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 is not needed, unless used
173 If PPCODE: directive is not used, C<void> return value should be used
174 only for subroutines which do not return a value, I<even if> CODE:
175 directive is used which sets ST(0) explicitly.
177 Older versions of this document recommended to use C<void> return
178 value in such cases. It was discovered that this could lead to
179 segfaults in cases when XSUB was I<truely> C<void>. This practice is
180 now deprecated, and may be not supported at some future version. Use
181 the return value C<SV *> in such cases. (Currently C<xsubpp> contains
182 some heuristic code which tries to disambiguate between "truely-void"
183 and "old-practice-declared-as-void" functions. Hence your code is at
184 mercy of this heuristics unless you use C<SV *> as return value.)
186 =head2 The MODULE Keyword
188 The MODULE keyword is used to start the XS code and to
189 specify the package of the functions which are being
190 defined. All text preceding the first MODULE keyword is
191 considered C code and is passed through to the output
192 untouched. Every XS module will have a bootstrap function
193 which is used to hook the XSUBs into Perl. The package name
194 of this bootstrap function will match the value of the last
195 MODULE statement in the XS source files. The value of
196 MODULE should always remain constant within the same XS
197 file, though this is not required.
199 The following example will start the XS code and will place
200 all functions in a package named RPC.
204 =head2 The PACKAGE Keyword
206 When functions within an XS source file must be separated into packages
207 the PACKAGE keyword should be used. This keyword is used with the MODULE
208 keyword and must follow immediately after it when used.
210 MODULE = RPC PACKAGE = RPC
212 [ XS code in package RPC ]
214 MODULE = RPC PACKAGE = RPCB
216 [ XS code in package RPCB ]
218 MODULE = RPC PACKAGE = RPC
220 [ XS code in package RPC ]
222 Although this keyword is optional and in some cases provides redundant
223 information it should always be used. This keyword will ensure that the
224 XSUBs appear in the desired package.
226 =head2 The PREFIX Keyword
228 The PREFIX keyword designates prefixes which should be
229 removed from the Perl function names. If the C function is
230 C<rpcb_gettime()> and the PREFIX value is C<rpcb_> then Perl will
231 see this function as C<gettime()>.
233 This keyword should follow the PACKAGE keyword when used.
234 If PACKAGE is not used then PREFIX should follow the MODULE
237 MODULE = RPC PREFIX = rpc_
239 MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_
241 =head2 The OUTPUT: Keyword
243 The OUTPUT: keyword indicates that certain function parameters should be
244 updated (new values made visible to Perl) when the XSUB terminates or that
245 certain values should be returned to the calling Perl function. For
246 simple functions, such as the sin() function above, the RETVAL variable is
247 automatically designated as an output value. In more complex functions
248 the B<xsubpp> compiler will need help to determine which variables are output
251 This keyword will normally be used to complement the CODE: keyword.
252 The RETVAL variable is not recognized as an output variable when the
253 CODE: keyword is present. The OUTPUT: keyword is used in this
254 situation to tell the compiler that RETVAL really is an output
257 The OUTPUT: keyword can also be used to indicate that function parameters
258 are output variables. This may be necessary when a parameter has been
259 modified within the function and the programmer would like the update to
263 rpcb_gettime(host,timep)
269 The OUTPUT: keyword will also allow an output parameter to
270 be mapped to a matching piece of code rather than to a
274 rpcb_gettime(host,timep)
278 timep sv_setnv(ST(1), (double)timep);
280 =head2 The CODE: Keyword
282 This keyword is used in more complicated XSUBs which require
283 special handling for the C function. The RETVAL variable is
284 available but will not be returned unless it is specified
285 under the OUTPUT: keyword.
287 The following XSUB is for a C function which requires special handling of
288 its parameters. The Perl usage is given first.
290 $status = rpcb_gettime( "localhost", $timep );
295 rpcb_gettime(host,timep)
299 RETVAL = rpcb_gettime( host, &timep );
304 =head2 The INIT: Keyword
306 The INIT: keyword allows initialization to be inserted into the XSUB before
307 the compiler generates the call to the C function. Unlike the CODE: keyword
308 above, this keyword does not affect the way the compiler handles RETVAL.
311 rpcb_gettime(host,timep)
315 printf("# Host is %s\n", host );
319 =head2 The NO_INIT Keyword
321 The NO_INIT keyword is used to indicate that a function
322 parameter is being used as only an output value. The B<xsubpp>
323 compiler will normally generate code to read the values of
324 all function parameters from the argument stack and assign
325 them to C variables upon entry to the function. NO_INIT
326 will tell the compiler that some parameters will be used for
327 output rather than for input and that they will be handled
328 before the function terminates.
330 The following example shows a variation of the rpcb_gettime() function.
331 This function uses the timep variable as only an output variable and does
332 not care about its initial contents.
335 rpcb_gettime(host,timep)
337 time_t &timep = NO_INIT
341 =head2 Initializing Function Parameters
343 Function parameters are normally initialized with their
344 values from the argument stack. The typemaps contain the
345 code segments which are used to transfer the Perl values to
346 the C parameters. The programmer, however, is allowed to
347 override the typemaps and supply alternate initialization
350 The following code demonstrates how to supply initialization code for
351 function parameters. The initialization code is eval'd by the compiler
352 before it is added to the output so anything which should be interpreted
353 literally, such as double quotes, must be protected with backslashes.
356 rpcb_gettime(host,timep)
357 char *host = (char *)SvPV(ST(0),na);
362 This should not be used to supply default values for parameters. One
363 would normally use this when a function parameter must be processed by
364 another library function before it can be used. Default parameters are
365 covered in the next section.
367 =head2 Default Parameter Values
369 Default values can be specified for function parameters by
370 placing an assignment statement in the parameter list. The
371 default value may be a number or a string. Defaults should
372 always be used on the right-most parameters only.
374 To allow the XSUB for rpcb_gettime() to have a default host
375 value the parameters to the XSUB could be rearranged. The
376 XSUB will then call the real rpcb_gettime() function with
377 the parameters in the correct order. Perl will call this
378 XSUB with either of the following statements.
380 $status = rpcb_gettime( $timep, $host );
382 $status = rpcb_gettime( $timep );
384 The XSUB will look like the code which follows. A CODE:
385 block is used to call the real rpcb_gettime() function with
386 the parameters in the correct order for that function.
389 rpcb_gettime(timep,host="localhost")
391 time_t timep = NO_INIT
393 RETVAL = rpcb_gettime( host, &timep );
398 =head2 The PREINIT: Keyword
400 The PREINIT: keyword allows extra variables to be declared before the
401 typemaps are expanded. If a variable is declared in a CODE: block then that
402 variable will follow any typemap code. This may result in a C syntax
403 error. To force the variable to be declared before the typemap code, place
404 it into a PREINIT: block. The PREINIT: keyword may be used one or more
405 times within an XSUB.
407 The following examples are equivalent, but if the code is using complex
408 typemaps then the first example is safer.
412 time_t timep = NO_INIT
414 char *host = "localhost";
416 RETVAL = rpcb_gettime( host, &timep );
421 A correct, but error-prone example.
425 time_t timep = NO_INIT
427 char *host = "localhost";
428 RETVAL = rpcb_gettime( host, &timep );
433 =head2 The SCOPE: Keyword
435 The SCOPE: keyword allows scoping to be enabled for a particular XSUB. If
436 enabled, the XSUB will invoke ENTER and LEAVE automatically.
438 To support potentially complex type mappings, if a typemap entry used
439 by this XSUB contains a comment like C</*scope*/> then scoping will
440 automatically be enabled for that XSUB.
450 =head2 The INPUT: Keyword
452 The XSUB's parameters are usually evaluated immediately after entering the
453 XSUB. The INPUT: keyword can be used to force those parameters to be
454 evaluated a little later. The INPUT: keyword can be used multiple times
455 within an XSUB and can be used to list one or more input variables. This
456 keyword is used with the PREINIT: keyword.
458 The following example shows how the input parameter C<timep> can be
459 evaluated late, after a PREINIT.
462 rpcb_gettime(host,timep)
469 RETVAL = rpcb_gettime( host, &tt );
475 The next example shows each input parameter evaluated late.
478 rpcb_gettime(host,timep)
489 RETVAL = rpcb_gettime( h, &tt );
495 =head2 Variable-length Parameter Lists
497 XSUBs can have variable-length parameter lists by specifying an ellipsis
498 C<(...)> in the parameter list. This use of the ellipsis is similar to that
499 found in ANSI C. The programmer is able to determine the number of
500 arguments passed to the XSUB by examining the C<items> variable which the
501 B<xsubpp> compiler supplies for all XSUBs. By using this mechanism one can
502 create an XSUB which accepts a list of parameters of unknown length.
504 The I<host> parameter for the rpcb_gettime() XSUB can be
505 optional so the ellipsis can be used to indicate that the
506 XSUB will take a variable number of parameters. Perl should
507 be able to call this XSUB with either of the following statements.
509 $status = rpcb_gettime( $timep, $host );
511 $status = rpcb_gettime( $timep );
513 The XS code, with ellipsis, follows.
516 rpcb_gettime(timep, ...)
517 time_t timep = NO_INIT
519 char *host = "localhost";
522 host = (char *)SvPV(ST(1), na);
523 RETVAL = rpcb_gettime( host, &timep );
528 =head2 The PPCODE: Keyword
530 The PPCODE: keyword is an alternate form of the CODE: keyword and is used
531 to tell the B<xsubpp> compiler that the programmer is supplying the code to
532 control the argument stack for the XSUBs return values. Occasionally one
533 will want an XSUB to return a list of values rather than a single value.
534 In these cases one must use PPCODE: and then explicitly push the list of
535 values on the stack. The PPCODE: and CODE: keywords are not used
536 together within the same XSUB.
538 The following XSUB will call the C rpcb_gettime() function
539 and will return its two output values, timep and status, to
540 Perl as a single list.
549 status = rpcb_gettime( host, &timep );
551 PUSHs(sv_2mortal(newSViv(status)));
552 PUSHs(sv_2mortal(newSViv(timep)));
554 Notice that the programmer must supply the C code necessary
555 to have the real rpcb_gettime() function called and to have
556 the return values properly placed on the argument stack.
558 The C<void> return type for this function tells the B<xsubpp> compiler that
559 the RETVAL variable is not needed or used and that it should not be created.
560 In most scenarios the void return type should be used with the PPCODE:
563 The EXTEND() macro is used to make room on the argument
564 stack for 2 return values. The PPCODE: directive causes the
565 B<xsubpp> compiler to create a stack pointer called C<sp>, and it
566 is this pointer which is being used in the EXTEND() macro.
567 The values are then pushed onto the stack with the PUSHs()
570 Now the rpcb_gettime() function can be used from Perl with
571 the following statement.
573 ($status, $timep) = rpcb_gettime("localhost");
575 =head2 Returning Undef And Empty Lists
577 Occasionally the programmer will want to return simply
578 C<undef> or an empty list if a function fails rather than a
579 separate status value. The rpcb_gettime() function offers
580 just this situation. If the function succeeds we would like
581 to have it return the time and if it fails we would like to
582 have undef returned. In the following Perl code the value
583 of $timep will either be undef or it will be a valid time.
585 $timep = rpcb_gettime( "localhost" );
587 The following XSUB uses the C<SV *> return type as a mneumonic only,
588 and uses a CODE: block to indicate to the compiler
589 that the programmer has supplied all the necessary code. The
590 sv_newmortal() call will initialize the return value to undef, making that
591 the default return value.
600 ST(0) = sv_newmortal();
601 if( rpcb_gettime( host, &timep ) )
602 sv_setnv( ST(0), (double)timep);
604 The next example demonstrates how one would place an explicit undef in the
605 return value, should the need arise.
614 ST(0) = sv_newmortal();
615 if( rpcb_gettime( host, &timep ) ){
616 sv_setnv( ST(0), (double)timep);
622 To return an empty list one must use a PPCODE: block and
623 then not push return values on the stack.
631 if( rpcb_gettime( host, &timep ) )
632 PUSHs(sv_2mortal(newSViv(timep)));
634 /* Nothing pushed on stack, so an empty */
635 /* list is implicitly returned. */
638 Some people may be inclined to include an explicit C<return> in the above
639 XSUB, rather than letting control fall through to the end. In those
640 situations C<XSRETURN_EMPTY> should be used, instead. This will ensure that
641 the XSUB stack is properly adjusted. Consult L<perlguts/"API LISTING"> for
642 other C<XSRETURN> macros.
644 =head2 The REQUIRE: Keyword
646 The REQUIRE: keyword is used to indicate the minimum version of the
647 B<xsubpp> compiler needed to compile the XS module. An XS module which
648 contains the following statement will compile with only B<xsubpp> version
653 =head2 The CLEANUP: Keyword
655 This keyword can be used when an XSUB requires special cleanup procedures
656 before it terminates. When the CLEANUP: keyword is used it must follow
657 any CODE:, PPCODE:, or OUTPUT: blocks which are present in the XSUB. The
658 code specified for the cleanup block will be added as the last statements
661 =head2 The BOOT: Keyword
663 The BOOT: keyword is used to add code to the extension's bootstrap
664 function. The bootstrap function is generated by the B<xsubpp> compiler and
665 normally holds the statements necessary to register any XSUBs with Perl.
666 With the BOOT: keyword the programmer can tell the compiler to add extra
667 statements to the bootstrap function.
669 This keyword may be used any time after the first MODULE keyword and should
670 appear on a line by itself. The first blank line after the keyword will
671 terminate the code block.
674 # The following message will be printed when the
675 # bootstrap function executes.
676 printf("Hello from the bootstrap!\n");
678 =head2 The VERSIONCHECK: Keyword
680 The VERSIONCHECK: keyword corresponds to B<xsubpp>'s C<-versioncheck> and
681 C<-noversioncheck> options. This keyword overrides the command line
682 options. Version checking is enabled by default. When version checking is
683 enabled the XS module will attempt to verify that its version matches the
684 version of the PM module.
686 To enable version checking:
690 To disable version checking:
692 VERSIONCHECK: DISABLE
694 =head2 The PROTOTYPES: Keyword
696 The PROTOTYPES: keyword corresponds to B<xsubpp>'s C<-prototypes> and
697 C<-noprototypes> options. This keyword overrides the command-line options.
698 Prototypes are enabled by default. When prototypes are enabled XSUBs will
699 be given Perl prototypes. This keyword may be used multiple times in an XS
700 module to enable and disable prototypes for different parts of the module.
702 To enable prototypes:
706 To disable prototypes:
710 =head2 The PROTOTYPE: Keyword
712 This keyword is similar to the PROTOTYPES: keyword above but can be used to
713 force B<xsubpp> to use a specific prototype for the XSUB. This keyword
714 overrides all other prototype options and keywords but affects only the
715 current XSUB. Consult L<perlsub/Prototypes> for information about Perl
719 rpcb_gettime(timep, ...)
720 time_t timep = NO_INIT
723 char *host = "localhost";
726 host = (char *)SvPV(ST(1), na);
727 RETVAL = rpcb_gettime( host, &timep );
732 =head2 The ALIAS: Keyword
734 The ALIAS: keyword allows an XSUB to have two more more unique Perl names
735 and to know which of those names was used when it was invoked. The Perl
736 names may be fully-qualified with package names. Each alias is given an
737 index. The compiler will setup a variable called C<ix> which contain the
738 index of the alias which was used. When the XSUB is called with its
739 declared name C<ix> will be 0.
741 The following example will create aliases C<FOO::gettime()> and
742 C<BAR::getit()> for this function.
745 rpcb_gettime(host,timep)
752 printf("# ix = %d\n", ix );
756 =head2 The INCLUDE: Keyword
758 This keyword can be used to pull other files into the XS module. The other
759 files may have XS code. INCLUDE: can also be used to run a command to
760 generate the XS code to be pulled into the module.
762 The file F<Rpcb1.xsh> contains our C<rpcb_gettime()> function:
765 rpcb_gettime(host,timep)
771 The XS module can use INCLUDE: to pull that file into it.
775 If the parameters to the INCLUDE: keyword are followed by a pipe (C<|>) then
776 the compiler will interpret the parameters as a command.
778 INCLUDE: cat Rpcb1.xsh |
780 =head2 The CASE: Keyword
782 The CASE: keyword allows an XSUB to have multiple distinct parts with each
783 part acting as a virtual XSUB. CASE: is greedy and if it is used then all
784 other XS keywords must be contained within a CASE:. This means nothing may
785 precede the first CASE: in the XSUB and anything following the last CASE: is
786 included in that case.
788 A CASE: might switch via a parameter of the XSUB, via the C<ix> ALIAS:
789 variable (see L<"The ALIAS: Keyword">), or maybe via the C<items> variable
790 (see L<"Variable-length Parameter Lists">). The last CASE: becomes the
791 B<default> case if it is not associated with a conditional. The following
792 example shows CASE switched via C<ix> with a function C<rpcb_gettime()>
793 having an alias C<x_gettime()>. When the function is called as
794 C<rpcb_gettime()> its parameters are the usual C<(char *host, time_t *timep)>,
795 but when the function is called as C<x_gettime()> its parameters are
796 reversed, C<(time_t *timep, char *host)>.
804 # 'a' is timep, 'b' is host
808 RETVAL = rpcb_gettime( b, &a );
813 # 'a' is host, 'b' is timep
820 That function can be called with either of the following statements. Note
821 the different argument lists.
823 $status = rpcb_gettime( $host, $timep );
825 $status = x_gettime( $timep, $host );
827 =head2 The & Unary Operator
829 The & unary operator is used to tell the compiler that it should dereference
830 the object when it calls the C function. This is used when a CODE: block is
831 not used and the object is a not a pointer type (the object is an C<int> or
832 C<long> but not a C<int*> or C<long*>).
834 The following XSUB will generate incorrect C code. The xsubpp compiler will
835 turn this into code which calls C<rpcb_gettime()> with parameters C<(char
836 *host, time_t timep)>, but the real C<rpcb_gettime()> wants the C<timep>
837 parameter to be of type C<time_t*> rather than C<time_t>.
840 rpcb_gettime(host,timep)
846 That problem is corrected by using the C<&> operator. The xsubpp compiler
847 will now turn this into code which calls C<rpcb_gettime()> correctly with
848 parameters C<(char *host, time_t *timep)>. It does this by carrying the
849 C<&> through, so the function call looks like C<rpcb_gettime(host, &timep)>.
852 rpcb_gettime(host,timep)
858 =head2 Inserting Comments and C Preprocessor Directives
860 C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
861 CODE:, PPCODE:, and CLEANUP: blocks, as well as outside the functions.
862 Comments are allowed anywhere after the MODULE keyword. The compiler
863 will pass the preprocessor directives through untouched and will remove
866 Comments can be added to XSUBs by placing a C<#> as the first
867 non-whitespace of a line. Care should be taken to avoid making the
868 comment look like a C preprocessor directive, lest it be interpreted as
869 such. The simplest way to prevent this is to put whitespace in front of
872 If you use preprocessor directives to choose one of two
873 versions of a function, use
876 #else /* ... version2 */
886 because otherwise xsubpp will believe that you made a duplicate
887 definition of the function. Also, put a blank line before the
888 #else/#endif so it will not be seen as part of the function body.
890 =head2 Using XS With C++
892 If a function is defined as a C++ method then it will assume
893 its first argument is an object pointer. The object pointer
894 will be stored in a variable called THIS. The object should
895 have been created by C++ with the new() function and should
896 be blessed by Perl with the sv_setref_pv() macro. The
897 blessing of the object by Perl can be handled by a typemap. An example
898 typemap is shown at the end of this section.
900 If the method is defined as static it will call the C++
901 function using the class::method() syntax. If the method is not static
902 the function will be called using the THIS-E<gt>method() syntax.
904 The next examples will use the following C++ class.
911 void set_blue( int );
917 The XSUBs for the blue() and set_blue() methods are defined with the class
918 name but the parameter for the object (THIS, or "self") is implicit and is
925 color::set_blue( val )
928 Both functions will expect an object as the first parameter. The xsubpp
929 compiler will call that object C<THIS> and will use it to call the specified
930 method. So in the C++ code the blue() and set_blue() methods will be called
931 in the following manner.
933 RETVAL = THIS->blue();
935 THIS->set_blue( val );
937 If the function's name is B<DESTROY> then the C++ C<delete> function will be
938 called and C<THIS> will be given as its parameter.
943 The C++ code will call C<delete>.
947 If the function's name is B<new> then the C++ C<new> function will be called
948 to create a dynamic C++ object. The XSUB will expect the class name, which
949 will be kept in a variable called C<CLASS>, to be given as the first
955 The C++ code will call C<new>.
957 RETVAL = new color();
959 The following is an example of a typemap that could be used for this C++
966 # The Perl object is blessed into 'CLASS', which should be a
967 # char* having the name of the package for the blessing.
969 sv_setref_pv( $arg, CLASS, (void*)$var );
973 if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
974 $var = ($type)SvIV((SV*)SvRV( $arg ));
976 warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
980 =head2 Interface Strategy
982 When designing an interface between Perl and a C library a straight
983 translation from C to XS is often sufficient. The interface will often be
984 very C-like and occasionally nonintuitive, especially when the C function
985 modifies one of its parameters. In cases where the programmer wishes to
986 create a more Perl-like interface the following strategy may help to
987 identify the more critical parts of the interface.
989 Identify the C functions which modify their parameters. The XSUBs for
990 these functions may be able to return lists to Perl, or may be
991 candidates to return undef or an empty list in case of failure.
993 Identify which values are used by only the C and XSUB functions
994 themselves. If Perl does not need to access the contents of the value
995 then it may not be necessary to provide a translation for that value
998 Identify the pointers in the C function parameter lists and return
999 values. Some pointers can be handled in XS with the & unary operator on
1000 the variable name while others will require the use of the * operator on
1001 the type name. In general it is easier to work with the & operator.
1003 Identify the structures used by the C functions. In many
1004 cases it may be helpful to use the T_PTROBJ typemap for
1005 these structures so they can be manipulated by Perl as
1008 =head2 Perl Objects And C Structures
1010 When dealing with C structures one should select either
1011 B<T_PTROBJ> or B<T_PTRREF> for the XS type. Both types are
1012 designed to handle pointers to complex objects. The
1013 T_PTRREF type will allow the Perl object to be unblessed
1014 while the T_PTROBJ type requires that the object be blessed.
1015 By using T_PTROBJ one can achieve a form of type-checking
1016 because the XSUB will attempt to verify that the Perl object
1017 is of the expected type.
1019 The following XS code shows the getnetconfigent() function which is used
1020 with ONC+ TIRPC. The getnetconfigent() function will return a pointer to a
1021 C structure and has the C prototype shown below. The example will
1022 demonstrate how the C pointer will become a Perl reference. Perl will
1023 consider this reference to be a pointer to a blessed object and will
1024 attempt to call a destructor for the object. A destructor will be
1025 provided in the XS source to free the memory used by getnetconfigent().
1026 Destructors in XS can be created by specifying an XSUB function whose name
1027 ends with the word B<DESTROY>. XS destructors can be used to free memory
1028 which may have been malloc'd by another XSUB.
1030 struct netconfig *getnetconfigent(const char *netid);
1032 A C<typedef> will be created for C<struct netconfig>. The Perl
1033 object will be blessed in a class matching the name of the C
1034 type, with the tag C<Ptr> appended, and the name should not
1035 have embedded spaces if it will be a Perl package name. The
1036 destructor will be placed in a class corresponding to the
1037 class of the object and the PREFIX keyword will be used to
1038 trim the name to the word DESTROY as Perl will expect.
1040 typedef struct netconfig Netconfig;
1042 MODULE = RPC PACKAGE = RPC
1045 getnetconfigent(netid)
1048 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1051 rpcb_DESTROY(netconf)
1054 printf("Now in NetconfigPtr::DESTROY\n");
1057 This example requires the following typemap entry. Consult the typemap
1058 section for more information about adding new typemaps for an extension.
1061 Netconfig * T_PTROBJ
1063 This example will be used with the following Perl statements.
1066 $netconf = getnetconfigent("udp");
1068 When Perl destroys the object referenced by $netconf it will send the
1069 object to the supplied XSUB DESTROY function. Perl cannot determine, and
1070 does not care, that this object is a C struct and not a Perl object. In
1071 this sense, there is no difference between the object created by the
1072 getnetconfigent() XSUB and an object created by a normal Perl subroutine.
1076 The typemap is a collection of code fragments which are used by the B<xsubpp>
1077 compiler to map C function parameters and values to Perl values. The
1078 typemap file may consist of three sections labeled C<TYPEMAP>, C<INPUT>, and
1079 C<OUTPUT>. The INPUT section tells the compiler how to translate Perl values
1080 into variables of certain C types. The OUTPUT section tells the compiler
1081 how to translate the values from certain C types into values Perl can
1082 understand. The TYPEMAP section tells the compiler which of the INPUT and
1083 OUTPUT code fragments should be used to map a given C type to a Perl value.
1084 Each of the sections of the typemap must be preceded by one of the TYPEMAP,
1085 INPUT, or OUTPUT keywords.
1087 The default typemap in the C<ext> directory of the Perl source contains many
1088 useful types which can be used by Perl extensions. Some extensions define
1089 additional typemaps which they keep in their own directory. These
1090 additional typemaps may reference INPUT and OUTPUT maps in the main
1091 typemap. The B<xsubpp> compiler will allow the extension's own typemap to
1092 override any mappings which are in the default typemap.
1094 Most extensions which require a custom typemap will need only the TYPEMAP
1095 section of the typemap file. The custom typemap used in the
1096 getnetconfigent() example shown earlier demonstrates what may be the typical
1097 use of extension typemaps. That typemap is used to equate a C structure
1098 with the T_PTROBJ typemap. The typemap used by getnetconfigent() is shown
1099 here. Note that the C type is separated from the XS type with a tab and
1100 that the C unary operator C<*> is considered to be a part of the C type name.
1103 Netconfig *<tab>T_PTROBJ
1107 File C<RPC.xs>: Interface to some ONC+ RPC bind library functions.
1113 #include <rpc/rpc.h>
1115 typedef struct netconfig Netconfig;
1117 MODULE = RPC PACKAGE = RPC
1120 rpcb_gettime(host="localhost")
1125 ST(0) = sv_newmortal();
1126 if( rpcb_gettime( host, &timep ) )
1127 sv_setnv( ST(0), (double)timep );
1130 getnetconfigent(netid="udp")
1133 MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
1136 rpcb_DESTROY(netconf)
1139 printf("NetconfigPtr::DESTROY\n");
1142 File C<typemap>: Custom typemap for RPC.xs.
1145 Netconfig * T_PTROBJ
1147 File C<RPC.pm>: Perl module for the RPC extension.
1153 @ISA = qw(Exporter DynaLoader);
1154 @EXPORT = qw(rpcb_gettime getnetconfigent);
1159 File C<rpctest.pl>: Perl test program for the RPC extension.
1163 $netconf = getnetconfigent();
1164 $a = rpcb_gettime();
1165 print "time = $a\n";
1166 print "netconf = $netconf\n";
1168 $netconf = getnetconfigent("tcp");
1169 $a = rpcb_gettime("poplar");
1170 print "time = $a\n";
1171 print "netconf = $netconf\n";
1176 This document covers features supported by C<xsubpp> 1.935.
1180 Dean Roehrich F<E<lt>roehrich@cray.comE<gt>>