return value
261 If the XSUB has a return type of C<void> then the compiler will
262 not declare a RETVAL variable for that function. When using
263 a PPCODE: section no manipulation of the RETVAL variable is required, the
264 section may use direct stack manipulation to place output values on the stack.
266 If PPCODE: directive is not used, C<void> return value should be used
267 only for subroutines which do not return a value, I<even if> CODE:
268 directive is used which sets ST(0) explicitly.
270 Older versions of this document recommended to use C<void> return
271 value in such cases. It was discovered that this could lead to
272 segfaults in cases when XSUB was I<truly> C<void>. This practice is
273 now deprecated, and may be not supported at some future version. Use
274 the return value C<SV *> in such cases. (Currently C<xsubpp> contains
275 some heuristic code which tries to disambiguate between "truly-void"
276 and "old-practice-declared-as-void" functions. Hence your code is at
277 mercy of this heuristics unless you use C<SV *> as return value.)
279 =head2 The MODULE Keyword
281 The MODULE keyword is used to start the XS code and to specify the package
282 of the functions which are being defined. All text preceding the first
283 MODULE keyword is considered C code and is passed through to the output with
284 POD stripped, but otherwise untouched. Every XS module will have a
285 bootstrap function which is used to hook the XSUBs into Perl. The package
286 name of this bootstrap function will match the value of the last MODULE
287 statement in the XS source files. The value of MODULE should always remain
288 constant within the same XS file, though this is not required.
290 The following example will start the XS code and will place
291 all functions in a package named RPC.
295 =head2 The PACKAGE Keyword
297 When functions within an XS source file must be separated into packages
298 the PACKAGE keyword should be used. This keyword is used with the MODULE
299 keyword and must follow immediately after it when used.
301 MODULE = RPC PACKAGE = RPC
303 [ XS code in package RPC ]
305 MODULE = RPC PACKAGE = RPCB
307 [ XS code in package RPCB ]
309 MODULE = RPC PACKAGE = RPC
311 [ XS code in package RPC ]
313 The same package name can be used more than once, allowing for
314 non-contiguous code. This is useful if you have a stronger ordering
315 principle than package names.
317 Although this keyword is optional and in some cases provides redundant
318 information it should always be used. This keyword will ensure that the
319 XSUBs appear in the desired package.
321 =head2 The PREFIX Keyword
323 The PREFIX keyword designates prefixes which should be
324 removed from the Perl function names. If the C function is
325 C<rpcb_gettime()> and the PREFIX value is C<rpcb_> then Perl will
326 see this function as C<gettime()>.
328 This keyword should follow the PACKAGE keyword when used.
329 If PACKAGE is not used then PREFIX should follow the MODULE
332 MODULE = RPC PREFIX = rpc_
334 MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_
336 =head2 The OUTPUT: Keyword
338 The OUTPUT: keyword indicates that certain function parameters should be
339 updated (new values made visible to Perl) when the XSUB terminates or that
340 certain values should be returned to the calling Perl function. For
341 simple functions which have no CODE: or PPCODE: section,
342 such as the sin() function above, the RETVAL variable is
343 automatically designated as an output value. For more complex functions
344 the B<xsubpp> compiler will need help to determine which variables are output
347 This keyword will normally be used to complement the CODE: keyword.
348 The RETVAL variable is not recognized as an output variable when the
349 CODE: keyword is present. The OUTPUT: keyword is used in this
350 situation to tell the compiler that RETVAL really is an output
353 The OUTPUT: keyword can also be used to indicate that function parameters
354 are output variables. This may be necessary when a parameter has been
355 modified within the function and the programmer would like the update to
359 rpcb_gettime(host,timep)
365 The OUTPUT: keyword will also allow an output parameter to
366 be mapped to a matching piece of code rather than to a
370 rpcb_gettime(host,timep)
374 timep sv_setnv(ST(1), (double)timep);
376 B<xsubpp> emits an automatic C<SvSETMAGIC()> for all parameters in the
377 OUTPUT section of the XSUB, except RETVAL. This is the usually desired
378 behavior, as it takes care of properly invoking 'set' magic on output
379 parameters (needed for hash or array element parameters that must be
380 created if they didn't exist). If for some reason, this behavior is
381 not desired, the OUTPUT section may contain a C<SETMAGIC: DISABLE> line
382 to disable it for the remainder of the parameters in the OUTPUT section.
383 Likewise, C<SETMAGIC: ENABLE> can be used to reenable it for the
384 remainder of the OUTPUT section. See L<perlguts> for more details
387 =head2 The NO_OUTPUT Keyword
389 The NO_OUTPUT can be placed as the first token of the XSUB. This keyword
390 indicates that while the C subroutine we provide an interface to has
391 a non-C<void> return type, the return value of this C subroutine should not
392 be returned from the generated Perl subroutine.
394 With this keyword present L<The RETVAL Variable> is created, and in the
395 generated call to the subroutine this variable is assigned to, but the value
396 of this variable is not going to be used in the auto-generated code.
398 This keyword makes sense only if C<RETVAL> is going to be accessed by the
399 user-supplied code. It is especially useful to make a function interface
400 more Perl-like, especially when the C return value is just an error condition
401 indicator. For example,
404 delete_file(char *name)
407 croak("Error %d while deleting file '%s'", RETVAL, name);
409 Here the generated XS function returns nothing on success, and will die()
410 with a meaningful error message on error.
412 =head2 The CODE: Keyword
414 This keyword is used in more complicated XSUBs which require
415 special handling for the C function. The RETVAL variable is
416 still declared, but it will not be returned unless it is specified
417 in the OUTPUT: section.
419 The following XSUB is for a C function which requires special handling of
420 its parameters. The Perl usage is given first.
422 $status = rpcb_gettime( "localhost", $timep );
427 rpcb_gettime(host,timep)
431 RETVAL = rpcb_gettime( host, &timep );
436 =head2 The INIT: Keyword
438 The INIT: keyword allows initialization to be inserted into the XSUB before
439 the compiler generates the call to the C function. Unlike the CODE: keyword
440 above, this keyword does not affect the way the compiler handles RETVAL.
443 rpcb_gettime(host,timep)
447 printf("# Host is %s\n", host );
451 Another use for the INIT: section is to check for preconditions before
452 making a call to the C function:
459 if (a == 0 && b == 0)
462 croak("lldiv: cannot divide by 0");
464 =head2 The NO_INIT Keyword
466 The NO_INIT keyword is used to indicate that a function
467 parameter is being used only as an output value. The B<xsubpp>
468 compiler will normally generate code to read the values of
469 all function parameters from the argument stack and assign
470 them to C variables upon entry to the function. NO_INIT
471 will tell the compiler that some parameters will be used for
472 output rather than for input and that they will be handled
473 before the function terminates.
475 The following example shows a variation of the rpcb_gettime() function.
476 This function uses the timep variable only as an output variable and does
477 not care about its initial contents.
480 rpcb_gettime(host,timep)
482 time_t &timep = NO_INIT
486 =head2 Initializing Function Parameters
488 C function parameters are normally initialized with their values from
489 the argument stack (which in turn contains the parameters that were
490 passed to the XSUB from Perl). The typemaps contain the
491 code segments which are used to translate the Perl values to
492 the C parameters. The programmer, however, is allowed to
493 override the typemaps and supply alternate (or additional)
494 initialization code. Initialization code starts with the first
495 C<=>, C<;> or C<+> on a line in the INPUT: section. The only
496 exception happens if this C<;> terminates the line, then this C<;>
499 The following code demonstrates how to supply initialization code for
500 function parameters. The initialization code is eval'd within double
501 quotes by the compiler before it is added to the output so anything
502 which should be interpreted literally [mainly C<$>, C<@>, or C<\\>]
503 must be protected with backslashes. The variables $var, $arg,
504 and $type can be used as in typemaps.
507 rpcb_gettime(host,timep)
508 char *host = (char *)SvPV($arg,PL_na);
513 This should not be used to supply default values for parameters. One
514 would normally use this when a function parameter must be processed by
515 another library function before it can be used. Default parameters are
516 covered in the next section.
518 If the initialization begins with C<=>, then it is output in
519 the declaration for the input variable, replacing the initialization
520 supplied by the typemap. If the initialization
521 begins with C<;> or C<+>, then it is performed after
522 all of the input variables have been declared. In the C<;>
523 case the initialization normally supplied by the typemap is not performed.
524 For the C<+> case, the declaration for the variable will include the
525 initialization from the typemap. A global
526 variable, C<%v>, is available for the truly rare case where
527 information from one initialization is needed in another
530 Here's a truly obscure example:
533 rpcb_gettime(host,timep)
534 time_t &timep ; /* \$v{timep}=@{[$v{timep}=$arg]} */
535 char *host + SvOK($v{timep}) ? SvPV($arg,PL_na) : NULL;
539 The construct C<\$v{timep}=@{[$v{timep}=$arg]}> used in the above
540 example has a two-fold purpose: first, when this line is processed by
541 B<xsubpp>, the Perl snippet C<$v{timep}=$arg> is evaluated. Second,
542 the text of the evaluated snippet is output into the generated C file
543 (inside a C comment)! During the processing of C<char *host> line,
544 $arg will evaluate to C<ST(0)>, and C<$v{timep}> will evaluate to
547 =head2 Default Parameter Values
549 Default values for XSUB arguments can be specified by placing an
550 assignment statement in the parameter list. The default value may
551 be a number, a string or the special string C<NO_INIT>. Defaults should
552 always be used on the right-most parameters only.
554 To allow the XSUB for rpcb_gettime() to have a default host
555 value the parameters to the XSUB could be rearranged. The
556 XSUB will then call the real rpcb_gettime() function with
557 the parameters in the correct order. This XSUB can be called
558 from Perl with either of the following statements:
560 $status = rpcb_gettime( $timep, $host );
562 $status = rpcb_gettime( $timep );
564 The XSUB will look like the code which follows. A CODE:
565 block is used to call the real rpcb_gettime() function with
566 the parameters in the correct order for that function.
569 rpcb_gettime(timep,host="localhost")
571 time_t timep = NO_INIT
573 RETVAL = rpcb_gettime( host, &timep );
578 =head2 The PREINIT: Keyword
580 The PREINIT: keyword allows extra variables to be declared immediately
581 before or after the declarations of the parameters from the INPUT: section
584 If a variable is declared inside a CODE: section it will follow any typemap
585 code that is emitted for the input parameters. This may result in the
586 declaration ending up after C code, which is C syntax error. Similar
587 errors may happen with an explicit C<;>-type or C<+>-type initialization of
588 parameters is used (see L<"Initializing Function Parameters">). Declaring
589 these variables in an INIT: section will not help.
591 In such cases, to force an additional variable to be declared together
592 with declarations of other variables, place the declaration into a
593 PREINIT: section. The PREINIT: keyword may be used one or more times
596 The following examples are equivalent, but if the code is using complex
597 typemaps then the first example is safer.
601 time_t timep = NO_INIT
603 char *host = "localhost";
605 RETVAL = rpcb_gettime( host, &timep );
610 For this particular case an INIT: keyword would generate the
611 same C code as the PREINIT: keyword. Another correct, but error-prone example:
615 time_t timep = NO_INIT
617 char *host = "localhost";
618 RETVAL = rpcb_gettime( host, &timep );
623 Another way to declare C<host> is to use a C block in the CODE: section:
627 time_t timep = NO_INIT
630 char *host = "localhost";
631 RETVAL = rpcb_gettime( host, &timep );
637 The ability to put additional declarations before the typemap entries are
638 processed is very handy in the cases when typemap conversions manipulate
644 MyState st = global_state;
648 reset_to(global_state, st);
650 Here we suppose that conversion to C<MyObject> in the INPUT: section and from
651 MyObject when processing RETVAL will modify a global variable C<global_state>.
652 After these conversions are performed, we restore the old value of
653 C<global_state> (to avoid memory leaks, for example).
655 There is another way to trade clarity for compactness: INPUT sections allow
656 declaration of C variables which do not appear in the parameter list of
657 a subroutine. Thus the above code for mutate() can be rewritten as
661 MyState st = global_state;
664 reset_to(global_state, st);
666 and the code for rpcb_gettime() can be rewritten as
670 time_t timep = NO_INIT
671 char *host = "localhost";
678 =head2 The SCOPE: Keyword
680 The SCOPE: keyword allows scoping to be enabled for a particular XSUB. If
681 enabled, the XSUB will invoke ENTER and LEAVE automatically.
683 To support potentially complex type mappings, if a typemap entry used
684 by an XSUB contains a comment like C</*scope*/> then scoping will
685 be automatically enabled for that XSUB.
695 =head2 The INPUT: Keyword
697 The XSUB's parameters are usually evaluated immediately after entering the
698 XSUB. The INPUT: keyword can be used to force those parameters to be
699 evaluated a little later. The INPUT: keyword can be used multiple times
700 within an XSUB and can be used to list one or more input variables. This
701 keyword is used with the PREINIT: keyword.
703 The following example shows how the input parameter C<timep> can be
704 evaluated late, after a PREINIT.
707 rpcb_gettime(host,timep)
714 RETVAL = rpcb_gettime( host, &tt );
720 The next example shows each input parameter evaluated late.
723 rpcb_gettime(host,timep)
734 RETVAL = rpcb_gettime( h, &tt );
740 Since INPUT sections allow declaration of C variables which do not appear
741 in the parameter list of a subroutine, this may be shortened to:
744 rpcb_gettime(host,timep)
750 RETVAL = rpcb_gettime( h, &tt );
756 (We used our knowledge that input conversion for C<char *> is a "simple" one,
757 thus C<host> is initialized on the declaration line, and our assignment
758 C<h = host> is not performed too early. Otherwise one would need to have the
759 assignment C<h = host> in a CODE: or INIT: section.)
761 =head2 The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
763 In the list of parameters for an XSUB, one can precede parameter names
764 by the C<IN>/C<OUTLIST>/C<IN_OUTLIST>/C<OUT>/C<IN_OUT> keywords.
765 C<IN> keyword is the default, the other keywords indicate how the Perl
766 interface should differ from the C interface.
768 Parameters preceded by C<OUTLIST>/C<IN_OUTLIST>/C<OUT>/C<IN_OUT>
769 keywords are considered to be used by the C subroutine I<via
770 pointers>. C<OUTLIST>/C<OUT> keywords indicate that the C subroutine
771 does not inspect the memory pointed by this parameter, but will write
772 through this pointer to provide additional return values.
774 Parameters preceded by C<OUTLIST> keyword do not appear in the usage
775 signature of the generated Perl function.
777 Parameters preceded by C<IN_OUTLIST>/C<IN_OUT>/C<OUT> I<do> appear as
778 parameters to the Perl function. With the exception of
779 C<OUT>-parameters, these parameters are converted to the corresponding
780 C type, then pointers to these data are given as arguments to the C
781 function. It is expected that the C function will write through these
784 The return list of the generated Perl function consists of the C return value
785 from the function (unless the XSUB is of C<void> return type or
786 C<The NO_OUTPUT Keyword> was used) followed by all the C<OUTLIST>
787 and C<IN_OUTLIST> parameters (in the order of appearance). On the
788 return from the XSUB the C<IN_OUT>/C<OUT> Perl parameter will be
789 modified to have the values written by the C function.
794 day_month(OUTLIST day, IN unix_time, OUTLIST month)
799 should be used from Perl as
801 my ($day, $month) = day_month(time);
803 The C signature of the corresponding function should be
805 void day_month(int *day, int unix_time, int *month);
807 The C<IN>/C<OUTLIST>/C<IN_OUTLIST>/C<IN_OUT>/C<OUT> keywords can be
808 mixed with ANSI-style declarations, as in
811 day_month(OUTLIST int day, int unix_time, OUTLIST int month)
813 (here the optional C<IN> keyword is omitted).
815 The C<IN_OUT> parameters are identical with parameters introduced with
816 L<The & Unary Operator> and put into the C<OUTPUT:> section (see
817 L<The OUTPUT: Keyword>). The C<IN_OUTLIST> parameters are very similar,
818 the only difference being that the value C function writes through the
819 pointer would not modify the Perl parameter, but is put in the output
822 The C<OUTLIST>/C<OUT> parameter differ from C<IN_OUTLIST>/C<IN_OUT>
823 parameters only by the initial value of the Perl parameter not
824 being read (and not being given to the C function - which gets some
825 garbage instead). For example, the same C function as above can be
828 void day_month(OUT int day, int unix_time, OUT int month);
833 day_month(day, unix_time, month)
841 However, the generated Perl function is called in very C-ish style:
844 day_month($day, time, $month);
846 =head2 The C<length(NAME)> Keyword
848 If one of the input arguments to the C function is the length of a string
849 argument C<NAME>, one can substitute the name of the length-argument by
850 C<length(NAME)> in the XSUB declaration. This argument must be omited when
851 the generated Perl function is called. E.g.,
854 dump_chars(char *s, short l)
858 printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
863 MODULE = x PACKAGE = x
865 void dump_chars(char *s, short length(s))
867 should be called as C<dump_chars($string)>.
869 This directive is supported with ANSI-type function declarations only.
871 =head2 Variable-length Parameter Lists
873 XSUBs can have variable-length parameter lists by specifying an ellipsis
874 C<(...)> in the parameter list. This use of the ellipsis is similar to that
875 found in ANSI C. The programmer is able to determine the number of
876 arguments passed to the XSUB by examining the C<items> variable which the
877 B<xsubpp> compiler supplies for all XSUBs. By using this mechanism one can
878 create an XSUB which accepts a list of parameters of unknown length.
880 The I<host> parameter for the rpcb_gettime() XSUB can be
881 optional so the ellipsis can be used to indicate that the
882 XSUB will take a variable number of parameters. Perl should
883 be able to call this XSUB with either of the following statements.
885 $status = rpcb_gettime( $timep, $host );
887 $status = rpcb_gettime( $timep );
889 The XS code, with ellipsis, follows.
892 rpcb_gettime(timep, ...)
893 time_t timep = NO_INIT
895 char *host = "localhost";
899 host = (char *)SvPV(ST(1), n_a);
900 RETVAL = rpcb_gettime( host, &timep );
905 =head2 The C_ARGS: Keyword
907 The C_ARGS: keyword allows creating of XSUBS which have different
908 calling sequence from Perl than from C, without a need to write
909 CODE: or PPCODE: section. The contents of the C_ARGS: paragraph is
910 put as the argument to the called C function without any change.
912 For example, suppose that a C function is declared as
914 symbolic nth_derivative(int n, symbolic function, int flags);
916 and that the default flags are kept in a global C variable
917 C<default_flags>. Suppose that you want to create an interface which
920 $second_deriv = $function->nth_derivative(2);
922 To do this, declare the XSUB as
925 nth_derivative(function, n)
929 n, function, default_flags
931 =head2 The PPCODE: Keyword
933 The PPCODE: keyword is an alternate form of the CODE: keyword and is used
934 to tell the B<xsubpp> compiler that the programmer is supplying the code to
935 control the argument stack for the XSUBs return values. Occasionally one
936 will want an XSUB to return a list of values rather than a single value.
937 In these cases one must use PPCODE: and then explicitly push the list of
938 values on the stack. The PPCODE: and CODE: keywords should not be used
939 together within the same XSUB.
941 The actual difference between PPCODE: and CODE: sections is in the
942 initialization of C<SP> macro (which stands for the I<current> Perl
943 stack pointer), and in the handling of data on the stack when returning
944 from an XSUB. In CODE: sections SP preserves the value which was on
945 entry to the XSUB: SP is on the function pointer (which follows the
946 last parameter). In PPCODE: sections SP is moved backward to the
947 beginning of the parameter list, which allows C<PUSH*()> macros
948 to place output values in the place Perl expects them to be when
949 the XSUB returns back to Perl.
951 The generated trailer for a CODE: section ensures that the number of return
952 values Perl will see is either 0 or 1 (depending on the C<void>ness of the
953 return value of the C function, and heuristics mentioned in
954 L<"The RETVAL Variable">). The trailer generated for a PPCODE: section
955 is based on the number of return values and on the number of times
956 C<SP> was updated by C<[X]PUSH*()> macros.
958 Note that macros C<ST(i)>, C<XST_m*()> and C<XSRETURN*()> work equally
959 well in CODE: sections and PPCODE: sections.
961 The following XSUB will call the C rpcb_gettime() function
962 and will return its two output values, timep and status, to
963 Perl as a single list.
972 status = rpcb_gettime( host, &timep );
974 PUSHs(sv_2mortal(newSViv(status)));
975 PUSHs(sv_2mortal(newSViv(timep)));
977 Notice that the programmer must supply the C code necessary
978 to have the real rpcb_gettime() function called and to have
979 the return values properly placed on the argument stack.
981 The C<void> return type for this function tells the B<xsubpp> compiler that
982 the RETVAL variable is not needed or used and that it should not be created.
983 In most scenarios the void return type should be used with the PPCODE:
986 The EXTEND() macro is used to make room on the argument
987 stack for 2 return values. The PPCODE: directive causes the
988 B<xsubpp> compiler to create a stack pointer available as C<SP>, and it
989 is this pointer which is being used in the EXTEND() macro.
990 The values are then pushed onto the stack with the PUSHs()
993 Now the rpcb_gettime() function can be used from Perl with
994 the following statement.
996 ($status, $timep) = rpcb_gettime("localhost");
998 When handling output parameters with a PPCODE section, be sure to handle
999 'set' magic properly. See L<perlguts> for details about 'set' magic.
1001 =head2 Returning Undef And Empty Lists
1003 Occasionally the programmer will want to return simply
1004 C<undef> or an empty list if a function fails rather than a
1005 separate status value. The rpcb_gettime() function offers
1006 just this situation. If the function succeeds we would like
1007 to have it return the time and if it fails we would like to
1008 have undef returned. In the following Perl code the value
1009 of $timep will either be undef or it will be a valid time.
1011 $timep = rpcb_gettime( "localhost" );
1013 The following XSUB uses the C<SV *> return type as a mnemonic only,
1014 and uses a CODE: block to indicate to the compiler
1015 that the programmer has supplied all the necessary code. The
1016 sv_newmortal() call will initialize the return value to undef, making that
1017 the default return value.
1026 ST(0) = sv_newmortal();
1027 if( rpcb_gettime( host, &timep ) )
1028 sv_setnv( ST(0), (double)timep);
1030 The next example demonstrates how one would place an explicit undef in the
1031 return value, should the need arise.
1040 ST(0) = sv_newmortal();
1041 if( rpcb_gettime( host, &timep ) ){
1042 sv_setnv( ST(0), (double)timep);
1045 ST(0) = &PL_sv_undef;
1048 To return an empty list one must use a PPCODE: block and
1049 then not push return values on the stack.
1057 if( rpcb_gettime( host, &timep ) )
1058 PUSHs(sv_2mortal(newSViv(timep)));
1060 /* Nothing pushed on stack, so an empty
1061 * list is implicitly returned. */
1064 Some people may be inclined to include an explicit C<return> in the above
1065 XSUB, rather than letting control fall through to the end. In those
1066 situations C<XSRETURN_EMPTY> should be used, instead. This will ensure that
1067 the XSUB stack is properly adjusted. Consult L<perlapi> for other
1070 Since C<XSRETURN_*> macros can be used with CODE blocks as well, one can
1071 rewrite this example as:
1079 RETVAL = rpcb_gettime( host, &timep );
1085 In fact, one can put this check into a POSTCALL: section as well. Together
1086 with PREINIT: simplifications, this leads to:
1096 =head2 The REQUIRE: Keyword
1098 The REQUIRE: keyword is used to indicate the minimum version of the
1099 B<xsubpp> compiler needed to compile the XS module. An XS module which
1100 contains the following statement will compile with only B<xsubpp> version
1105 =head2 The CLEANUP: Keyword
1107 This keyword can be used when an XSUB requires special cleanup procedures
1108 before it terminates. When the CLEANUP: keyword is used it must follow
1109 any CODE:, PPCODE:, or OUTPUT: blocks which are present in the XSUB. The
1110 code specified for the cleanup block will be added as the last statements
1113 =head2 The POSTCALL: Keyword
1115 This keyword can be used when an XSUB requires special procedures
1116 executed after the C subroutine call is performed. When the POSTCALL:
1117 keyword is used it must precede OUTPUT: and CLEANUP: blocks which are
1118 present in the XSUB.
1120 See examples in L<"The NO_OUTPUT Keyword"> and L<"Returning Undef And Empty Lists">.
1122 The POSTCALL: block does not make a lot of sense when the C subroutine
1123 call is supplied by user by providing either CODE: or PPCODE: section.
1125 =head2 The BOOT: Keyword
1127 The BOOT: keyword is used to add code to the extension's bootstrap
1128 function. The bootstrap function is generated by the B<xsubpp> compiler and
1129 normally holds the statements necessary to register any XSUBs with Perl.
1130 With the BOOT: keyword the programmer can tell the compiler to add extra
1131 statements to the bootstrap function.
1133 This keyword may be used any time after the first MODULE keyword and should
1134 appear on a line by itself. The first blank line after the keyword will
1135 terminate the code block.
1138 # The following message will be printed when the
1139 # bootstrap function executes.
1140 printf("Hello from the bootstrap!\n");
1142 =head2 The VERSIONCHECK: Keyword
1144 The VERSIONCHECK: keyword corresponds to B<xsubpp>'s C<-versioncheck> and
1145 C<-noversioncheck> options. This keyword overrides the command line
1146 options. Version checking is enabled by default. When version checking is
1147 enabled the XS module will attempt to verify that its version matches the
1148 version of the PM module.
1150 To enable version checking:
1152 VERSIONCHECK: ENABLE
1154 To disable version checking:
1156 VERSIONCHECK: DISABLE
1158 =head2 The PROTOTYPES: Keyword
1160 The PROTOTYPES: keyword corresponds to B<xsubpp>'s C<-prototypes> and
1161 C<-noprototypes> options. This keyword overrides the command line options.
1162 Prototypes are enabled by default. When prototypes are enabled XSUBs will
1163 be given Perl prototypes. This keyword may be used multiple times in an XS
1164 module to enable and disable prototypes for different parts of the module.
1166 To enable prototypes:
1170 To disable prototypes:
1174 =head2 The PROTOTYPE: Keyword
1176 This keyword is similar to the PROTOTYPES: keyword above but can be used to
1177 force B<xsubpp> to use a specific prototype for the XSUB. This keyword
1178 overrides all other prototype options and keywords but affects only the
1179 current XSUB. Consult L<perlsub/Prototypes> for information about Perl
1183 rpcb_gettime(timep, ...)
1184 time_t timep = NO_INIT
1187 char *host = "localhost";
1191 host = (char *)SvPV(ST(1), n_a);
1192 RETVAL = rpcb_gettime( host, &timep );
1197 If the prototypes are enabled, you can disable it locally for a given
1198 XSUB as in the following example:
1201 rpcb_gettime_noproto()
1205 =head2 The ALIAS: Keyword
1207 The ALIAS: keyword allows an XSUB to have two or more unique Perl names
1208 and to know which of those names was used when it was invoked. The Perl
1209 names may be fully-qualified with package names. Each alias is given an
1210 index. The compiler will setup a variable called C<ix> which contain the
1211 index of the alias which was used. When the XSUB is called with its
1212 declared name C<ix> will be 0.
1214 The following example will create aliases C<FOO::gettime()> and
1215 C<BAR::getit()> for this function.
1218 rpcb_gettime(host,timep)
1225 printf("# ix = %d\n", ix );
1229 =head2 The OVERLOAD: Keyword
1231 Instead of writing an overloaded interface using pure Perl, you
1232 can also use the OVERLOAD keyword to define additional Perl names
1233 for your functions (like the ALIAS: keyword above). However, the
1234 overloaded functions must be defined with three parameters (except
1235 for the nomethod() function which needs four parameters). If any
1236 function has the OVERLOAD: keyword, several additional lines
1237 will be defined in the c file generated by xsubpp in order to
1238 register with the overload magic.
1240 Since blessed objects are actually stored as RV's, it is useful
1241 to use the typemap features to preprocess parameters and extract
1242 the actual SV stored within the blessed RV. See the sample for
1243 T_PTROBJ_SPECIAL below.
1245 To use the OVERLOAD: keyword, create an XS function which takes
1246 three input parameters ( or use the c style '...' definition) like
1250 cmp (lobj, robj, swap)
1255 { /* function defined here */}
1257 In this case, the function will overload both of the three way
1258 comparison operators. For all overload operations using non-alpha
1259 characters, you must type the parameter without quoting, seperating
1260 multiple overloads with whitespace. Note that "" (the stringify
1261 overload) should be entered as \"\" (i.e. escaped).
1263 =head2 The FALLBACK: Keyword
1265 In addition to the OVERLOAD keyword, if you need to control how
1266 Perl autogenerates missing overloaded operators, you can set the
1267 FALLBACK keyword in the module header section, like this:
1269 MODULE = RPC PACKAGE = RPC
1274 where FALLBACK can take any of the three values TRUE, FALSE, or
1275 UNDEF. If you do not set any FALLBACK value when using OVERLOAD,
1276 it defaults to UNDEF. FALLBACK is not used except when one or
1277 more functions using OVERLOAD have been defined. Please see
1278 L<overload/Fallback> for more details.
1280 =head2 The INTERFACE: Keyword
1282 This keyword declares the current XSUB as a keeper of the given
1283 calling signature. If some text follows this keyword, it is
1284 considered as a list of functions which have this signature, and
1285 should be attached to the current XSUB.
1287 For example, if you have 4 C functions multiply(), divide(), add(),
1288 subtract() all having the signature:
1290 symbolic f(symbolic, symbolic);
1292 you can make them all to use the same XSUB using this:
1295 interface_s_ss(arg1, arg2)
1302 (This is the complete XSUB code for 4 Perl functions!) Four generated
1303 Perl function share names with corresponding C functions.
1305 The advantage of this approach comparing to ALIAS: keyword is that there
1306 is no need to code a switch statement, each Perl function (which shares
1307 the same XSUB) knows which C function it should call. Additionally, one
1308 can attach an extra function remainder() at runtime by using
1310 CV *mycv = newXSproto("Symbolic::remainder",
1311 XS_Symbolic_interface_s_ss, __FILE__, "$$");
1312 XSINTERFACE_FUNC_SET(mycv, remainder);
1314 say, from another XSUB. (This example supposes that there was no
1315 INTERFACE_MACRO: section, otherwise one needs to use something else instead of
1316 C<XSINTERFACE_FUNC_SET>, see the next section.)
1318 =head2 The INTERFACE_MACRO: Keyword
1320 This keyword allows one to define an INTERFACE using a different way
1321 to extract a function pointer from an XSUB. The text which follows
1322 this keyword should give the name of macros which would extract/set a
1323 function pointer. The extractor macro is given return type, C<CV*>,
1324 and C<XSANY.any_dptr> for this C<CV*>. The setter macro is given cv,
1325 and the function pointer.
1327 The default value is C<XSINTERFACE_FUNC> and C<XSINTERFACE_FUNC_SET>.
1328 An INTERFACE keyword with an empty list of functions can be omitted if
1329 INTERFACE_MACRO keyword is used.
1331 Suppose that in the previous example functions pointers for
1332 multiply(), divide(), add(), subtract() are kept in a global C array
1333 C<fp[]> with offsets being C<multiply_off>, C<divide_off>, C<add_off>,
1334 C<subtract_off>. Then one can use
1336 #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
1337 ((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32])
1338 #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
1339 CvXSUBANY(cv).any_i32 = CAT2( f, _off )
1344 interface_s_ss(arg1, arg2)
1348 XSINTERFACE_FUNC_BYOFFSET
1349 XSINTERFACE_FUNC_BYOFFSET_set
1356 =head2 The INCLUDE: Keyword
1358 This keyword can be used to pull other files into the XS module. The other
1359 files may have XS code. INCLUDE: can also be used to run a command to
1360 generate the XS code to be pulled into the module.
1362 The file F<Rpcb1.xsh> contains our C<rpcb_gettime()> function:
1365 rpcb_gettime(host,timep)
1371 The XS module can use INCLUDE: to pull that file into it.
1375 If the parameters to the INCLUDE: keyword are followed by a pipe (C<|>) then
1376 the compiler will interpret the parameters as a command.
1378 INCLUDE: cat Rpcb1.xsh |
1380 =head2 The CASE: Keyword
1382 The CASE: keyword allows an XSUB to have multiple distinct parts with each
1383 part acting as a virtual XSUB. CASE: is greedy and if it is used then all
1384 other XS keywords must be contained within a CASE:. This means nothing may
1385 precede the first CASE: in the XSUB and anything following the last CASE: is
1386 included in that case.
1388 A CASE: might switch via a parameter of the XSUB, via the C<ix> ALIAS:
1389 variable (see L<"The ALIAS: Keyword">), or maybe via the C<items> variable
1390 (see L<"Variable-length Parameter Lists">). The last CASE: becomes the
1391 B<default> case if it is not associated with a conditional. The following
1392 example shows CASE switched via C<ix> with a function C<rpcb_gettime()>
1393 having an alias C<x_gettime()>. When the function is called as
1394 C<rpcb_gettime()> its parameters are the usual C<(char *host, time_t *timep)>,
1395 but when the function is called as C<x_gettime()> its parameters are
1396 reversed, C<(time_t *timep, char *host)>.
1404 # 'a' is timep, 'b' is host
1408 RETVAL = rpcb_gettime( b, &a );
1413 # 'a' is host, 'b' is timep
1420 That function can be called with either of the following statements. Note
1421 the different argument lists.
1423 $status = rpcb_gettime( $host, $timep );
1425 $status = x_gettime( $timep, $host );
1427 =head2 The & Unary Operator
1429 The C<&> unary operator in the INPUT: section is used to tell B<xsubpp>
1430 that it should convert a Perl value to/from C using the C type to the left
1431 of C<&>, but provide a pointer to this value when the C function is called.
1433 This is useful to avoid a CODE: block for a C function which takes a parameter
1434 by reference. Typically, the parameter should be not a pointer type (an
1435 C<int> or C<long> but not an C<int*> or C<long*>).
1437 The following XSUB will generate incorrect C code. The B<xsubpp> compiler will
1438 turn this into code which calls C<rpcb_gettime()> with parameters C<(char
1439 *host, time_t timep)>, but the real C<rpcb_gettime()> wants the C<timep>
1440 parameter to be of type C<time_t*> rather than C<time_t>.
1443 rpcb_gettime(host,timep)
1449 That problem is corrected by using the C<&> operator. The B<xsubpp> compiler
1450 will now turn this into code which calls C<rpcb_gettime()> correctly with
1451 parameters C<(char *host, time_t *timep)>. It does this by carrying the
1452 C<&> through, so the function call looks like C<rpcb_gettime(host, &timep)>.
1455 rpcb_gettime(host,timep)
1461 =head2 Inserting POD, Comments and C Preprocessor Directives
1463 C preprocessor directives are allowed within BOOT:, PREINIT: INIT:, CODE:,
1464 PPCODE:, POSTCALL:, and CLEANUP: blocks, as well as outside the functions.
1465 Comments are allowed anywhere after the MODULE keyword. The compiler will
1466 pass the preprocessor directives through untouched and will remove the
1467 commented lines. POD documentation is allowed at any point, both in the
1468 C and XS language sections. POD must be terminated with a C<=cut> command;
1469 C<xsubpp> will exit with an error if it does not. It is very unlikely that
1470 human generated C code will be mistaken for POD, as most indenting styles
1471 result in whitespace in front of any line starting with C<=>. Machine
1472 generated XS files may fall into this trap unless care is taken to
1473 ensure that a space breaks the sequence "\n=".
1475 Comments can be added to XSUBs by placing a C<#> as the first
1476 non-whitespace of a line. Care should be taken to avoid making the
1477 comment look like a C preprocessor directive, lest it be interpreted as
1478 such. The simplest way to prevent this is to put whitespace in front of
1481 If you use preprocessor directives to choose one of two
1482 versions of a function, use
1485 #else /* ... version2 */
1495 because otherwise B<xsubpp> will believe that you made a duplicate
1496 definition of the function. Also, put a blank line before the
1497 #else/#endif so it will not be seen as part of the function body.
1499 =head2 Using XS With C++
1501 If an XSUB name contains C<::>, it is considered to be a C++ method.
1502 The generated Perl function will assume that
1503 its first argument is an object pointer. The object pointer
1504 will be stored in a variable called THIS. The object should
1505 have been created by C++ with the new() function and should
1506 be blessed by Perl with the sv_setref_pv() macro. The
1507 blessing of the object by Perl can be handled by a typemap. An example
1508 typemap is shown at the end of this section.
1510 If the return type of the XSUB includes C<static>, the method is considered
1511 to be a static method. It will call the C++
1512 function using the class::method() syntax. If the method is not static
1513 the function will be called using the THIS-E<gt>method() syntax.
1515 The next examples will use the following C++ class.
1522 void set_blue( int );
1528 The XSUBs for the blue() and set_blue() methods are defined with the class
1529 name but the parameter for the object (THIS, or "self") is implicit and is
1536 color::set_blue( val )
1539 Both Perl functions will expect an object as the first parameter. In the
1540 generated C++ code the object is called C<THIS>, and the method call will
1541 be performed on this object. So in the C++ code the blue() and set_blue()
1542 methods will be called as this:
1544 RETVAL = THIS->blue();
1546 THIS->set_blue( val );
1548 You could also write a single get/set method using an optional argument:
1551 color::blue( val = NO_INIT )
1556 THIS->set_blue( val );
1557 RETVAL = THIS->blue();
1561 If the function's name is B<DESTROY> then the C++ C<delete> function will be
1562 called and C<THIS> will be given as its parameter. The generated C++ code for
1567 will look like this:
1569 color *THIS = ...; // Initialized as in typemap
1573 If the function's name is B<new> then the C++ C<new> function will be called
1574 to create a dynamic C++ object. The XSUB will expect the class name, which
1575 will be kept in a variable called C<CLASS>, to be given as the first
1581 The generated C++ code will call C<new>.
1583 RETVAL = new color();
1585 The following is an example of a typemap that could be used for this C++
1592 # The Perl object is blessed into 'CLASS', which should be a
1593 # char* having the name of the package for the blessing.
1595 sv_setref_pv( $arg, CLASS, (void*)$var );
1599 if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
1600 $var = ($type)SvIV((SV*)SvRV( $arg ));
1602 warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
1606 =head2 Interface Strategy
1608 When designing an interface between Perl and a C library a straight
1609 translation from C to XS (such as created by C<h2xs -x>) is often sufficient.
1610 However, sometimes the interface will look
1611 very C-like and occasionally nonintuitive, especially when the C function
1612 modifies one of its parameters, or returns failure inband (as in "negative
1613 return values mean failure"). In cases where the programmer wishes to
1614 create a more Perl-like interface the following strategy may help to
1615 identify the more critical parts of the interface.
1617 Identify the C functions with input/output or output parameters. The XSUBs for
1618 these functions may be able to return lists to Perl.
1620 Identify the C functions which use some inband info as an indication
1621 of failure. They may be
1622 candidates to return undef or an empty list in case of failure. If the
1623 failure may be detected without a call to the C function, you may want to use
1624 an INIT: section to report the failure. For failures detectable after the C
1625 function returns one may want to use a POSTCALL: section to process the
1626 failure. In more complicated cases use CODE: or PPCODE: sections.
1628 If many functions use the same failure indication based on the return value,
1629 you may want to create a special typedef to handle this situation. Put
1631 typedef int negative_is_failure;
1633 near the beginning of XS file, and create an OUTPUT typemap entry
1634 for C<negative_is_failure> which converts negative values to C<undef>, or
1635 maybe croak()s. After this the return value of type C<negative_is_failure>
1636 will create more Perl-like interface.
1638 Identify which values are used by only the C and XSUB functions
1639 themselves, say, when a parameter to a function should be a contents of a
1640 global variable. If Perl does not need to access the contents of the value
1641 then it may not be necessary to provide a translation for that value
1644 Identify the pointers in the C function parameter lists and return
1645 values. Some pointers may be used to implement input/output or
1646 output parameters, they can be handled in XS with the C<&> unary operator,
1647 and, possibly, using the NO_INIT keyword.
1648 Some others will require handling of types like C<int *>, and one needs
1649 to decide what a useful Perl translation will do in such a case. When
1650 the semantic is clear, it is advisable to put the translation into a typemap
1653 Identify the structures used by the C functions. In many
1654 cases it may be helpful to use the T_PTROBJ typemap for
1655 these structures so they can be manipulated by Perl as
1656 blessed objects. (This is handled automatically by C<h2xs -x>.)
1658 If the same C type is used in several different contexts which require
1659 different translations, C<typedef> several new types mapped to this C type,
1660 and create separate F<typemap> entries for these new types. Use these
1661 types in declarations of return type and parameters to XSUBs.
1663 =head2 Perl Objects And C Structures
1665 When dealing with C structures one should select either
1666 B<T_PTROBJ> or B<T_PTRREF> for the XS type. Both types are
1667 designed to handle pointers to complex objects. The
1668 T_PTRREF type will allow the Perl object to be unblessed
1669 while the T_PTROBJ type requires that the object be blessed.
1670 By using T_PTROBJ one can achieve a form of type-checking
1671 because the XSUB will attempt to verify that the Perl object
1672 is of the expected type.
1674 The following XS code shows the getnetconfigent() function which is used
1675 with ONC+ TIRPC. The getnetconfigent() function will return a pointer to a
1676 C structure and has the C prototype shown below. The example will
1677 demonstrate how the C pointer will become a Perl reference. Perl will
1678 consider this reference to be a pointer to a blessed object and will
1679 attempt to call a destructor for the object. A destructor will be
1680 provided in the XS source to free the memory used by getnetconfigent().
1681 Destructors in XS can be created by specifying an XSUB function whose name
1682 ends with the word B<DESTROY>. XS destructors can be used to free memory
1683 which may have been malloc'd by another XSUB.
1685 struct netconfig *getnetconfigent(const char *netid);
1687 A C<typedef> will be created for C<struct netconfig>. The Perl
1688 object will be blessed in a class matching the name of the C
1689 type, with the tag C<Ptr> appended, and the name should not
1690 have embedded spaces if it will be a Perl package name. The
1691 destructor will be placed in a class corresponding to the
1692 class of the object and the PREFIX keyword will be used to
1693 trim the name to the word DESTROY as Perl will expect.
1695 typedef struct netconfig Netconfig;
1697 MODULE = RPC PACKAGE = RPC
1700 getnetconfigent(netid)
1703 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1706 rpcb_DESTROY(netconf)
1709 printf("Now in NetconfigPtr::DESTROY\n");
1712 This example requires the following typemap entry. Consult the typemap
1713 section for more information about adding new typemaps for an extension.
1716 Netconfig * T_PTROBJ
1718 This example will be used with the following Perl statements.
1721 $netconf = getnetconfigent("udp");
1723 When Perl destroys the object referenced by $netconf it will send the
1724 object to the supplied XSUB DESTROY function. Perl cannot determine, and
1725 does not care, that this object is a C struct and not a Perl object. In
1726 this sense, there is no difference between the object created by the
1727 getnetconfigent() XSUB and an object created by a normal Perl subroutine.
1731 The typemap is a collection of code fragments which are used by the B<xsubpp>
1732 compiler to map C function parameters and values to Perl values. The
1733 typemap file may consist of three sections labelled C<TYPEMAP>, C<INPUT>, and
1734 C<OUTPUT>. An unlabelled initial section is assumed to be a C<TYPEMAP>
1735 section. The INPUT section tells
1736 the compiler how to translate Perl values
1737 into variables of certain C types. The OUTPUT section tells the compiler
1738 how to translate the values from certain C types into values Perl can
1739 understand. The TYPEMAP section tells the compiler which of the INPUT and
1740 OUTPUT code fragments should be used to map a given C type to a Perl value.
1741 The section labels C<TYPEMAP>, C<INPUT>, or C<OUTPUT> must begin
1742 in the first column on a line by themselves, and must be in uppercase.
1744 The default typemap in the C<lib/ExtUtils> directory of the Perl source
1745 contains many useful types which can be used by Perl extensions. Some
1746 extensions define additional typemaps which they keep in their own directory.
1747 These additional typemaps may reference INPUT and OUTPUT maps in the main
1748 typemap. The B<xsubpp> compiler will allow the extension's own typemap to
1749 override any mappings which are in the default typemap.
1751 Most extensions which require a custom typemap will need only the TYPEMAP
1752 section of the typemap file. The custom typemap used in the
1753 getnetconfigent() example shown earlier demonstrates what may be the typical
1754 use of extension typemaps. That typemap is used to equate a C structure
1755 with the T_PTROBJ typemap. The typemap used by getnetconfigent() is shown
1756 here. Note that the C type is separated from the XS type with a tab and
1757 that the C unary operator C<*> is considered to be a part of the C type name.
1760 Netconfig *<tab>T_PTROBJ
1762 Here's a more complicated example: suppose that you wanted C<struct
1763 netconfig> to be blessed into the class C<Net::Config>. One way to do
1764 this is to use underscores (_) to separate package names, as follows:
1766 typedef struct netconfig * Net_Config;
1768 And then provide a typemap entry C<T_PTROBJ_SPECIAL> that maps underscores to
1769 double-colons (::), and declare C<Net_Config> to be of that type:
1773 Net_Config T_PTROBJ_SPECIAL
1777 if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
1778 IV tmp = SvIV((SV*)SvRV($arg));
1782 croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")
1786 sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",
1789 The INPUT and OUTPUT sections substitute underscores for double-colons
1790 on the fly, giving the desired effect. This example demonstrates some
1791 of the power and versatility of the typemap facility.
1793 =head2 Safely Storing Static Data in XS
1795 Starting with Perl 5.8, a macro framework has been defined to allow
1796 static data to be safely stored in XS modules that will be accessed from
1797 a multi-threaded Perl.
1799 Although primarily designed for use with multi-threaded Perl, the macros
1800 have been designed so that they will work with non-threaded Perl as well.
1802 It is therefore strongly recommended that these macros be used by all
1803 XS modules that make use of static data.
1805 The easiest way to get a template set of macros to use is by specifying
1806 the C<-g> (C<--global>) option with h2xs (see L<h2xs>).
1808 Below is an example module that makes use of the macros.
1816 #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION
1825 MODULE = BlindMice PACKAGE = BlindMice
1831 strcpy(MY_CXT.name[0], "None");
1832 strcpy(MY_CXT.name[1], "None");
1833 strcpy(MY_CXT.name[2], "None");
1837 newMouse(char * name)
1842 if (MY_CXT.count >= 3) {
1843 warn("Already have 3 blind mice") ;
1847 RETVAL = ++ MY_CXT.count;
1848 strcpy(MY_CXT.name[MY_CXT.count - 1], name);
1852 get_mouse_name(index)
1856 RETVAL = MY_CXT.lives ++;
1857 if (index > MY_CXT.count)
1858 croak("There are only 3 blind mice.");
1860 RETVAL = newSVpv(MY_CXT.name[index - 1]);
1869 This macro is used to define a unique key to refer to the static data
1870 for an XS module. The suggested naming scheme, as used by h2xs, is to
1871 use a string that consists of the module name, the string "::_guts"
1872 and the module version number.
1874 #define MY_CXT_KEY "MyModule::_guts" XS_VERSION
1876 =item typedef my_cxt_t
1878 This struct typedef I<must> always be called C<my_cxt_t> -- the other
1879 C<CXT*> macros assume the existence of the C<my_cxt_t> typedef name.
1881 Declare a typedef named C<my_cxt_t> that is a structure that contains
1882 all the data that needs to be interpreter-local.
1890 Always place the START_MY_CXT macro directly after the declaration
1895 The MY_CXT_INIT macro initialises storage for the C<my_cxt_t> struct.
1897 It I<must> be called exactly once -- typically in a BOOT: section.
1901 Use the dMY_CXT macro (a declaration) in all the functions that access
1906 Use the MY_CXT macro to access members of the C<my_cxt_t> struct. For
1907 example, if C<my_cxt_t> is
1913 then use this to access the C<index> member
1922 File C<RPC.xs>: Interface to some ONC+ RPC bind library functions.
1928 #include <rpc/rpc.h>
1930 typedef struct netconfig Netconfig;
1932 MODULE = RPC PACKAGE = RPC
1935 rpcb_gettime(host="localhost")
1940 ST(0) = sv_newmortal();
1941 if( rpcb_gettime( host, &timep ) )
1942 sv_setnv( ST(0), (double)timep );
1945 getnetconfigent(netid="udp")
1948 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1951 rpcb_DESTROY(netconf)
1954 printf("NetconfigPtr::DESTROY\n");
1957 File C<typemap>: Custom typemap for RPC.xs.
1960 Netconfig * T_PTROBJ
1962 File C<RPC.pm>: Perl module for the RPC extension.
1968 @ISA = qw(Exporter DynaLoader);
1969 @EXPORT = qw(rpcb_gettime getnetconfigent);
1974 File C<rpctest.pl>: Perl test program for the RPC extension.
1978 $netconf = getnetconfigent();
1979 $a = rpcb_gettime();
1980 print "time = $a\n";
1981 print "netconf = $netconf\n";
1983 $netconf = getnetconfigent("tcp");
1984 $a = rpcb_gettime("poplar");
1985 print "time = $a\n";
1986 print "netconf = $netconf\n";
1991 This document covers features supported by C<xsubpp> 1.935.
1995 Originally written by Dean Roehrich <F<roehrich@cray.com>>.
1997 Maintained since 1996 by The Perl Porters <F<perlbug@perl.org>>.