*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->OpenBSD man pages -> perlxs (1)              



NAME    [Toc]    [Back]

       perlxs - XS language reference manual

DESCRIPTION    [Toc]    [Back]


       XS is an interface description file format used to create
       an extension interface between Perl and C code (or a C
       library) which one wishes to use with Perl.  The XS interface
 is combined with the library to create a new library
       which can then be either dynamically loaded or statically
       linked into perl.  The XS interface description is written
       in the XS language and is the core component of the Perl
       extension interface.

       An XSUB forms the basic unit of the XS interface.  After
       compilation by the xsubpp compiler, each XSUB amounts to a
       C function definition which will provide the glue between
       Perl calling conventions and C calling conventions.

       The glue code pulls the arguments from the Perl stack,
       converts these Perl values to the formats expected by a C
       function, call this C function, transfers the return values
 of the C function back to Perl.  Return values here
       may be a conventional C return value or any C function
       arguments that may serve as output parameters.  These
       return values may be passed back to Perl either by putting
       them on the Perl stack, or by modifying the arguments supplied
 from the Perl side.

       The above is a somewhat simplified view of what really
       happens.  Since Perl allows more flexible calling conventions
 than C, XSUBs may do much more in practice, such as
       checking input parameters for validity, throwing exceptions
 (or returning undef/empty list) if the return value
       from the C function indicates failure, calling different C
       functions based on numbers and types of the arguments,
       providing an object-oriented interface, etc.

       Of course, one could write such glue code directly in C.
       However, this would be a tedious task, especially if one
       needs to write glue for multiple C functions, and/or one
       is not familiar enough with the Perl stack discipline and
       other such arcana.  XS comes to the rescue here: instead
       of writing this glue C code in long-hand, one can write a
       more concise short-hand description of what should be done
       by the glue, and let the XS compiler xsubpp handle the

       The XS language allows one to describe the mapping between
       how the C routine is used, and how the corresponding Perl
       routine is used.  It also allows creation of Perl routines
       which are directly translated to C code and which are not
       related to a pre-existing C function.  In cases when the C
       interface coincides with the Perl interface, the XSUB declaration
 is almost identical to a declaration of a C function
 (in K&R style).  In such circumstances, there is
       another tool called "h2xs" that is able to translate an
       entire C header file into a corresponding XS file that
       will provide glue to the functions/macros described in the
       header file.

       The XS compiler is called xsubpp.  This compiler creates
       the constructs necessary to let an XSUB manipulate Perl
       values, and creates the glue necessary to let Perl call
       the XSUB.  The compiler uses typemaps to determine how to
       map C function parameters and output values to Perl values
       and back.  The default typemap (which comes with Perl)
       handles many common C types.  A supplementary typemap may
       also be needed to handle any special structures and types
       for the library being linked.

       A file in XS format starts with a C language section which
       goes until the first "MODULE =" directive.  Other XS
       directives and XSUB definitions may follow this line.  The
       "language" used in this part of the file is usually
       referred to as the XS language.  xsubpp recognizes and
       skips POD (see perlpod) in both the C and XS language sections,
 which allows the XS file to contain embedded documentation.

       See perlxstut for a tutorial on the whole extension creation

       Note: For some extensions, Dave Beazley's SWIG system may
       provide a significantly more convenient mechanism for creating
 the extension glue code.  See http://www.swig.org/
       for more information.

       On The Road    [Toc]    [Back]

       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

       From C this function will be called with the following

            #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
       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_get-
       time() 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

                 char *host
                 time_t &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;

       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 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    [Toc]    [Back]

       The simplest XSUBs consist of 3 parts: a description of
       the return value, the name of the XSUB routine and the
       names of its arguments, and a description of types or formats
 of the arguments.

       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 x

       Optionally, one can merge the description of types and the
       list of argument names, rewriting this as

            sin(double x)

       This makes this XSUB look similar to an ANSI C declaration.
  An optional semicolon is allowed after the argument
       list, as in

            sin(double x);

       Parameters with C pointer types can have different semantic:
 C functions with similar declarations

            bool string_looks_as_a_number(char *s);
            bool make_char_uppercase(char *c);

       are used in absolutely incompatible manner.  Parameters to
       these functions could be described xsubpp like this:

            char *  s
            char    &c

       Both these XS declarations correspond to the "char*" C
       type, but they have different semantics, see "The & Unary
       It is convenient to think that the indirection operator
       "*" should be considered as a part of the type and the
       address operator "&" should be considered part of the
       variable.  See "The Typemap" for more info about handling
       qualifiers and unary operators in C types.

       The function name and the return type must be placed on
       separate lines and should be flush left-adjusted.

         INCORRECT                        CORRECT

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

       The rest of the function description 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 for better readability.


         double x

       More complicated XSUBs may contain many other sections.
       Each section of an XSUB starts with the corresponding keyword,
 such as INIT: or CLEANUP:.  However, the first two
       lines of an XSUB always contain the same data: descriptions
 of the return type and the names of the function and
       its parameters.  Whatever immediately follows these is
       considered to be an INPUT: section unless explicitly
       marked  with another keyword.  (See "The INPUT: Keyword".)

       An XSUB section continues until another section-start keyword
 is found.

       The Argument Stack    [Toc]    [Back]

       The Perl argument stack is used to store the values which
       are sent as parameters to the XSUB and to store the XSUB's
       return value(s).  In reality all Perl functions (including
       non-XSUB ones) keep their values on this stack all 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 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    [Toc]    [Back]

       The RETVAL variable is a special C variable that is
       declared automatically for you.  The C type of RETVAL
       matches the return type of the C library function.  The
       xsubpp compiler will declare this variable in each XSUB
       with non-"void" return type.  By default the generated C
       function will use RETVAL 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 declare a RETVAL variable for that function.
       When using a PPCODE: section no manipulation of the RETVAL
       variable is required, the section may use direct stack
       manipulation to place output values on the stack.

       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)

       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.)

       Returning SVs, AVs and HVs through RETVAL

       When you're using RETVAL to return an "SV *", there's some
       magic going on behind the scenes that should be mentioned.
       When you're manipulating the argument stack using the
       ST(x) macro, for example, you usually have to pay special
       attention to reference counts. (For more about reference
       counts, see perlguts.) To make your life easier, the
       typemap file automatically makes "RETVAL" mortal when
       you're returning an "SV *". Thus, the following two XSUBs
       are more or less equivalent:
                 ST(0) = newSVpv("Hello World",0);

         SV *
                 RETVAL = newSVpv("Hello World",0);

       This is quite useful as it usually improves readability.
       While this works fine for an "SV *", it's unfortunately
       not as easy to have "AV *" or "HV *" as a return value.
       You should be able to write:

         AV *
                 RETVAL = newAV();
                 /* do something with RETVAL */

       But due to an unfixable bug (fixing it would break lots of
       existing CPAN modules) in the typemap file, the reference
       count of the "AV *" is not properly decremented. Thus, the
       above XSUB would leak memory whenever it is being  called.
       The same problem exists for "HV *".

       When you're returning an "AV *" or a "HV *", you have make
       sure their reference count is decremented by making the AV
       or HV mortal:

         AV *
                 RETVAL = newAV();
                 /* do something with RETVAL */

       And also remember that you don't have to do this for an
       "SV *".

       The MODULE Keyword    [Toc]    [Back]

       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 with
       POD stripped, but otherwise 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 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    [Toc]    [Back]

       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 ]

       The same package name can be used more than once, allowing
       for non-contiguous code. This is useful if you have a
       stronger ordering principle than package names.

       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    [Toc]    [Back]

       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

            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 Perl)
       when the XSUB terminates or that certain values should be
       returned to the calling Perl function.  For simple functions
 which have no CODE: or PPCODE: section, such as the
       sin() function above, the RETVAL variable is automatically
       designated as an output value.  For 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.

                 char *host
                 time_t &timep

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

                 char *host
                 time_t &timep
                 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 perlguts for more
       details about 'set' magic.

       The NO_OUTPUT Keyword    [Toc]    [Back]

       The NO_OUTPUT can be placed as the first token of the
       XSUB.  This keyword indicates that while the C subroutine
       we provide an interface to has a non-"void" return type,
       the return value of this C subroutine should not be
       returned from the generated Perl subroutine.

       With this keyword present "The RETVAL Variable" is created,
 and in the generated call to the subroutine this
       variable is assigned to, but the value of this variable is
       not going to be used in the auto-generated code.

       This keyword makes sense only if "RETVAL" is going to be
       accessed by the user-supplied code.  It is especially useful
 to make a function interface more Perl-like, especially
 when the C return value is just an error condition
       indicator.  For example,

         NO_OUTPUT int
         delete_file(char *name)
             if (RETVAL != 0)
                 croak("Error %d while deleting file '%s'",  RETVAL, name);

       Here the generated XS function returns nothing on success,
       and will die() with a meaningful error message on error.

       The CODE: Keyword

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

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

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

       The XSUB follows.
                 char *host
                 time_t timep
                      RETVAL = rpcb_gettime( host, &timep );

       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.

                 char *host
                 time_t &timep
                 printf("# Host is %s0, host );

       Another use for the INIT: section is to check for preconditions
 before making a call to the C function:

           long long
               long long a
               long long b
               if (a == 0 && b == 0)
               if (b == 0)
                   croak("lldiv: cannot divide by 0");

       The NO_INIT Keyword    [Toc]    [Back]

       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.

       The following example shows a variation of the rpcb_get-
       time() function.  This function uses the timep variable
       only as an output variable and does not care about its
       initial contents.
                 char *host
                 time_t &timep = NO_INIT

       Initializing Function Parameters    [Toc]    [Back]

       C function parameters are normally initialized with their
       values from the argument stack (which in turn contains the
       parameters that were passed to the XSUB from Perl).  The
       typemaps contain the code segments which are used to
       translate the Perl values to the C parameters.  The programmer,
 however, is allowed to override the typemaps and
       supply alternate (or additional) initialization code.
       Initialization code starts with the first "=", ";" or "+"
       on a line in the INPUT: section.  The only exception happens
 if this ";" terminates the line, then this ";" is
       quietly ignored.

       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.

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

       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
       in the declaration for the input variable, replacing the
       initialization supplied by the typemap.  If the initialization
 begins with ";" or "+", then it is performed after
       all of the input variables have been declared.  In the ";"
       case the initialization normally supplied by the typemap
       is not performed.  For the "+" case, the declaration for
       the variable will include the initialization from the
       typemap.  A global variable, %v, is available for the
       truly rare case where information from one initialization
       is needed in another initialization.
       Here's a truly obscure example:

                 time_t  &timep  ; /* {timep}=@{[$v{timep}=$arg]}
                 char *host + SvOK($v{timep}) ?  SvPV($arg,PL_na)

       The construct "{timep}=@{[$v{timep}=$arg]}" used in the
       above example has a two-fold purpose: first, when this
       line is processed by xsubpp, the Perl snippet
       "$v{timep}=$arg" is evaluated.  Second, the text of the
       evaluated snippet is output into the generated C file
       (inside a C comment)!  During the processing of "char
       *host" line, $arg will evaluate to ST(0), and $v{timep}
       will evaluate to ST(1).

       Default Parameter Values    [Toc]    [Back]

       Default values for XSUB arguments can be specified by
       placing an assignment statement in the parameter list.
       The default value may be a number, a string or the special
       string "NO_INIT".  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.  This XSUB can
       be called from Perl 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

                 char *host
                 time_t timep = NO_INIT
                      RETVAL = rpcb_gettime( host, &timep );
       The PREINIT: Keyword

       The PREINIT: keyword allows extra variables to be declared
       immediately before or after the declarations of the parameters
 from the INPUT: section are emitted.

       If a variable is declared inside a CODE: section it will
       follow any typemap code that is emitted for the input
       parameters.  This may result in the declaration ending up
       after C code, which is C syntax error.  Similar errors may
       happen with an explicit ";"-type or "+"-type initialization
 of parameters is used (see "Initializing Function
       Parameters").  Declaring these variables in an INIT: section
 will not help.

       In such cases, to force an additional variable to be
       declared together with declarations of other variables,
       place the declaration into a PREINIT: section.  The
       PREINIT: keyword may be used one or more times within an

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

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

       For this particular case an INIT: keyword would generate
       the same C code as the PREINIT: keyword.  Another correct,
       but error-prone example:

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

       Another way to declare "host" is to use a C block in the
       CODE: section:
                 time_t timep = NO_INIT
                   char *host = "localhost";
                   RETVAL = rpcb_gettime( host, &timep );

       The ability to put additional declarations before the
       typemap entries are processed is very handy in the cases
       when typemap conversions manipulate some global state:

                   MyState st = global_state;
                   MyObject o;
                   reset_to(global_state, st);

       Here we suppose that conversion to "MyObject" in the
       INPUT: section and from MyObject when processing RETVAL
       will modify a global variable "global_state".  After these
       conversions are performed, we restore the old value of
       "global_state" (to avoid memory leaks, for example).

       There is another way to trade clarity for compactness:
       INPUT sections allow declaration of C variables which do
       not appear in the parameter list of a subroutine.  Thus
       the above code for mutate() can be rewritten as

                 MyState st = global_state;
                 MyObject o;
                 reset_to(global_state, st);

       and the code for rpcb_gettime() can be rewritten as

                 time_t timep = NO_INIT
                 char *host = "localhost";
                 host, &timep
       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 an XSUB contains a comment like "/*scope*/"
       then  scoping will be automatically 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.

                 char *host
                 time_t tt;
                 time_t timep
                      RETVAL = rpcb_gettime( host, &tt );
                      timep = tt;

       The next example shows each input parameter evaluated
                 time_t tt;
                 char *host
                 char *h;
                 time_t timep
                      h = host;
                      RETVAL = rpcb_gettime( h, &tt );
                      timep = tt;

       Since INPUT sections allow declaration of C variables
       which do not appear in the parameter list of a subroutine,
       this may be shortened to:

                 time_t tt;
                 char *host;
                 char *h = host;
                 time_t timep;
                 RETVAL = rpcb_gettime( h, &tt );
                 timep = tt;

       (We used our knowledge that input conversion for "char *"
       is a "simple" one, thus "host" is initialized on the declaration
 line, and our assignment "h = host" is not performed
 too early.  Otherwise one would need to have the
       assignment "h = host" in a CODE: or INIT: section.)


       In the list of parameters for an XSUB, one can precede
       parameter names by the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT"
 keywords.  "IN" keyword is the
       default, the other keywords indicate how the Perl interface
 should differ from the C interface.

       Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT"
 keywords are considered to be used by
       the C subroutine via pointers.  "OUTLIST"/"OUT" keywords
       indicate that the C subroutine does not inspect the memory
       pointed by this parameter, but will write through this
       pointer to provide additional return values.

       Parameters preceded by "OUTLIST" keyword do not appear in
       the usage signature of the generated Perl function.

       Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do
       appear as parameters to the Perl function.  With the
       exception of "OUT"-parameters, these parameters are converted
 to the corresponding C type, then pointers to these
       data are given as arguments to the C function.  It is
       expected that the C function will write through these

       The return list of the generated Perl function consists of
       the C return value from the function (unless the XSUB is
       of "void" return type or "The NO_OUTPUT Keyword" was used)
       followed by all the "OUTLIST" and "IN_OUTLIST" parameters
       (in the order of appearance).  On the return from the XSUB
       the "IN_OUT"/"OUT" Perl parameter will be modified to have
       the values written by the C function.

       For example, an XSUB

         day_month(OUTLIST day, IN unix_time, OUTLIST month)
           int day
           int unix_time
           int month

       should be used from Perl as

         my ($day, $month) = day_month(time);

       The C signature of the corresponding function should be

         void day_month(int *day, int unix_time, int *month);

       The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords
       can be mixed with ANSI-style declarations, as in

         day_month(OUTLIST int day, int  unix_time,  OUTLIST  int

       (here the optional "IN" keyword is omitted).

       The "IN_OUT" parameters are identical with parameters
       introduced with "The & Unary Operator" and put into the
       "OUTPUT:" section (see "The OUTPUT: Keyword").  The
       "IN_OUTLIST" parameters are very similar, the only difference
 being that the value C function writes through the
       pointer would not modify the Perl parameter, but is put in
       the output list.

       The "OUTLIST"/"OUT" parameter differ from
       "IN_OUTLIST"/"IN_OUT" parameters only by the initial value
       of the Perl parameter not being read (and not being given
       to the C function - which gets some garbage instead).  For
       example, the same C function as above can be interfaced
       with as

         void  day_month(OUT  int  day,  int  unix_time,  OUT int


         day_month(day, unix_time, month)
             int &day = NO_INIT
             int  unix_time
             int &month = NO_INIT

       However, the generated Perl function is called in very
       C-ish style:

         my ($day, $month);
         day_month($day, time, $month);

       The "length(NAME)" Keyword

       If one of the input arguments to the C function is the
       length of a string argument "NAME", one can substitute the
       name of the length-argument by "length(NAME)" in the XSUB
       declaration.  This argument must be omited when the generated
 Perl function is called.  E.g.,

         dump_chars(char *s, short l)
           short n = 0;
           while (n < l) {
               printf("s[%d] =

         MODULE = x            PACKAGE = x

         void dump_chars(char *s, short length(s))

       should be called as "dump_chars($string)".

       This directive is supported with ANSI-type function declarations
       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

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

            $status = rpcb_gettime( $timep );

       The XS code, with ellipsis, follows.

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

       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 PPCODE: 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 a C function is declared as

           symbolic nth_derivative(int n, symbolic function,  int

       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

           nth_derivative(function, n)
               symbolic        function
               int             n
               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 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 should not be used together
       within the same XSUB.

       The actual difference between PPCODE: and CODE: sections
       is in the initialization of "SP" macro (which stands for
       the current Perl stack pointer), and in the handling of
       data on the stack when returning from an XSUB.  In CODE:
       sections SP preserves the value which was on entry to the
       XSUB: SP is on the function pointer (which follows the
       last parameter).  In PPCODE: sections SP is moved backward
       to the beginning of the parameter list, which allows
       "PUSH*()" macros to place output values in the place Perl
       expects them to be when the XSUB returns back to Perl.

       The generated trailer for a CODE: section ensures that the
       number of return values Perl will see is either 0 or 1
       (depending on the "void"ness of the return value of the C
       function, and heuristics mentioned in "The RETVAL Variable").
  The trailer generated for a PPCODE: section is
       based on the number of return values and on the number of
       times "SP" was updated by "[X]PUSH*()" macros.

       Note that macros ST(i), "XST_m*()" and "XSRETURN*()" work
       equally well in CODE: sections and PPCODE: sections.

       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.
                 char *host
                 time_t  timep;
                 bool_t  status;
                 status = rpcb_gettime( host, &timep );
                 EXTEND(SP, 2);

       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

       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 perlguts for
       details about 'set' magic.

       Returning Undef And Empty Lists    [Toc]    [Back]

       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
       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

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

                 char *host
                 time_t  timep;
                 if( rpcb_gettime( host, &timep ) )
                     /* 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 perlapi
       for other "XSRETURN" macros.

       Since "XSRETURN_*" macros can be used with CODE blocks as
       well, one can rewrite this example as:

                 char *host
                 time_t  timep;
                 RETVAL = rpcb_gettime( host, &timep );
                 if (RETVAL == 0)

       In fact, one can put this check into a POSTCALL: section
       as well.  Together with PREINIT: simplifications, this
       leads to:

                 char *host
                 time_t  timep;
                 if (RETVAL == 0)

       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 POSTCALL: Keyword

       This keyword can be used when an XSUB requires special
       procedures executed after the C subroutine call is performed.
  When the POSTCALL: keyword is used it must precede
 OUTPUT: and CLEANUP: blocks which are present in the

       See examples in "The NO_OUTPUT Keyword" and "Returning
       Undef And Empty Lists".

       The POSTCALL: block does not make a lot of sense when the
       C subroutine call is supplied by user by providing either
       CODE: or PPCODE: section.

       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

            # The following message will be printed when the
            # bootstrap function executes.
            printf("Hello from the bootstrap!0);

       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:


       To disable version checking:


       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:


       To disable prototypes:


       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 "Prototypes" in perlsub for information
       about Perl prototypes.

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

       If the prototypes are enabled, you can disable it locally
       for a given XSUB as in the following example:

               PROTOTYPE: DISABLE

       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 fullyqualified
 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.
                 char *host
                 time_t &timep
                   FOO::gettime = 1
                   BAR::getit = 2
                 printf("# ix = %d0, ix );

       The OVERLOAD: Keyword

       Instead of writing an overloaded interface using pure
       Perl, you can also use the OVERLOAD keyword to define
       additional Perl names for your functions (like the ALIAS:
       keyword above).  However, the overloaded functions must be
       defined with three parameters (except for the nomethod()
       function which needs four parameters).  If any function
       has the OVERLOAD: keyword, several additional lines will
       be defined in the c file generated by xsubpp in order to
       register with the overload magic.

       Since blessed objects are actually stored as RV's, it is
       useful to use the typemap features to preprocess parameters
 and extract the actual SV stored within the blessed
       RV. See the sample for T_PTROBJ_SPECIAL below.

       To use the OVERLOAD: keyword, create an XS function which
       takes three input parameters ( or use the c style '...'
       definition) like this:

           SV *
           cmp (lobj, robj, swap)
           My_Module_obj    lobj
           My_Module_obj    robj
           IV               swap
           OVERLOAD: cmp <=>
           { /* function defined here */}

       In this case, the function will overload both of the three
       way comparison operators.  For all overload operations
       using non-alpha characters, you must type the parameter
       without quoting, seperating multiple overloads with
       whitespace.  Note that "" (the stringify overload) should
       be entered as

       The FALLBACK: Keyword

       In addition to the OVERLOAD keyword, if you need to control
 how Perl autogenerates missing overloaded operators,
       you can set the FALLBACK keyword in the module header section,
 like this:
           MODULE = RPC  PACKAGE = RPC

           FALLBACK: TRUE

       where FALLBACK can take any of the three values TRUE,
       FALSE, or UNDEF.  If you do not set any FALLBACK value
       when using OVERLOAD, it defaults to UNDEF.  FALLBACK is
       not used except when one or more functions using OVERLOAD
       have been defined.  Please see "Fallback" in overload for
       more details.

       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 the current

       For example, if you have 4 C functions multiply(),
       divide(), add(), subtract() all having the signature:

           symbolic f(symbolic, symbolic);

       you can make them all to use the same XSUB using this:

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

       (This is the complete XSUB code for 4 Perl functions!)
       Four generated Perl function share names with corresponding
 C functions.

       The advantage of this approach comparing to ALIAS: keyword
       is that there is no need to code a switch statement, each
       Perl function (which shares the same XSUB) knows which C
       function it should call.  Additionally, one can attach an
       extra function remainder() at runtime by using

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

       say, from another XSUB.  (This example supposes that there
       was no INTERFACE_MACRO: section, otherwise one needs to
       use something else instead of "XSINTERFACE_FUNC_SET", see
       the next section.)
       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

       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)
           #define            XSINTERFACE_FUNC_BYOFFSET_set(cv,f)
CvXSUBANY(cv).any_i32 = CAT2( f, _off )

       in C section,

           interface_s_ss(arg1, arg2)
               symbolic        arg1
               symbolic        arg2
               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 t

 Similar pages
Name OS Title
man IRIX print entries from the on-line reference manuals; find manual entries by keyword
groff NetBSD a short reference for the GNU roff language
groff FreeBSD a short reference for the GNU roff language
man HP-UX find manual information by keywords; print out a manual entry
geocustoms HP-UX configure system language on multi-language systems
catman HP-UX create the cat files for the manual
getNAME FreeBSD get name sections from manual pages
dtmanaction HP-UX CDE manual page actions
manctl FreeBSD manipulating manual pages
fixman HP-UX fix manual pages for faster viewing with man(1)
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service