


PERLXS(1)        Perl Programmers Reference Guide       PERLXS(1)


NAME
       perlxs - XS language reference manual

DESCRIPTION
       Introduction

       XS is a language used to create an extension interface
       between Perl and some C library which one wishes to use
       with Perl.  The XS interface is combined with the library
       to create a new library which can be linked to Perl.  An
       XSUB is a function in the XS language and is the core
       component of the Perl application interface.

       The XS compiler is called xsubpp.  This compiler will
       embed the constructs necessary to let an XSUB, which is
       really a C function in disguise, manipulate Perl values
       and creates the glue necessary to let Perl access the
       XSUB.  The compiler uses typemaps to determine how to map
       C function parameters and variables to Perl values.  The
       default typemap handles many common C types.  A supplement
       typemap must be created to handle special structures and
       types for the library being linked.

       See the perlxstut manpage for a tutorial on the whole
       extension creation process.

       Note: For many extensions, Dave Beazley's SWIG system
       provides a significantly more convenient mechanism for
       creating the XS glue code. See the section on
       /www.cs.utah.edu/~beazley/SWIG in the http: manpage for
       more information.

       On The Road

       Many of the examples which follow will concentrate on
       creating an interface between Perl and the ONC+ RPC bind
       library functions.  The rpcb_gettime() function is used to
       demonstrate many features of the XS language.  This
       function has two parameters; the first is an input
       parameter and the second is an output parameter.  The
       function also returns a status value.

               bool_t rpcb_gettime(const char *host, time_t *timep);

       From C this function will be called with the following
       statements.

            #include <rpc/rpc.h>
            bool_t status;
            time_t timep;
            status = rpcb_gettime( "localhost", &timep );

       If an XSUB is created to offer a direct translation
       between this function and Perl, then this XSUB will be



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       used from Perl with the following code.  The $status and
       $timep variables will contain the output of the function.

            use RPC;
            $status = rpcb_gettime( "localhost", $timep );

       The following XS file shows an XS subroutine, or XSUB,
       which demonstrates one possible interface to the
       rpcb_gettime() function.  This XSUB represents a direct
       translation between C and Perl and so preserves the
       interface even from Perl.  This XSUB will be invoked from
       Perl with the usage shown above.  Note that the first
       three #include statements, for EXTERN.h, perl.h, and
       XSUB.h, will always be present at the beginning of an XS
       file.  This approach and others will be expanded later in
       this document.

            #include "EXTERN.h"
            #include "perl.h"
            #include "XSUB.h"
            #include <rpc/rpc.h>

            MODULE = RPC  PACKAGE = RPC

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 OUTPUT:
                 timep

       Any extension to Perl, including those containing XSUBs,
       should have a Perl module to serve as the bootstrap which
       pulls the extension into Perl.  This module will export
       the extension's functions and variables to the Perl
       program and will cause the extension's XSUBs to be linked
       into Perl.  The following module will be used for most of
       the examples in this document and should be used from Perl
       with the use command as shown earlier.  Perl modules are
       explained in more detail later in this document.

            package RPC;

            require Exporter;
            require DynaLoader;
            @ISA = qw(Exporter DynaLoader);
            @EXPORT = qw( rpcb_gettime );

            bootstrap RPC;
            1;

       Throughout this document a variety of interfaces to the
       rpcb_gettime() XSUB will be explored.  The XSUBs will take
       their parameters in different orders or will take



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       different numbers of parameters.  In each case the XSUB is
       an abstraction between Perl and the real C rpcb_gettime()
       function, and the XSUB must always ensure that the real
       rpcb_gettime() function is called with the correct
       parameters.  This abstraction will allow the programmer to
       create a more Perl-like interface to the C function.

       The Anatomy of an XSUB

       The following XSUB allows a Perl program to access a C
       library function called sin().  The XSUB will imitate the
       C function which takes a single argument and returns a
       single value.

            double
            sin(x)
              double x

       When using C pointers the indirection operator * should be
       considered part of the type and the address operator &
       should be considered part of the variable, as is
       demonstrated in the rpcb_gettime() function above.  See
       the section on typemaps for more about handling qualifiers
       and unary operators in C types.

       The function name and the return type must be placed on
       separate lines.

         INCORRECT                        CORRECT

         double sin(x)                    double
           double x                       sin(x)
                                            double x

       The function body may be indented or left-adjusted.  The
       following example shows a function with its body left-
       adjusted.  Most examples in this document will indent the
       body.

         CORRECT

         double
         sin(x)
         double x


       The Argument Stack

       The argument stack is used to store the values which are
       sent as parameters to the XSUB and to store the XSUB's
       return value.  In reality all Perl functions keep their
       values on this stack at the same time, each limited to its
       own range of positions on the stack.  In this document the
       first position on that stack which belongs to the active



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       function will be referred to as position 0 for that
       function.

       XSUBs refer to their stack arguments with the macro ST(x),
       where x refers to a position in this XSUB's part of the
       stack.  Position 0 for that function would be known to the
       XSUB as ST(0).  The XSUB's incoming parameters and
       outgoing return values always begin at ST(0).  For many
       simple cases the xsubpp compiler will generate the code
       necessary to handle the argument stack by embedding code
       fragments found in the typemaps.  In more complex cases
       the programmer must supply the code.

       The RETVAL Variable

       The RETVAL variable is a magic variable which always
       matches the return type of the C library function.  The
       xsubpp compiler will supply this variable in each XSUB and
       by default will use it to hold the return value of the C
       library function being called.  In simple cases the value
       of RETVAL will be placed in ST(0) of the argument stack
       where it can be received by Perl as the return value of
       the XSUB.

       If the XSUB has a return type of void then the compiler
       will not supply a RETVAL variable for that function.  When
       using the PPCODE: directive the RETVAL variable is not
       needed, unless used explicitly.

       If PPCODE: directive is not used, void return value should
       be used only for subroutines which do not return a value,
       even if CODE: directive is used which sets ST(0)
       explicitly.

       Older versions of this document recommended to use void
       return value in such cases. It was discovered that this
       could lead to segfaults in cases when XSUB was truly void.
       This practice is now deprecated, and may be not supported
       at some future version. Use the return value SV * in such
       cases. (Currently xsubpp contains some heuristic code
       which tries to disambiguate between "truly-void" and "old-
       practice-declared-as-void" functions. Hence your code is
       at mercy of this heuristics unless you use SV * as return
       value.)

       The MODULE Keyword

       The MODULE keyword is used to start the XS code and to
       specify the package of the functions which are being
       defined.  All text preceding the first MODULE keyword is
       considered C code and is passed through to the output
       untouched.  Every XS module will have a bootstrap function
       which is used to hook the XSUBs into Perl.  The package
       name of this bootstrap function will match the value of



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       the last MODULE statement in the XS source files.  The
       value of MODULE should always remain constant within the
       same XS file, though this is not required.

       The following example will start the XS code and will
       place all functions in a package named RPC.

            MODULE = RPC


       The PACKAGE Keyword

       When functions within an XS source file must be separated
       into packages the PACKAGE keyword should be used.  This
       keyword is used with the MODULE keyword and must follow
       immediately after it when used.

            MODULE = RPC  PACKAGE = RPC

            [ XS code in package RPC ]

            MODULE = RPC  PACKAGE = RPCB

            [ XS code in package RPCB ]

            MODULE = RPC  PACKAGE = RPC

            [ XS code in package RPC ]

       Although this keyword is optional and in some cases
       provides redundant information it should always be used.
       This keyword will ensure that the XSUBs appear in the
       desired package.

       The PREFIX Keyword

       The PREFIX keyword designates prefixes which should be
       removed from the Perl function names.  If the C function
       is rpcb_gettime() and the PREFIX value is rpcb_ then Perl
       will see this function as gettime().

       This keyword should follow the PACKAGE keyword when used.
       If PACKAGE is not used then PREFIX should follow the
       MODULE keyword.

            MODULE = RPC  PREFIX = rpc_

            MODULE = RPC  PACKAGE = RPCB  PREFIX = rpcb_


       The OUTPUT: Keyword

       The OUTPUT: keyword indicates that certain function
       parameters should be updated (new values made visible to



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       Perl) when the XSUB terminates or that certain values
       should be returned to the calling Perl function.  For
       simple functions, such as the sin() function above, the
       RETVAL variable is automatically designated as an output
       value.  In more complex functions the xsubpp compiler will
       need help to determine which variables are output
       variables.

       This keyword will normally be used to complement the CODE:
       keyword.  The RETVAL variable is not recognized as an
       output variable when the CODE: keyword is present.  The
       OUTPUT:  keyword is used in this situation to tell the
       compiler that RETVAL really is an output variable.

       The OUTPUT: keyword can also be used to indicate that
       function parameters are output variables.  This may be
       necessary when a parameter has been modified within the
       function and the programmer would like the update to be
       seen by Perl.

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 OUTPUT:
                 timep

       The OUTPUT: keyword will also allow an output parameter to
       be mapped to a matching piece of code rather than to a
       typemap.

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 OUTPUT:
                 timep sv_setnv(ST(1), (double)timep);

       xsubpp emits an automatic SvSETMAGIC() for all parameters
       in the OUTPUT section of the XSUB, except RETVAL.  This is
       the usually desired behavior, as it takes care of properly
       invoking 'set' magic on output parameters (needed for hash
       or array element parameters that must be created if they
       didn't exist).  If for some reason, this behavior is not
       desired, the OUTPUT section may contain a SETMAGIC:
       DISABLE line to disable it for the remainder of the
       parameters in the OUTPUT section.  Likewise,  SETMAGIC:
       ENABLE can be used to reenable it for the remainder of the
       OUTPUT section.  See the perlguts manpage for more details
       about 'set' magic.







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       The CODE: Keyword

       This keyword is used in more complicated XSUBs which
       require special handling for the C function.  The RETVAL
       variable is available but will not be returned unless it
       is specified under the OUTPUT: keyword.

       The following XSUB is for a C function which requires
       special handling of its parameters.  The Perl usage is
       given first.

            $status = rpcb_gettime( "localhost", $timep );

       The XSUB follows.

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t timep
                 CODE:
                      RETVAL = rpcb_gettime( host, &timep );
                 OUTPUT:
                 timep
                 RETVAL


       The INIT: Keyword

       The INIT: keyword allows initialization to be inserted
       into the XSUB before the compiler generates the call to
       the C function.  Unlike the CODE: keyword above, this
       keyword does not affect the way the compiler handles
       RETVAL.

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 INIT:
                 printf("# Host is %s\n", host );
                 OUTPUT:
                 timep


       The NO_INIT Keyword

       The NO_INIT keyword is used to indicate that a function
       parameter is being used only as an output value.  The
       xsubpp compiler will normally generate code to read the
       values of all function parameters from the argument stack
       and assign them to C variables upon entry to the function.
       NO_INIT will tell the compiler that some parameters will
       be used for output rather than for input and that they
       will be handled before the function terminates.



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       The following example shows a variation of the
       rpcb_gettime() function.  This function uses the timep
       variable only as an output variable and does not care
       about its initial contents.

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep = NO_INIT
                 OUTPUT:
                 timep


       Initializing Function Parameters

       Function parameters are normally initialized with their
       values from the argument stack.  The typemaps contain the
       code segments which are used to transfer the Perl values
       to the C parameters.  The programmer, however, is allowed
       to override the typemaps and supply alternate (or
       additional) initialization code.

       The following code demonstrates how to supply
       initialization code for function parameters.  The
       initialization code is eval'd within double quotes by the
       compiler before it is added to the output so anything
       which should be interpreted literally [mainly $, @, or \\]
       must be protected with backslashes.  The variables $var,
       $arg, and $type can be used as in typemaps.

            bool_t
            rpcb_gettime(host,timep)
                 char *host = (char *)SvPV($arg,PL_na);
                 time_t &timep = 0;
                 OUTPUT:
                 timep

       This should not be used to supply default values for
       parameters.  One would normally use this when a function
       parameter must be processed by another library function
       before it can be used.  Default parameters are covered in
       the next section.

       If the initialization begins with =, then it is output on
       the same line where the input variable is declared.  If
       the initialization begins with ; or +, then it is output
       after all of the input variables have been declared.  The
       = and ; cases replace the initialization normally supplied
       from the typemap.  For the + case, the initialization from
       the typemap will precede the initialization code included
       after the +.  A global variable, %v, is available for the
       truly rare case where information from one initialization
       is needed in another initialization.




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            bool_t
            rpcb_gettime(host,timep)
                 time_t &timep ; /*\$v{time}=@{[$v{time}=$arg]}*/
                 char *host + SvOK($v{time}) ? SvPV($arg,PL_na) : NULL;
                 OUTPUT:
                 timep


       Default Parameter Values

       Default values can be specified for function parameters by
       placing an assignment statement in the parameter list.
       The default value may be a number or a string.  Defaults
       should always be used on the right-most parameters only.

       To allow the XSUB for rpcb_gettime() to have a default
       host value the parameters to the XSUB could be rearranged.
       The XSUB will then call the real rpcb_gettime() function
       with the parameters in the correct order.  Perl will call
       this XSUB with either of the following statements.

            $status = rpcb_gettime( $timep, $host );

            $status = rpcb_gettime( $timep );

       The XSUB will look like the code  which  follows.   A
       CODE: block  is used to call the real rpcb_gettime()
       function with the parameters in the correct order for that
       function.

            bool_t
            rpcb_gettime(timep,host="localhost")
                 char *host
                 time_t timep = NO_INIT
                 CODE:
                      RETVAL = rpcb_gettime( host, &timep );
                 OUTPUT:
                 timep
                 RETVAL


       The PREINIT: Keyword

       The PREINIT: keyword allows extra variables to be declared
       before the typemaps are expanded.  If a variable is
       declared in a CODE: block then that variable will follow
       any typemap code.  This may result in a C syntax error.
       To force the variable to be declared before the typemap
       code, place it into a PREINIT: block.  The PREINIT:
       keyword may be used one or more times within an XSUB.

       The following examples are equivalent, but if the code is
       using complex typemaps then the first example is safer.




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            bool_t
            rpcb_gettime(timep)
                 time_t timep = NO_INIT
                 PREINIT:
                 char *host = "localhost";
                 CODE:
                 RETVAL = rpcb_gettime( host, &timep );
                 OUTPUT:
                 timep
                 RETVAL

       A correct, but error-prone example.

            bool_t
            rpcb_gettime(timep)
                 time_t timep = NO_INIT
                 CODE:
                 char *host = "localhost";
                 RETVAL = rpcb_gettime( host, &timep );
                 OUTPUT:
                 timep
                 RETVAL


       The SCOPE: Keyword

       The SCOPE: keyword allows scoping to be enabled for a
       particular XSUB. If enabled, the XSUB will invoke ENTER
       and LEAVE automatically.

       To support potentially complex type mappings, if a typemap
       entry used by this XSUB contains a comment like /*scope*/
       then scoping will automatically be enabled for that XSUB.

       To enable scoping:

           SCOPE: ENABLE

       To disable scoping:

           SCOPE: DISABLE


       The INPUT: Keyword

       The XSUB's parameters are usually evaluated immediately
       after entering the XSUB.  The INPUT: keyword can be used
       to force those parameters to be evaluated a little later.
       The INPUT: keyword can be used multiple times within an
       XSUB and can be used to list one or more input variables.
       This keyword is used with the PREINIT: keyword.

       The following example shows how the input parameter timep
       can be evaluated late, after a PREINIT.



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           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 PREINIT:
                 time_t tt;
                 INPUT:
                 time_t timep
                 CODE:
                      RETVAL = rpcb_gettime( host, &tt );
                      timep = tt;
                 OUTPUT:
                 timep
                 RETVAL

       The next example shows each input parameter evaluated
       late.

           bool_t
           rpcb_gettime(host,timep)
                 PREINIT:
                 time_t tt;
                 INPUT:
                 char *host
                 PREINIT:
                 char *h;
                 INPUT:
                 time_t timep
                 CODE:
                      h = host;
                      RETVAL = rpcb_gettime( h, &tt );
                      timep = tt;
                 OUTPUT:
                 timep
                 RETVAL


       Variable-length Parameter Lists

       XSUBs can have variable-length parameter lists by
       specifying an ellipsis (...) in the parameter list.  This
       use of the ellipsis is similar to that found in ANSI C.
       The programmer is able to determine the number of
       arguments passed to the XSUB by examining the items
       variable which the xsubpp compiler supplies for all XSUBs.
       By using this mechanism one can create an XSUB which
       accepts a list of parameters of unknown length.

       The host parameter for the rpcb_gettime() XSUB can be
       optional so the ellipsis can be used to indicate that the
       XSUB will take a variable number of parameters.  Perl
       should be able to call this XSUB with either of the
       following statements.

            $status = rpcb_gettime( $timep, $host );



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            $status = rpcb_gettime( $timep );

       The XS code, with ellipsis, follows.

            bool_t
            rpcb_gettime(timep, ...)
                 time_t timep = NO_INIT
                 PREINIT:
                 char *host = "localhost";
                 STRLEN n_a;
                 CODE:
                         if( items > 1 )
                              host = (char *)SvPV(ST(1), n_a);
                         RETVAL = rpcb_gettime( host, &timep );
                 OUTPUT:
                 timep
                 RETVAL


       The C_ARGS: Keyword

       The C_ARGS: keyword allows creating of XSUBS which have
       different calling sequence from Perl than from C, without
       a need to write CODE: or CPPCODE: section.  The contents
       of the C_ARGS: paragraph is put as the argument to the
       called C function without any change.

       For example, suppose that C function is declared as

           symbolic nth_derivative(int n, symbolic function, int flags);

       and that the default flags are kept in a global C variable
       default_flags.  Suppose that you want to create an
       interface which is called as

           $second_deriv = $function->nth_derivative(2);

       To do this, declare the XSUB as

           symbolic
           nth_derivative(function, n)
               symbolic        function
               int             n
           C_ARGS:
               n, function, default_flags


       The PPCODE: Keyword

       The PPCODE: keyword is an alternate form of the CODE:
       keyword and is used to tell the xsubpp compiler that the
       programmer is supplying the code to control the argument
       stack for the XSUBs return values.  Occasionally one will
       want an XSUB to return a list of values rather than a



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       single value.  In these cases one must use PPCODE: and
       then explicitly push the list of values on the stack.  The
       PPCODE: and CODE:  keywords are not used together within
       the same XSUB.

       The following XSUB will call the C rpcb_gettime() function
       and will return its two output values, timep and status,
       to Perl as a single list.

            void
            rpcb_gettime(host)
                 char *host
                 PREINIT:
                 time_t  timep;
                 bool_t  status;
                 PPCODE:
                 status = rpcb_gettime( host, &timep );
                 EXTEND(SP, 2);
                 PUSHs(sv_2mortal(newSViv(status)));
                 PUSHs(sv_2mortal(newSViv(timep)));

       Notice that the programmer must supply the C code
       necessary to have the real rpcb_gettime() function called
       and to have the return values properly placed on the
       argument stack.

       The void return type for this function tells the xsubpp
       compiler that the RETVAL variable is not needed or used
       and that it should not be created.  In most scenarios the
       void return type should be used with the PPCODE:
       directive.

       The EXTEND() macro is used to make room on the argument
       stack for 2 return values.  The PPCODE: directive causes
       the xsubpp compiler to create a stack pointer available as
       SP, and it is this pointer which is being used in the
       EXTEND() macro.  The values are then pushed onto the stack
       with the PUSHs() macro.

       Now the rpcb_gettime() function can be used from Perl with
       the following statement.

            ($status, $timep) = rpcb_gettime("localhost");

       When handling output parameters with a PPCODE section, be
       sure to handle 'set' magic properly.  See the perlguts
       manpage for details about 'set' magic.

       Returning Undef And Empty Lists

       Occasionally the programmer will want to return simply
       undef or an empty list if a function fails rather than a
       separate status value.  The rpcb_gettime() function offers
       just this situation.  If the function succeeds we would



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       like to have it return the time and if it fails we would
       like to have undef returned.  In the following Perl code
       the value of $timep will either be undef or it will be a
       valid time.

            $timep = rpcb_gettime( "localhost" );

       The following XSUB uses the SV * return type as a mnemonic
       only, and uses a CODE: block to indicate to the compiler
       that the programmer has supplied all the necessary code.
       The sv_newmortal() call will initialize the return value
       to undef, making that the default return value.

            SV *
            rpcb_gettime(host)
                 char *  host
                 PREINIT:
                 time_t  timep;
                 bool_t x;
                 CODE:
                 ST(0) = sv_newmortal();
                 if( rpcb_gettime( host, &timep ) )
                      sv_setnv( ST(0), (double)timep);

       The next example demonstrates how one would place an
       explicit undef in the return value, should the need arise.

            SV *
            rpcb_gettime(host)
                 char *  host
                 PREINIT:
                 time_t  timep;
                 bool_t x;
                 CODE:
                 ST(0) = sv_newmortal();
                 if( rpcb_gettime( host, &timep ) ){
                      sv_setnv( ST(0), (double)timep);
                 }
                 else{
                      ST(0) = &PL_sv_undef;
                 }

       To return an empty list one must use a PPCODE: block and
       then not push return values on the stack.













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            void
            rpcb_gettime(host)
                 char *host
                 PREINIT:
                 time_t  timep;
                 PPCODE:
                 if( rpcb_gettime( host, &timep ) )
                      PUSHs(sv_2mortal(newSViv(timep)));
                 else{
                 /* Nothing pushed on stack, so an empty */
                 /* list is implicitly returned. */
                 }

       Some people may be inclined to include an explicit return
       in the above XSUB, rather than letting control fall
       through to the end.  In those situations XSRETURN_EMPTY
       should be used, instead.  This will ensure that the XSUB
       stack is properly adjusted.  Consult the section on API
       LISTING in the perlguts manpage for other XSRETURN macros.

       The REQUIRE: Keyword

       The REQUIRE: keyword is used to indicate the minimum
       version of the xsubpp compiler needed to compile the XS
       module.  An XS module which contains the following
       statement will compile with only xsubpp version 1.922 or
       greater:

               REQUIRE: 1.922


       The CLEANUP: Keyword

       This keyword can be used when an XSUB requires special
       cleanup procedures before it terminates.  When the
       CLEANUP:  keyword is used it must follow any CODE:,
       PPCODE:, or OUTPUT: blocks which are present in the XSUB.
       The code specified for the cleanup block will be added as
       the last statements in the XSUB.

       The BOOT: Keyword

       The BOOT: keyword is used to add code to the extension's
       bootstrap function.  The bootstrap function is generated
       by the xsubpp compiler and normally holds the statements
       necessary to register any XSUBs with Perl.  With the BOOT:
       keyword the programmer can tell the compiler to add extra
       statements to the bootstrap function.

       This keyword may be used any time after the first MODULE
       keyword and should appear on a line by itself.  The first
       blank line after the keyword will terminate the code
       block.




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            BOOT:
            # The following message will be printed when the
            # bootstrap function executes.
            printf("Hello from the bootstrap!\n");


       The VERSIONCHECK: Keyword

       The VERSIONCHECK: keyword corresponds to xsubpp's
       -versioncheck and -noversioncheck options.  This keyword
       overrides the command line options.  Version checking is
       enabled by default.  When version checking is enabled the
       XS module will attempt to verify that its version matches
       the version of the PM module.

       To enable version checking:

           VERSIONCHECK: ENABLE

       To disable version checking:

           VERSIONCHECK: DISABLE


       The PROTOTYPES: Keyword

       The PROTOTYPES: keyword corresponds to xsubpp's
       -prototypes and -noprototypes options.  This keyword
       overrides the command line options.  Prototypes are
       enabled by default.  When prototypes are enabled XSUBs
       will be given Perl prototypes.  This keyword may be used
       multiple times in an XS module to enable and disable
       prototypes for different parts of the module.

       To enable prototypes:

           PROTOTYPES: ENABLE

       To disable prototypes:

           PROTOTYPES: DISABLE


       The PROTOTYPE: Keyword

       This keyword is similar to the PROTOTYPES: keyword above
       but can be used to force xsubpp to use a specific
       prototype for the XSUB.  This keyword overrides all other
       prototype options and keywords but affects only the
       current XSUB.  Consult the Prototypes entry in the perlsub
       manpage for information about Perl prototypes.






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           bool_t
           rpcb_gettime(timep, ...)
                 time_t timep = NO_INIT
                 PROTOTYPE: $;$
                 PREINIT:
                 char *host = "localhost";
                 STRLEN n_a;
                 CODE:
                         if( items > 1 )
                              host = (char *)SvPV(ST(1), n_a);
                         RETVAL = rpcb_gettime( host, &timep );
                 OUTPUT:
                 timep
                 RETVAL


       The ALIAS: Keyword

       The ALIAS: keyword allows an XSUB to have two or more
       unique Perl names and to know which of those names was
       used when it was invoked.  The Perl names may be fully-
       qualified with package names.  Each alias is given an
       index.  The compiler will setup a variable called ix which
       contain the index of the alias which was used.  When the
       XSUB is called with its declared name ix will be 0.

       The following example will create aliases FOO::gettime()
       and BAR::getit() for this function.

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 ALIAS:
                   FOO::gettime = 1
                   BAR::getit = 2
                 INIT:
                 printf("# ix = %d\n", ix );
                 OUTPUT:
                 timep


       The INTERFACE: Keyword

       This keyword declares the current XSUB as a keeper of the
       given calling signature.  If some text follows this
       keyword, it is considered as a list of functions which
       have this signature, and should be attached to XSUBs.

       Say, if you have 4 functions multiply(), divide(), add(),
       subtract() all having the signature

           symbolic f(symbolic, symbolic);




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       you code them all by using XSUB

           symbolic
           interface_s_ss(arg1, arg2)
               symbolic        arg1
               symbolic        arg2
           INTERFACE:
               multiply divide
               add subtract

       The advantage of this approach comparing to ALIAS: keyword
       is that one can attach an extra function remainder() at
       runtime by using

           CV *mycv = newXSproto("Symbolic::remainder",
                        XS_Symbolic_interface_s_ss, __FILE__,
       "$$");
           XSINTERFACE_FUNC_SET(mycv, remainder);

       (This example supposes that there was no INTERFACE_MACRO:
       section, otherwise one needs to use something else instead
       of XSINTERFACE_FUNC_SET.)

       The INTERFACE_MACRO: Keyword

       This keyword allows one to define an INTERFACE using a
       different way to extract a function pointer from an XSUB.
       The text which follows this keyword should give the name
       of macros which would extract/set a function pointer.  The
       extractor macro is given return type, CV*, and
       XSANY.any_dptr for this CV*.  The setter macro is given
       cv, and the function pointer.

       The default value is XSINTERFACE_FUNC and
       XSINTERFACE_FUNC_SET.  An INTERFACE keyword with an empty
       list of functions can be omitted if INTERFACE_MACRO
       keyword is used.

       Suppose that in the previous example functions pointers
       for multiply(), divide(), add(), subtract() are kept in a
       global C array fp[] with offsets being multiply_off,
       divide_off, add_off, subtract_off.  Then one can use

           #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
               ((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32])
           #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
               CvXSUBANY(cv).any_i32 = CAT2( f, _off )

       in C section,








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           symbolic
           interface_s_ss(arg1, arg2)
               symbolic        arg1
               symbolic        arg2
           INTERFACE_MACRO:
               XSINTERFACE_FUNC_BYOFFSET
               XSINTERFACE_FUNC_BYOFFSET_set
           INTERFACE:
               multiply divide
               add subtract

       in XSUB section.

       The INCLUDE: Keyword

       This keyword can be used to pull other files into the XS
       module.  The other files may have XS code.  INCLUDE: can
       also be used to run a command to generate the XS code to
       be pulled into the module.

       The file Rpcb1.xsh contains our rpcb_gettime() function:

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 OUTPUT:
                 timep

       The XS module can use INCLUDE: to pull that file into it.

           INCLUDE: Rpcb1.xsh

       If the parameters to the INCLUDE: keyword are followed by
       a pipe (|) then the compiler will interpret the parameters
       as a command.

           INCLUDE: cat Rpcb1.xsh |


       The CASE: Keyword

       The CASE: keyword allows an XSUB to have multiple distinct
       parts with each part acting as a virtual XSUB.  CASE: is
       greedy and if it is used then all other XS keywords must
       be contained within a CASE:.  This means nothing may
       precede the first CASE: in the XSUB and anything following
       the last CASE: is included in that case.

       A CASE: might switch via a parameter of the XSUB, via the
       ix ALIAS: variable (see the section on The ALIAS:
       Keyword), or maybe via the items variable (see the section
       on Variable-length Parameter Lists).  The last CASE:
       becomes the default case if it is not associated with a



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       conditional.  The following example shows CASE switched
       via ix with a function rpcb_gettime() having an alias
       x_gettime().  When the function is called as
       rpcb_gettime() its parameters are the usual (char *host,
       time_t *timep), but when the function is called as
       x_gettime() its parameters are reversed, (time_t *timep,
       char *host).

           long
           rpcb_gettime(a,b)
             CASE: ix == 1
                 ALIAS:
                 x_gettime = 1
                 INPUT:
                 # 'a' is timep, 'b' is host
                 char *b
                 time_t a = NO_INIT
                 CODE:
                      RETVAL = rpcb_gettime( b, &a );
                 OUTPUT:
                 a
                 RETVAL
             CASE:
                 # 'a' is host, 'b' is timep
                 char *a
                 time_t &b = NO_INIT
                 OUTPUT:
                 b
                 RETVAL

       That function can be called with either of the following
       statements.  Note the different argument lists.

               $status = rpcb_gettime( $host, $timep );

               $status = x_gettime( $timep, $host );


       The & Unary Operator

       The & unary operator is used to tell the compiler that it
       should dereference the object when it calls the C
       function.  This is used when a CODE: block is not used and
       the object is a not a pointer type (the object is an int
       or long but not a int* or long*).

       The following XSUB will generate incorrect C code.  The
       xsubpp compiler will turn this into code which calls
       rpcb_gettime() with parameters (char *host, time_t timep),
       but the real rpcb_gettime() wants the timep parameter to
       be of type time_t* rather than time_t.






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           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t timep
                 OUTPUT:
                 timep

       That problem is corrected by using the & operator.  The
       xsubpp compiler will now turn this into code which calls
       rpcb_gettime() correctly with parameters (char *host,
       time_t *timep).  It does this by carrying the & through,
       so the function call looks like rpcb_gettime(host,
       &timep).

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
                 OUTPUT:
                 timep


       Inserting Comments and C Preprocessor Directives

       C preprocessor directives are allowed within BOOT:,
       PREINIT: INIT:, CODE:, PPCODE:, and CLEANUP: blocks, as
       well as outside the functions.  Comments are allowed
       anywhere after the MODULE keyword.  The compiler will pass
       the preprocessor directives through untouched and will
       remove the commented lines.

       Comments can be added to XSUBs by placing a # as the first
       non-whitespace of a line.  Care should be taken to avoid
       making the comment look like a C preprocessor directive,
       lest it be interpreted as such.  The simplest way to
       prevent this is to put whitespace in front of the #.

       If you use preprocessor directives to choose one of two
       versions of a function, use

           #if ... version1
           #else /* ... version2  */
           #endif

       and not

           #if ... version1
           #endif
           #if ... version2
           #endif

       because otherwise xsubpp will believe that you made a
       duplicate definition of the function.  Also, put a blank
       line before the #else/#endif so it will not be seen as



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       part of the function body.

       Using XS With C++

       If a function is defined as a C++ method then it will
       assume its first argument is an object pointer.  The
       object pointer will be stored in a variable called THIS.
       The object should have been created by C++ with the new()
       function and should be blessed by Perl with the
       sv_setref_pv() macro.  The blessing of the object by Perl
       can be handled by a typemap.  An example typemap is shown
       at the end of this section.

       If the method is defined as static it will call the C++
       function using the class::method() syntax.  If the method
       is not static the function will be called using the
       THIS->method() syntax.

       The next examples will use the following C++ class.

            class color {
                 public:
                 color();
                 ~color();
                 int blue();
                 void set_blue( int );

                 private:
                 int c_blue;
            };

       The XSUBs for the blue() and set_blue() methods are
       defined with the class name but the parameter for the
       object (THIS, or "self") is implicit and is not listed.

            int
            color::blue()

            void
            color::set_blue( val )
                 int val

       Both functions will expect an object as the first
       parameter.  The xsubpp compiler will call that object THIS
       and will use it to call the specified method.  So in the
       C++ code the blue() and set_blue() methods will be called
       in the following manner.

            RETVAL = THIS->blue();

            THIS->set_blue( val );

       If the function's name is DESTROY then the C++ delete
       function will be called and THIS will be given as its



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       parameter.

            void
            color::DESTROY()

       The C++ code will call delete.

            delete THIS;

       If the function's name is new then the C++ new function
       will be called to create a dynamic C++ object.  The XSUB
       will expect the class name, which will be kept in a
       variable called CLASS, to be given as the first argument.

            color *
            color::new()

       The C++ code will call new.

               RETVAL = new color();

       The following is an example of a typemap that could be
       used for this C++ example.

           TYPEMAP
           color *             O_OBJECT

           OUTPUT
           # The Perl object is blessed into 'CLASS', which should be a
           # char* having the name of the package for the blessing.
           O_OBJECT
               sv_setref_pv( $arg, CLASS, (void*)$var );

           INPUT
           O_OBJECT
               if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
                       $var = ($type)SvIV((SV*)SvRV( $arg ));
               else{
                       warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
                       XSRETURN_UNDEF;
               }


       Interface Strategy

       When designing an interface between Perl and a C library a
       straight translation from C to XS is often sufficient.
       The interface will often be very C-like and occasionally
       nonintuitive, especially when the C function modifies one
       of its parameters.  In cases where the programmer wishes
       to create a more Perl-like interface the following
       strategy may help to identify the more critical parts of
       the interface.




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       Identify the C functions which modify their parameters.
       The XSUBs for these functions may be able to return lists
       to Perl, or may be candidates to return undef or an empty
       list in case of failure.

       Identify which values are used by only the C and XSUB
       functions themselves.  If Perl does not need to access the
       contents of the value then it may not be necessary to
       provide a translation for that value from C to Perl.

       Identify the pointers in the C function parameter lists
       and return values.  Some pointers can be handled in XS
       with the & unary operator on the variable name while
       others will require the use of the * operator on the type
       name.  In general it is easier to work with the &
       operator.

       Identify the structures used by the C functions.  In many
       cases it may be helpful to use the T_PTROBJ typemap for
       these structures so they can be manipulated by Perl as
       blessed objects.

       Perl Objects And C Structures

       When dealing with C structures one should select either
       T_PTROBJ or T_PTRREF for the XS type.  Both types are
       designed to handle pointers to complex objects.  The
       T_PTRREF type will allow the Perl object to be unblessed
       while the T_PTROBJ type requires that the object be
       blessed.  By using T_PTROBJ one can achieve a form of
       type-checking because the XSUB will attempt to verify that
       the Perl object is of the expected type.

       The following XS code shows the getnetconfigent() function
       which is used with ONC+ TIRPC.  The getnetconfigent()
       function will return a pointer to a C structure and has
       the C prototype shown below.  The example will demonstrate
       how the C pointer will become a Perl reference.  Perl will
       consider this reference to be a pointer to a blessed
       object and will attempt to call a destructor for the
       object.  A destructor will be provided in the XS source to
       free the memory used by getnetconfigent().  Destructors in
       XS can be created by specifying an XSUB function whose
       name ends with the word DESTROY.  XS destructors can be
       used to free memory which may have been malloc'd by
       another XSUB.

            struct netconfig *getnetconfigent(const char *netid);

       A typedef will be created for struct netconfig.  The Perl
       object will be blessed in a class matching the name of the
       C type, with the tag Ptr appended, and the name should not
       have embedded spaces if it will be a Perl package name.
       The destructor will be placed in a class corresponding to



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       the class of the object and the PREFIX keyword will be
       used to trim the name to the word DESTROY as Perl will
       expect.

            typedef struct netconfig Netconfig;

            MODULE = RPC  PACKAGE = RPC

            Netconfig *
            getnetconfigent(netid)
                 char *netid

            MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

            void
            rpcb_DESTROY(netconf)
                 Netconfig *netconf
                 CODE:
                 printf("Now in NetconfigPtr::DESTROY\n");
                 free( netconf );

       This example requires the following typemap entry.
       Consult the typemap section for more information about
       adding new typemaps for an extension.

            TYPEMAP
            Netconfig *  T_PTROBJ

       This example will be used with the following Perl
       statements.

            use RPC;
            $netconf = getnetconfigent("udp");

       When Perl destroys the object referenced by $netconf it
       will send the object to the supplied XSUB DESTROY
       function.  Perl cannot determine, and does not care, that
       this object is a C struct and not a Perl object.  In this
       sense, there is no difference between the object created
       by the getnetconfigent() XSUB and an object created by a
       normal Perl subroutine.

       The Typemap

       The typemap is a collection of code fragments which are
       used by the xsubpp compiler to map C function parameters
       and values to Perl values.  The typemap file may consist
       of three sections labeled TYPEMAP, INPUT, and OUTPUT.  Any
       unlabelled initial section is assumed to be a TYPEMAP
       section if a name is not explicitly specified.  The INPUT
       section tells the compiler how to translate Perl values
       into variables of certain C types.  The OUTPUT section
       tells the compiler how to translate the values from
       certain C types into values Perl can understand.  The



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       TYPEMAP section tells the compiler which of the INPUT and
       OUTPUT code fragments should be used to map a given C type
       to a Perl value.  The section labels TYPEMAP, INPUT, or
       OUTPUT must begin in the first column on a line by
       themselves, and must be in uppercase.

       The default typemap in the ext directory of the Perl
       source contains many useful types which can be used by
       Perl extensions.  Some extensions define additional
       typemaps which they keep in their own directory.  These
       additional typemaps may reference INPUT and OUTPUT maps in
       the main typemap.  The xsubpp compiler will allow the
       extension's own typemap to override any mappings which are
       in the default typemap.

       Most extensions which require a custom typemap will need
       only the TYPEMAP section of the typemap file.  The custom
       typemap used in the getnetconfigent() example shown
       earlier demonstrates what may be the typical use of
       extension typemaps.  That typemap is used to equate a C
       structure with the T_PTROBJ typemap.  The typemap used by
       getnetconfigent() is shown here.  Note that the C type is
       separated from the XS type with a tab and that the C unary
       operator * is considered to be a part of the C type name.

            TYPEMAP
            Netconfig *<tab>T_PTROBJ

       Here's a more complicated example: suppose that you wanted
       struct netconfig to be blessed into the class Net::Config.
       One way to do this is to use underscores (_) to separate
       package names, as follows:

               typedef struct netconfig * Net_Config;

       And then provide a typemap entry T_PTROBJ_SPECIAL that
       maps underscores to double-colons (::), and declare
       Net_Config to be of that type:

               TYPEMAP
               Net_Config      T_PTROBJ_SPECIAL

               INPUT
               T_PTROBJ_SPECIAL
                       if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
                               IV tmp = SvIV((SV*)SvRV($arg));
                       $var = ($type) tmp;
                       }
                       else
                               croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")







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               OUTPUT
               T_PTROBJ_SPECIAL
                       sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",
                       (void*)$var);

       The INPUT and OUTPUT sections substitute underscores for
       double-colons on the fly, giving the desired effect.  This
       example demonstrates some of the power and versatility of
       the typemap facility.

EXAMPLES
       File RPC.xs: Interface to some ONC+ RPC bind library
       functions.

            #include "EXTERN.h"
            #include "perl.h"
            #include "XSUB.h"

            #include <rpc/rpc.h>

            typedef struct netconfig Netconfig;

            MODULE = RPC  PACKAGE = RPC

            SV *
            rpcb_gettime(host="localhost")
                 char *host
                 PREINIT:
                 time_t  timep;
                 CODE:
                 ST(0) = sv_newmortal();
                 if( rpcb_gettime( host, &timep ) )
                      sv_setnv( ST(0), (double)timep );

            Netconfig *
            getnetconfigent(netid="udp")
                 char *netid

            MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

            void
            rpcb_DESTROY(netconf)
                 Netconfig *netconf
                 CODE:
                 printf("NetconfigPtr::DESTROY\n");
                 free( netconf );

       File typemap: Custom typemap for RPC.xs.

            TYPEMAP
            Netconfig *  T_PTROBJ

       File RPC.pm: Perl module for the RPC extension.




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            package RPC;

            require Exporter;
            require DynaLoader;
            @ISA = qw(Exporter DynaLoader);
            @EXPORT = qw(rpcb_gettime getnetconfigent);

            bootstrap RPC;
            1;

       File rpctest.pl: Perl test program for the RPC extension.

            use RPC;

            $netconf = getnetconfigent();
            $a = rpcb_gettime();
            print "time = $a\n";
            print "netconf = $netconf\n";

            $netconf = getnetconfigent("tcp");
            $a = rpcb_gettime("poplar");
            print "time = $a\n";
            print "netconf = $netconf\n";


XS VERSION
       This document covers features supported by xsubpp 1.935.

AUTHOR
       Dean Roehrich <roehrich@cray.com> Jul 8, 1996



























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