3 perlxs - XS language reference manual
9 XS is a language used to create an extension interface
10 between Perl and some C library which one wishes to use with
11 Perl. The XS interface is combined with the library to
12 create a new library which can be linked to Perl. An B<XSUB>
13 is a function in the XS language and is the core component
14 of the Perl application interface.
16 The XS compiler is called B<xsubpp>. This compiler will embed
17 the constructs necessary to let an XSUB, which is really a C
18 function in disguise, manipulate Perl values and creates the
19 glue necessary to let Perl access the XSUB. The compiler
20 uses B<typemaps> to determine how to map C function parameters
21 and variables to Perl values. The default typemap handles
22 many common C types. A supplement typemap must be created
23 to handle special structures and types for the library being
26 See L<perlxstut> for a tutorial on the whole extension creation process.
30 Many of the examples which follow will concentrate on creating an interface
31 between Perl and the ONC+ RPC bind library functions. The rpcb_gettime()
32 function is used to demonstrate many features of the XS language. This
33 function has two parameters; the first is an input parameter and the second
34 is an output parameter. The function also returns a status value.
36 bool_t rpcb_gettime(const char *host, time_t *timep);
38 From C this function will be called with the following
44 status = rpcb_gettime( "localhost", &timep );
46 If an XSUB is created to offer a direct translation between this function
47 and Perl, then this XSUB will be used from Perl with the following code.
48 The $status and $timep variables will contain the output of the function.
51 $status = rpcb_gettime( "localhost", $timep );
53 The following XS file shows an XS subroutine, or XSUB, which
54 demonstrates one possible interface to the rpcb_gettime()
55 function. This XSUB represents a direct translation between
56 C and Perl and so preserves the interface even from Perl.
57 This XSUB will be invoked from Perl with the usage shown
58 above. Note that the first three #include statements, for
59 C<EXTERN.h>, C<perl.h>, and C<XSUB.h>, will always be present at the
60 beginning of an XS file. This approach and others will be
61 expanded later in this document.
68 MODULE = RPC PACKAGE = RPC
71 rpcb_gettime(host,timep)
77 Any extension to Perl, including those containing XSUBs,
78 should have a Perl module to serve as the bootstrap which
79 pulls the extension into Perl. This module will export the
80 extension's functions and variables to the Perl program and
81 will cause the extension's XSUBs to be linked into Perl.
82 The following module will be used for most of the examples
83 in this document and should be used from Perl with the C<use>
84 command as shown earlier. Perl modules are explained in
85 more detail later in this document.
91 @ISA = qw(Exporter DynaLoader);
92 @EXPORT = qw( rpcb_gettime );
97 Throughout this document a variety of interfaces to the rpcb_gettime()
98 XSUB will be explored. The XSUBs will take their parameters in different
99 orders or will take different numbers of parameters. In each case the
100 XSUB is an abstraction between Perl and the real C rpcb_gettime()
101 function, and the XSUB must always ensure that the real rpcb_gettime()
102 function is called with the correct parameters. This abstraction will
103 allow the programmer to create a more Perl-like interface to the C
106 =head2 The Anatomy of an XSUB
108 The following XSUB allows a Perl program to access a C library function
109 called sin(). The XSUB will imitate the C function which takes a single
110 argument and returns a single value.
116 When using C pointers the indirection operator C<*> should be considered
117 part of the type and the address operator C<&> should be considered part of
118 the variable, as is demonstrated in the rpcb_gettime() function above. See
119 the section on typemaps for more about handling qualifiers and unary
120 operators in C types.
122 The function name and the return type must be placed on
131 The function body may be indented or left-adjusted. The following example
132 shows a function with its body left-adjusted. Most examples in this
133 document will indent the body.
141 =head2 The Argument Stack
143 The argument stack is used to store the values which are
144 sent as parameters to the XSUB and to store the XSUB's
145 return value. In reality all Perl functions keep their
146 values on this stack at the same time, each limited to its
147 own range of positions on the stack. In this document the
148 first position on that stack which belongs to the active
149 function will be referred to as position 0 for that function.
151 XSUBs refer to their stack arguments with the macro B<ST(x)>, where I<x>
152 refers to a position in this XSUB's part of the stack. Position 0 for that
153 function would be known to the XSUB as ST(0). The XSUB's incoming
154 parameters and outgoing return values always begin at ST(0). For many
155 simple cases the B<xsubpp> compiler will generate the code necessary to
156 handle the argument stack by embedding code fragments found in the
157 typemaps. In more complex cases the programmer must supply the code.
159 =head2 The RETVAL Variable
161 The RETVAL variable is a magic variable which always matches
162 the return type of the C library function. The B<xsubpp> compiler will
163 supply this variable in each XSUB and by default will use it to hold the
164 return value of the C library function being called. In simple cases the
165 value of RETVAL will be placed in ST(0) of the argument stack where it can
166 be received by Perl as the return value of the XSUB.
168 If the XSUB has a return type of C<void> then the compiler will
169 not supply a RETVAL variable for that function. When using
170 the PPCODE: directive the RETVAL variable may not be needed.
172 =head2 The MODULE Keyword
174 The MODULE keyword is used to start the XS code and to
175 specify the package of the functions which are being
176 defined. All text preceding the first MODULE keyword is
177 considered C code and is passed through to the output
178 untouched. Every XS module will have a bootstrap function
179 which is used to hook the XSUBs into Perl. The package name
180 of this bootstrap function will match the value of the last
181 MODULE statement in the XS source files. The value of
182 MODULE should always remain constant within the same XS
183 file, though this is not required.
185 The following example will start the XS code and will place
186 all functions in a package named RPC.
190 =head2 The PACKAGE Keyword
192 When functions within an XS source file must be separated into packages
193 the PACKAGE keyword should be used. This keyword is used with the MODULE
194 keyword and must follow immediately after it when used.
196 MODULE = RPC PACKAGE = RPC
198 [ XS code in package RPC ]
200 MODULE = RPC PACKAGE = RPCB
202 [ XS code in package RPCB ]
204 MODULE = RPC PACKAGE = RPC
206 [ XS code in package RPC ]
208 Although this keyword is optional and in some cases provides redundant
209 information it should always be used. This keyword will ensure that the
210 XSUBs appear in the desired package.
212 =head2 The PREFIX Keyword
214 The PREFIX keyword designates prefixes which should be
215 removed from the Perl function names. If the C function is
216 C<rpcb_gettime()> and the PREFIX value is C<rpcb_> then Perl will
217 see this function as C<gettime()>.
219 This keyword should follow the PACKAGE keyword when used.
220 If PACKAGE is not used then PREFIX should follow the MODULE
223 MODULE = RPC PREFIX = rpc_
225 MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_
227 =head2 The OUTPUT: Keyword
229 The OUTPUT: keyword indicates that certain function parameters should be
230 updated (new values made visible to Perl) when the XSUB terminates or that
231 certain values should be returned to the calling Perl function. For
232 simple functions, such as the sin() function above, the RETVAL variable is
233 automatically designated as an output value. In more complex functions
234 the B<xsubpp> compiler will need help to determine which variables are output
237 This keyword will normally be used to complement the CODE: keyword.
238 The RETVAL variable is not recognized as an output variable when the
239 CODE: keyword is present. The OUTPUT: keyword is used in this
240 situation to tell the compiler that RETVAL really is an output
243 The OUTPUT: keyword can also be used to indicate that function parameters
244 are output variables. This may be necessary when a parameter has been
245 modified within the function and the programmer would like the update to
249 rpcb_gettime(host,timep)
255 The OUTPUT: keyword will also allow an output parameter to
256 be mapped to a matching piece of code rather than to a
260 rpcb_gettime(host,timep)
264 timep sv_setnv(ST(1), (double)timep);
266 =head2 The CODE: Keyword
268 This keyword is used in more complicated XSUBs which require
269 special handling for the C function. The RETVAL variable is
270 available but will not be returned unless it is specified
271 under the OUTPUT: keyword.
273 The following XSUB is for a C function which requires special handling of
274 its parameters. The Perl usage is given first.
276 $status = rpcb_gettime( "localhost", $timep );
281 rpcb_gettime(host,timep)
285 RETVAL = rpcb_gettime( host, &timep );
290 =head2 The INIT: Keyword
292 The INIT: keyword allows initialization to be inserted into the XSUB before
293 the compiler generates the call to the C function. Unlike the CODE: keyword
294 above, this keyword does not affect the way the compiler handles RETVAL.
297 rpcb_gettime(host,timep)
301 printf("# Host is %s\n", host );
305 =head2 The NO_INIT Keyword
307 The NO_INIT keyword is used to indicate that a function
308 parameter is being used as only an output value. The B<xsubpp>
309 compiler will normally generate code to read the values of
310 all function parameters from the argument stack and assign
311 them to C variables upon entry to the function. NO_INIT
312 will tell the compiler that some parameters will be used for
313 output rather than for input and that they will be handled
314 before the function terminates.
316 The following example shows a variation of the rpcb_gettime() function.
317 This function uses the timep variable as only an output variable and does
318 not care about its initial contents.
321 rpcb_gettime(host,timep)
323 time_t &timep = NO_INIT
327 =head2 Initializing Function Parameters
329 Function parameters are normally initialized with their
330 values from the argument stack. The typemaps contain the
331 code segments which are used to transfer the Perl values to
332 the C parameters. The programmer, however, is allowed to
333 override the typemaps and supply alternate initialization
336 The following code demonstrates how to supply initialization code for
337 function parameters. The initialization code is eval'd by the compiler
338 before it is added to the output so anything which should be interpreted
339 literally, such as double quotes, must be protected with backslashes.
342 rpcb_gettime(host,timep)
343 char *host = (char *)SvPV(ST(0),na);
348 This should not be used to supply default values for parameters. One
349 would normally use this when a function parameter must be processed by
350 another library function before it can be used. Default parameters are
351 covered in the next section.
353 =head2 Default Parameter Values
355 Default values can be specified for function parameters by
356 placing an assignment statement in the parameter list. The
357 default value may be a number or a string. Defaults should
358 always be used on the right-most parameters only.
360 To allow the XSUB for rpcb_gettime() to have a default host
361 value the parameters to the XSUB could be rearranged. The
362 XSUB will then call the real rpcb_gettime() function with
363 the parameters in the correct order. Perl will call this
364 XSUB with either of the following statements.
366 $status = rpcb_gettime( $timep, $host );
368 $status = rpcb_gettime( $timep );
370 The XSUB will look like the code which follows. A CODE:
371 block is used to call the real rpcb_gettime() function with
372 the parameters in the correct order for that function.
375 rpcb_gettime(timep,host="localhost")
377 time_t timep = NO_INIT
379 RETVAL = rpcb_gettime( host, &timep );
384 =head2 The PREINIT: Keyword
386 The PREINIT: keyword allows extra variables to be declared before the
387 typemaps are expanded. If a variable is declared in a CODE: block then that
388 variable will follow any typemap code. This may result in a C syntax
389 error. To force the variable to be declared before the typemap code, place
390 it into a PREINIT: block. The PREINIT: keyword may be used one or more
391 times within an XSUB.
393 The following examples are equivalent, but if the code is using complex
394 typemaps then the first example is safer.
398 time_t timep = NO_INIT
400 char *host = "localhost";
402 RETVAL = rpcb_gettime( host, &timep );
407 A correct, but error-prone example.
411 time_t timep = NO_INIT
413 char *host = "localhost";
414 RETVAL = rpcb_gettime( host, &timep );
419 =head2 The INPUT: Keyword
421 The XSUB's parameters are usually evaluated immediately after entering the
422 XSUB. The INPUT: keyword can be used to force those parameters to be
423 evaluated a little later. The INPUT: keyword can be used multiple times
424 within an XSUB and can be used to list one or more input variables. This
425 keyword is used with the PREINIT: keyword.
427 The following example shows how the input parameter C<timep> can be
428 evaluated late, after a PREINIT.
431 rpcb_gettime(host,timep)
438 RETVAL = rpcb_gettime( host, &tt );
444 The next example shows each input parameter evaluated late.
447 rpcb_gettime(host,timep)
458 RETVAL = rpcb_gettime( h, &tt );
464 =head2 Variable-length Parameter Lists
466 XSUBs can have variable-length parameter lists by specifying an ellipsis
467 C<(...)> in the parameter list. This use of the ellipsis is similar to that
468 found in ANSI C. The programmer is able to determine the number of
469 arguments passed to the XSUB by examining the C<items> variable which the
470 B<xsubpp> compiler supplies for all XSUBs. By using this mechanism one can
471 create an XSUB which accepts a list of parameters of unknown length.
473 The I<host> parameter for the rpcb_gettime() XSUB can be
474 optional so the ellipsis can be used to indicate that the
475 XSUB will take a variable number of parameters. Perl should
476 be able to call this XSUB with either of the following statements.
478 $status = rpcb_gettime( $timep, $host );
480 $status = rpcb_gettime( $timep );
482 The XS code, with ellipsis, follows.
485 rpcb_gettime(timep, ...)
486 time_t timep = NO_INIT
488 char *host = "localhost";
491 host = (char *)SvPV(ST(1), na);
492 RETVAL = rpcb_gettime( host, &timep );
497 =head2 The PPCODE: Keyword
499 The PPCODE: keyword is an alternate form of the CODE: keyword and is used
500 to tell the B<xsubpp> compiler that the programmer is supplying the code to
501 control the argument stack for the XSUBs return values. Occasionally one
502 will want an XSUB to return a list of values rather than a single value.
503 In these cases one must use PPCODE: and then explicitly push the list of
504 values on the stack. The PPCODE: and CODE: keywords are not used
505 together within the same XSUB.
507 The following XSUB will call the C rpcb_gettime() function
508 and will return its two output values, timep and status, to
509 Perl as a single list.
518 status = rpcb_gettime( host, &timep );
520 PUSHs(sv_2mortal(newSViv(status)));
521 PUSHs(sv_2mortal(newSViv(timep)));
523 Notice that the programmer must supply the C code necessary
524 to have the real rpcb_gettime() function called and to have
525 the return values properly placed on the argument stack.
527 The C<void> return type for this function tells the B<xsubpp> compiler that
528 the RETVAL variable is not needed or used and that it should not be created.
529 In most scenarios the void return type should be used with the PPCODE:
532 The EXTEND() macro is used to make room on the argument
533 stack for 2 return values. The PPCODE: directive causes the
534 B<xsubpp> compiler to create a stack pointer called C<sp>, and it
535 is this pointer which is being used in the EXTEND() macro.
536 The values are then pushed onto the stack with the PUSHs()
539 Now the rpcb_gettime() function can be used from Perl with
540 the following statement.
542 ($status, $timep) = rpcb_gettime("localhost");
544 =head2 Returning Undef And Empty Lists
546 Occasionally the programmer will want to simply return
547 C<undef> or an empty list if a function fails rather than a
548 separate status value. The rpcb_gettime() function offers
549 just this situation. If the function succeeds we would like
550 to have it return the time and if it fails we would like to
551 have undef returned. In the following Perl code the value
552 of $timep will either be undef or it will be a valid time.
554 $timep = rpcb_gettime( "localhost" );
556 The following XSUB uses the C<void> return type to disable the generation of
557 the RETVAL variable and uses a CODE: block to indicate to the compiler
558 that the programmer has supplied all the necessary code. The
559 sv_newmortal() call will initialize the return value to undef, making that
560 the default return value.
569 ST(0) = sv_newmortal();
570 if( rpcb_gettime( host, &timep ) )
571 sv_setnv( ST(0), (double)timep);
573 The next example demonstrates how one would place an explicit undef in the
574 return value, should the need arise.
583 ST(0) = sv_newmortal();
584 if( rpcb_gettime( host, &timep ) ){
585 sv_setnv( ST(0), (double)timep);
591 To return an empty list one must use a PPCODE: block and
592 then not push return values on the stack.
600 if( rpcb_gettime( host, &timep ) )
601 PUSHs(sv_2mortal(newSViv(timep)));
603 /* Nothing pushed on stack, so an empty */
604 /* list is implicitly returned. */
607 Some people may be inclined to include an explicit C<return> in the above
608 XSUB, rather than letting control fall through to the end. In those
609 situations C<XSRETURN_EMPTY> should be used, instead. This will ensure that
610 the XSUB stack is properly adjusted. Consult L<perlguts/"API LISTING"> for
611 other C<XSRETURN> macros.
613 =head2 The REQUIRE: Keyword
615 The REQUIRE: keyword is used to indicate the minimum version of the
616 B<xsubpp> compiler needed to compile the XS module. An XS module which
617 contains the following statement will only compile with B<xsubpp> version
622 =head2 The CLEANUP: Keyword
624 This keyword can be used when an XSUB requires special cleanup procedures
625 before it terminates. When the CLEANUP: keyword is used it must follow
626 any CODE:, PPCODE:, or OUTPUT: blocks which are present in the XSUB. The
627 code specified for the cleanup block will be added as the last statements
630 =head2 The BOOT: Keyword
632 The BOOT: keyword is used to add code to the extension's bootstrap
633 function. The bootstrap function is generated by the B<xsubpp> compiler and
634 normally holds the statements necessary to register any XSUBs with Perl.
635 With the BOOT: keyword the programmer can tell the compiler to add extra
636 statements to the bootstrap function.
638 This keyword may be used any time after the first MODULE keyword and should
639 appear on a line by itself. The first blank line after the keyword will
640 terminate the code block.
643 # The following message will be printed when the
644 # bootstrap function executes.
645 printf("Hello from the bootstrap!\n");
647 =head2 The VERSIONCHECK: Keyword
649 The VERSIONCHECK: keyword corresponds to B<xsubpp>'s C<-versioncheck> and
650 C<-noversioncheck> options. This keyword overrides the commandline
651 options. Version checking is enabled by default. When version checking is
652 enabled the XS module will attempt to verify that its version matches the
653 version of the PM module.
655 To enable version checking:
659 To disable version checking:
661 VERSIONCHECK: DISABLE
663 =head2 The PROTOTYPES: Keyword
665 The PROTOTYPES: keyword corresponds to B<xsubpp>'s C<-prototypes> and
666 C<-noprototypes> options. This keyword overrides the commandline options.
667 Prototypes are enabled by default. When prototypes are enabled XSUBs will
668 be given Perl prototypes. This keyword may be used multiple times in an XS
669 module to enable and disable prototypes for different parts of the module.
671 To enable prototypes:
675 To disable prototypes:
679 =head2 The PROTOTYPE: Keyword
681 This keyword is similar to the PROTOTYPES: keyword above but can be used to
682 force B<xsubpp> to use a specific prototype for the XSUB. This keyword
683 overrides all other prototype options and keywords but affects only the
684 current XSUB. Consult L<perlsub/Prototypes> for information about Perl
688 rpcb_gettime(timep, ...)
689 time_t timep = NO_INIT
692 char *host = "localhost";
695 host = (char *)SvPV(ST(1), na);
696 RETVAL = rpcb_gettime( host, &timep );
701 =head2 The ALIAS: Keyword
703 The ALIAS: keyword allows an XSUB to have two more more unique Perl names
704 and to know which of those names was used when it was invoked. The Perl
705 names may be fully-qualified with package names. Each alias is given an
706 index. The compiler will setup a variable called C<ix> which contain the
707 index of the alias which was used. When the XSUB is called with its
708 declared name C<ix> will be 0.
710 The following example will create aliases C<FOO::gettime()> and
711 C<BAR::getit()> for this function.
714 rpcb_gettime(host,timep)
721 printf("# ix = %d\n", ix );
725 =head2 The INCLUDE: Keyword
727 This keyword can be used to pull other files into the XS module. The other
728 files may have XS code. INCLUDE: can also be used to run a command to
729 generate the XS code to be pulled into the module.
731 The file F<Rpcb1.xsh> contains our C<rpcb_gettime()> function:
734 rpcb_gettime(host,timep)
740 The XS module can use INCLUDE: to pull that file into it.
744 If the parameters to the INCLUDE: keyword are followed by a pipe (C<|>) then
745 the compiler will interpret the parameters as a command.
747 INCLUDE: cat Rpcb1.xsh |
749 =head2 The CASE: Keyword
751 The CASE: keyword allows an XSUB to have multiple distinct parts with each
752 part acting as a virtual XSUB. CASE: is greedy and if it is used then all
753 other XS keywords must be contained within a CASE:. This means nothing may
754 precede the first CASE: in the XSUB and anything following the last CASE: is
755 included in that case.
757 A CASE: might switch via a parameter of the XSUB, via the C<ix> ALIAS:
758 variable (see L<"The ALIAS: Keyword">), or maybe via the C<items> variable
759 (see L<"Variable-length Parameter Lists">). The last CASE: becomes the
760 B<default> case if it is not associated with a conditional. The following
761 example shows CASE switched via C<ix> with a function C<rpcb_gettime()>
762 having an alias C<x_gettime()>. When the function is called as
763 C<rpcb_gettime()> it's parameters are the usual C<(char *host, time_t *timep)>,
764 but when the function is called as C<x_gettime()> is parameters are
765 reversed, C<(time_t *timep, char *host)>.
773 # 'a' is timep, 'b' is host
777 RETVAL = rpcb_gettime( b, &a );
782 # 'a' is host, 'b' is timep
789 That function can be called with either of the following statements. Note
790 the different argument lists.
792 $status = rpcb_gettime( $host, $timep );
794 $status = x_gettime( $timep, $host );
796 =head2 The & Unary Operator
798 The & unary operator is used to tell the compiler that it should dereference
799 the object when it calls the C function. This is used when a CODE: block is
800 not used and the object is a not a pointer type (the object is an C<int> or
801 C<long> but not a C<int*> or C<long*>).
803 The following XSUB will generate incorrect C code. The xsubpp compiler will
804 turn this into code which calls C<rpcb_gettime()> with parameters C<(char
805 *host, time_t timep)>, but the real C<rpcb_gettime()> wants the C<timep>
806 parameter to be of type C<time_t*> rather than C<time_t>.
809 rpcb_gettime(host,timep)
815 That problem is corrected by using the C<&> operator. The xsubpp compiler
816 will now turn this into code which calls C<rpcb_gettime()> correctly with
817 parameters C<(char *host, time_t *timep)>. It does this by carrying the
818 C<&> through, so the function call looks like C<rpcb_gettime(host, &timep)>.
821 rpcb_gettime(host,timep)
827 =head2 Inserting Comments and C Preprocessor Directives
829 C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
830 CODE:, PPCODE: and CLEANUP: blocks, as well as outside the functions.
831 Comments are allowed anywhere after the MODULE keyword. The compiler
832 will pass the preprocessor directives through untouched and will remove
834 Comments can be added to XSUBs by placing a C<#> as the first
835 non-whitespace of a line. Care should be taken to avoid making the
836 comment look like a C preprocessor directive, lest it be interpreted as
837 such. The simplest way to prevent this is to put whitespace in front of
841 If you use preprocessor directives to choose one of two
842 versions of a function, use
845 #else /* ... version2 */
855 because otherwise xsubpp will believe that you made a duplicate
856 definition of the function. Also, put a blank line before the
857 #else/#endif so it will not be seen as part of the function body.
859 =head2 Using XS With C++
861 If a function is defined as a C++ method then it will assume
862 its first argument is an object pointer. The object pointer
863 will be stored in a variable called THIS. The object should
864 have been created by C++ with the new() function and should
865 be blessed by Perl with the sv_setref_pv() macro. The
866 blessing of the object by Perl can be handled by a typemap. An example
867 typemap is shown at the end of this section.
869 If the method is defined as static it will call the C++
870 function using the class::method() syntax. If the method is not static
871 the function will be called using the THIS-E<gt>method() syntax.
873 The next examples will use the following C++ class.
880 void set_blue( int );
886 The XSUBs for the blue() and set_blue() methods are defined with the class
887 name but the parameter for the object (THIS, or "self") is implicit and is
894 color::set_blue( val )
897 Both functions will expect an object as the first parameter. The xsubpp
898 compiler will call that object C<THIS> and will use it to call the specified
899 method. So in the C++ code the blue() and set_blue() methods will be called
900 in the following manner.
902 RETVAL = THIS->blue();
904 THIS->set_blue( val );
906 If the function's name is B<DESTROY> then the C++ C<delete> function will be
907 called and C<THIS> will be given as its parameter.
912 The C++ code will call C<delete>.
916 If the function's name is B<new> then the C++ C<new> function will be called
917 to create a dynamic C++ object. The XSUB will expect the class name, which
918 will be kept in a variable called C<CLASS>, to be given as the first
924 The C++ code will call C<new>.
926 RETVAL = new color();
928 The following is an example of a typemap that could be used for this C++
935 # The Perl object is blessed into 'CLASS', which should be a
936 # char* having the name of the package for the blessing.
938 sv_setref_pv( $arg, CLASS, (void*)$var );
942 if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
943 $var = ($type)SvIV((SV*)SvRV( $arg ));
945 warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
949 =head2 Interface Strategy
951 When designing an interface between Perl and a C library a straight
952 translation from C to XS is often sufficient. The interface will often be
953 very C-like and occasionally nonintuitive, especially when the C function
954 modifies one of its parameters. In cases where the programmer wishes to
955 create a more Perl-like interface the following strategy may help to
956 identify the more critical parts of the interface.
958 Identify the C functions which modify their parameters. The XSUBs for
959 these functions may be able to return lists to Perl, or may be
960 candidates to return undef or an empty list in case of failure.
962 Identify which values are used by only the C and XSUB functions
963 themselves. If Perl does not need to access the contents of the value
964 then it may not be necessary to provide a translation for that value
967 Identify the pointers in the C function parameter lists and return
968 values. Some pointers can be handled in XS with the & unary operator on
969 the variable name while others will require the use of the * operator on
970 the type name. In general it is easier to work with the & operator.
972 Identify the structures used by the C functions. In many
973 cases it may be helpful to use the T_PTROBJ typemap for
974 these structures so they can be manipulated by Perl as
977 =head2 Perl Objects And C Structures
979 When dealing with C structures one should select either
980 B<T_PTROBJ> or B<T_PTRREF> for the XS type. Both types are
981 designed to handle pointers to complex objects. The
982 T_PTRREF type will allow the Perl object to be unblessed
983 while the T_PTROBJ type requires that the object be blessed.
984 By using T_PTROBJ one can achieve a form of type-checking
985 because the XSUB will attempt to verify that the Perl object
986 is of the expected type.
988 The following XS code shows the getnetconfigent() function which is used
989 with ONC+ TIRPC. The getnetconfigent() function will return a pointer to a
990 C structure and has the C prototype shown below. The example will
991 demonstrate how the C pointer will become a Perl reference. Perl will
992 consider this reference to be a pointer to a blessed object and will
993 attempt to call a destructor for the object. A destructor will be
994 provided in the XS source to free the memory used by getnetconfigent().
995 Destructors in XS can be created by specifying an XSUB function whose name
996 ends with the word B<DESTROY>. XS destructors can be used to free memory
997 which may have been malloc'd by another XSUB.
999 struct netconfig *getnetconfigent(const char *netid);
1001 A C<typedef> will be created for C<struct netconfig>. The Perl
1002 object will be blessed in a class matching the name of the C
1003 type, with the tag C<Ptr> appended, and the name should not
1004 have embedded spaces if it will be a Perl package name. The
1005 destructor will be placed in a class corresponding to the
1006 class of the object and the PREFIX keyword will be used to
1007 trim the name to the word DESTROY as Perl will expect.
1009 typedef struct netconfig Netconfig;
1011 MODULE = RPC PACKAGE = RPC
1014 getnetconfigent(netid)
1017 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1020 rpcb_DESTROY(netconf)
1023 printf("Now in NetconfigPtr::DESTROY\n");
1026 This example requires the following typemap entry. Consult the typemap
1027 section for more information about adding new typemaps for an extension.
1030 Netconfig * T_PTROBJ
1032 This example will be used with the following Perl statements.
1035 $netconf = getnetconfigent("udp");
1037 When Perl destroys the object referenced by $netconf it will send the
1038 object to the supplied XSUB DESTROY function. Perl cannot determine, and
1039 does not care, that this object is a C struct and not a Perl object. In
1040 this sense, there is no difference between the object created by the
1041 getnetconfigent() XSUB and an object created by a normal Perl subroutine.
1045 The typemap is a collection of code fragments which are used by the B<xsubpp>
1046 compiler to map C function parameters and values to Perl values. The
1047 typemap file may consist of three sections labeled C<TYPEMAP>, C<INPUT>, and
1048 C<OUTPUT>. The INPUT section tells the compiler how to translate Perl values
1049 into variables of certain C types. The OUTPUT section tells the compiler
1050 how to translate the values from certain C types into values Perl can
1051 understand. The TYPEMAP section tells the compiler which of the INPUT and
1052 OUTPUT code fragments should be used to map a given C type to a Perl value.
1053 Each of the sections of the typemap must be preceded by one of the TYPEMAP,
1054 INPUT, or OUTPUT keywords.
1056 The default typemap in the C<ext> directory of the Perl source contains many
1057 useful types which can be used by Perl extensions. Some extensions define
1058 additional typemaps which they keep in their own directory. These
1059 additional typemaps may reference INPUT and OUTPUT maps in the main
1060 typemap. The B<xsubpp> compiler will allow the extension's own typemap to
1061 override any mappings which are in the default typemap.
1063 Most extensions which require a custom typemap will need only the TYPEMAP
1064 section of the typemap file. The custom typemap used in the
1065 getnetconfigent() example shown earlier demonstrates what may be the typical
1066 use of extension typemaps. That typemap is used to equate a C structure
1067 with the T_PTROBJ typemap. The typemap used by getnetconfigent() is shown
1068 here. Note that the C type is separated from the XS type with a tab and
1069 that the C unary operator C<*> is considered to be a part of the C type name.
1072 Netconfig *<tab>T_PTROBJ
1076 File C<RPC.xs>: Interface to some ONC+ RPC bind library functions.
1082 #include <rpc/rpc.h>
1084 typedef struct netconfig Netconfig;
1086 MODULE = RPC PACKAGE = RPC
1089 rpcb_gettime(host="localhost")
1094 ST(0) = sv_newmortal();
1095 if( rpcb_gettime( host, &timep ) )
1096 sv_setnv( ST(0), (double)timep );
1099 getnetconfigent(netid="udp")
1102 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1105 rpcb_DESTROY(netconf)
1108 printf("NetconfigPtr::DESTROY\n");
1111 File C<typemap>: Custom typemap for RPC.xs.
1114 Netconfig * T_PTROBJ
1116 File C<RPC.pm>: Perl module for the RPC extension.
1122 @ISA = qw(Exporter DynaLoader);
1123 @EXPORT = qw(rpcb_gettime getnetconfigent);
1128 File C<rpctest.pl>: Perl test program for the RPC extension.
1132 $netconf = getnetconfigent();
1133 $a = rpcb_gettime();
1134 print "time = $a\n";
1135 print "netconf = $netconf\n";
1137 $netconf = getnetconfigent("tcp");
1138 $a = rpcb_gettime("poplar");
1139 print "time = $a\n";
1140 print "netconf = $netconf\n";
1145 This document covers features supported by C<xsubpp> 1.935.
1149 Dean Roehrich F<E<lt>roehrich@cray.comE<gt>>