


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


NAME
       perlxs - XS language reference manual

DESCRIPTION
   Introduction
       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
       rest.

       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



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       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 process.

       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
       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



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       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_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



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       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
       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
            sin(x)
              double x

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

            double
            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

            double
            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
       Operator".

       It is convenient to think that the indirection operator
       "*" should be considered as a part of the type and the



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       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.

         CORRECT

         double
         sin(x)
         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
       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



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       fragments found in the typemaps.  In more complex cases
       the programmer must supply the code.

   The RETVAL Variable
       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)
       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.)

   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:

         void
         alpha()
             PPCODE:
                 ST(0) = newSVpv("Hello World",0);
                 sv_2mortal(ST(0));
                 XSRETURN(1);



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         SV *
         beta()
             CODE:
                 RETVAL = newSVpv("Hello World",0);
             OUTPUT:
                 RETVAL

       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 *
         array()
             CODE:
                 RETVAL = newAV();
                 /* do something with RETVAL */
             OUTPUT:
                 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 to
       make sure their reference count is decremented by making
       the AV or HV mortal:

         AV *
         array()
             CODE:
                 RETVAL = newAV();
                 sv_2mortal((SV*)RETVAL);
                 /* do something with RETVAL */
             OUTPUT:
                 RETVAL

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

   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 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



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       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 ]

       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
       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
       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,



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       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.

            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 perlguts for more
       details about 'set' magic.

   The NO_OUTPUT Keyword
       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.



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       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)
           POSTCALL:
             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 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.







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

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

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

   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.

       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
       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



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       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'ed 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_nolen($arg);
                 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
       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:

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

       The construct "\$v{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).



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   Default Parameter Values
       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
       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
       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
       XSUB.

       The following examples are equivalent, but if the code is



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       using complex typemaps then the first example is safer.

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

       For this particular case an INIT: keyword would generate
       the same C code as the PREINIT: keyword.  Another 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

       Another way to declare "host" is to use a C block in the
       CODE: section:

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

       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:

           MyObject
           mutate(o)
               PREINIT:
                   MyState st = global_state;
               INPUT:
                   MyObject o;
               CLEANUP:
                   reset_to(global_state, st);



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       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

           MyObject
           mutate(o)
                 MyState st = global_state;
                 MyObject o;
               CLEANUP:
                 reset_to(global_state, st);

       and the code for rpcb_gettime() can be rewritten as

            bool_t
            rpcb_gettime(timep)
                 time_t timep = NO_INIT
                 char *host = "localhost";
               C_ARGS:
                 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 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.



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

           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

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

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



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       (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.)

   The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
       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
       pointers.

       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

         void
         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




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         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

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

       (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 month);

       or

         void
         day_month(day, unix_time, month)
             int &day = NO_INIT
             int  unix_time
             int &month = NO_INIT
           OUTPUT:
             day
             month

       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 omitted when the
       generated Perl function is called.  E.g.,







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         void
         dump_chars(char *s, short l)
         {
           short n = 0;
           while (n < l) {
               printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
               n++;
           }
         }

         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 only.

   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 );

            $status = rpcb_gettime( $timep );

       The XS code, with ellipsis, follows.

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




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   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 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
       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



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       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.

            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 perlguts 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
       like to have it return the time and if it fails we would



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       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:
                 if( rpcb_gettime( host, &timep ) ){
                      ST(0) = sv_newmortal();
                      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
       perlapi for other "XSRETURN" macros.

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

            int
            rpcb_gettime(host)
                 char *host
               PREINIT:
                 time_t  timep;
               CODE:
                 RETVAL = rpcb_gettime( host, &timep );
                 if (RETVAL == 0)
                       XSRETURN_UNDEF;
               OUTPUT:
                 RETVAL

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

            int
            rpcb_gettime(host)
                 char *host
                 time_t  timep;
               POSTCALL:
                 if (RETVAL == 0)
                       XSRETURN_UNDEF;

   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:




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               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 XSUB.

       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
       block.

            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




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       To disable version checking:

           VERSIONCHECK: DISABLE

       Note that if the version of the PM module is an NV (a
       floating point number), it will be stringified with a
       possible loss of precision (currently chopping to nine
       decimal places) so that it may not match the version of
       the XS module anymore. Quoting the $VERSION declaration to
       make it a string is recommended if long version numbers
       are used.

   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 "Prototypes" in perlsub for
       information about Perl prototypes.

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

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




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           void
           rpcb_gettime_noproto()
               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 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 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:









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           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, separating multiple overloads with
       whitespace.  Note that "" (the stringify overload) should
       be entered as \"\" (i.e. escaped).

   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
       XSUB.

       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:

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



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       (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",
                                 XS_Symbolic_interface_s_ss, __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 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_ANON(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 ALIAS: Keyword"), or maybe
       via the "items" variable (see "Variable-length Parameter
       Lists").  The last CASE: becomes the default case if it is
       not associated with a 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



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       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 in the INPUT: section is used to
       tell xsubpp that it should convert a Perl value to/from C
       using the C type to the left of "&", but provide a pointer
       to this value when the C function is called.

       This is useful to avoid a CODE: block for a C function
       which takes a parameter by reference.  Typically, the
       parameter should be not a pointer type (an "int" or "long"
       but not an "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 POD, Comments and C Preprocessor Directives
       C preprocessor directives are allowed within BOOT:,
       PREINIT: INIT:, CODE:, PPCODE:, POSTCALL:, 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. POD documentation is
       allowed at any point, both in the C and XS language
       sections. POD must be terminated with a "=cut" command;
       "xsubpp" will exit with an error if it does not. It is
       very unlikely that human generated C code will be mistaken
       for POD, as most indenting styles result in whitespace in
       front of any line starting with "=". Machine generated XS
       files may fall into this trap unless care is taken to
       ensure that a space breaks the sequence "\n=".

       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





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           #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
       part of the function body.

   Using XS With C++
       If an XSUB name contains "::", it is considered to be a
       C++ method.  The generated Perl function will assume that
       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 return type of the XSUB includes "static", the
       method is considered to be a static method.  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 Perl functions will expect an object as the first
       parameter.  In the generated C++ code the object is called
       "THIS", and the method call will be performed on this



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       object.  So in the C++ code the blue() and set_blue()
       methods will be called as this:

            RETVAL = THIS->blue();

            THIS->set_blue( val );

       You could also write a single get/set method using an
       optional argument:

            int
            color::blue( val = NO_INIT )
                int val
                PROTOTYPE $;$
                CODE:
                    if (items > 1)
                        THIS->set_blue( val );
                    RETVAL = THIS->blue();
                OUTPUT:
                    RETVAL

       If the function's name is DESTROY then the C++ "delete"
       function will be called and "THIS" will be given as its
       parameter.  The generated C++ code for

            void
            color::DESTROY()

       will look like this:

            color *THIS = ...; // Initialized as in typemap

            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 generated 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



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           # 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 (such as created by
       "h2xs -x") is often sufficient.  However, sometimes the
       interface will look very C-like and occasionally
       nonintuitive, especially when the C function modifies one
       of its parameters, or returns failure inband (as in
       "negative return values mean failure").  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.

       Identify the C functions with input/output or output
       parameters.  The XSUBs for these functions may be able to
       return lists to Perl.

       Identify the C functions which use some inband info as an
       indication of failure.  They may be candidates to return
       undef or an empty list in case of failure.  If the failure
       may be detected without a call to the C function, you may
       want to use an INIT: section to report the failure.  For
       failures detectable after the C function returns one may
       want to use a POSTCALL: section to process the failure.
       In more complicated cases use CODE: or PPCODE: sections.

       If many functions use the same failure indication based on
       the return value, you may want to create a special typedef
       to handle this situation.  Put

         typedef int negative_is_failure;

       near the beginning of XS file, and create an OUTPUT
       typemap entry for "negative_is_failure" which converts
       negative values to "undef", or maybe croak()s.  After this
       the return value of type "negative_is_failure" will create
       more Perl-like interface.

       Identify which values are used by only the C and XSUB
       functions themselves, say, when a parameter to a function
       should be a contents of a global variable.  If Perl does



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       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 may be used to implement
       input/output or output parameters, they can be handled in
       XS with the "&" unary operator, and, possibly, using the
       NO_INIT keyword.  Some others will require handling of
       types like "int *", and one needs to decide what a useful
       Perl translation will do in such a case.  When the
       semantic is clear, it is advisable to put the translation
       into a typemap file.

       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.  (This is handled automatically by "h2xs
       -x".)

       If the same C type is used in several different contexts
       which require different translations, "typedef" several
       new types mapped to this C type, and create separate
       typemap entries for these new types.  Use these types in
       declarations of return type and parameters to XSUBs.

   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



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       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 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 labelled "TYPEMAP", "INPUT", and
       "OUTPUT".  An unlabelled initial section is assumed to be
       a "TYPEMAP" section.  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



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       translate the values from certain C types into values Perl
       can understand.  The 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 "lib/ExtUtils" 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));
                               $var = INT2PTR($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.

       The INT2PTR macro (defined in perl.h) casts an integer to
       a pointer, of a given type, taking care of the possible
       different size of integers and pointers.  There are also
       PTR2IV, PTR2UV, PTR2NV macros, to map the other way, which
       may be useful in OUTPUT sections.

   Safely Storing Static Data in XS
       Starting with Perl 5.8, a macro framework has been defined
       to allow static data to be safely stored in XS modules
       that will be accessed from a multi-threaded Perl.

       Although primarily designed for use with multi-threaded
       Perl, the macros have been designed so that they will work
       with non-threaded Perl as well.

       It is therefore strongly recommended that these macros be
       used by all XS modules that make use of static data.

       The easiest way to get a template set of macros to use is
       by specifying the "-g" ("--global") option with h2xs (see
       h2xs).

       Below is an example module that makes use of the macros.

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

           /* Global Data */

           #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION

           typedef struct {
               int count;
               char name[3][100];
           } my_cxt_t;

           START_MY_CXT

           MODULE = BlindMice           PACKAGE = BlindMice

           BOOT:
           {
               MY_CXT_INIT;



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               MY_CXT.count = 0;
               strcpy(MY_CXT.name[0], "None");
               strcpy(MY_CXT.name[1], "None");
               strcpy(MY_CXT.name[2], "None");
           }

           int
           newMouse(char * name)
               char * name;
               PREINIT:
                 dMY_CXT;
               CODE:
                 if (MY_CXT.count >= 3) {
                     warn("Already have 3 blind mice");
                     RETVAL = 0;
                 }
                 else {
                     RETVAL = ++ MY_CXT.count;
                     strcpy(MY_CXT.name[MY_CXT.count - 1], name);
                 }

           char *
           get_mouse_name(index)
             int index
             CODE:
               dMY_CXT;
               RETVAL = MY_CXT.lives ++;
               if (index > MY_CXT.count)
                 croak("There are only 3 blind mice.");
               else
                 RETVAL = newSVpv(MY_CXT.name[index - 1]);

           void
           CLONE(...)
               CODE:
               MY_CXT_CLONE;

       REFERENCE

       MY_CXT_KEY
            This macro is used to define a unique key to refer to
            the static data for an XS module. The suggested
            naming scheme, as used by h2xs, is to use a string
            that consists of the module name, the string
            "::_guts" and the module version number.

                #define MY_CXT_KEY "MyModule::_guts" XS_VERSION

       typedef my_cxt_t
            This struct typedef must always be called "my_cxt_t"
            -- the other "CXT*" macros assume the existence of
            the "my_cxt_t" typedef name.

            Declare a typedef named "my_cxt_t" that is a



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            structure that contains all the data that needs to be
            interpreter-local.

                typedef struct {
                    int some_value;
                } my_cxt_t;

       START_MY_CXT
            Always place the START_MY_CXT macro directly after
            the declaration of "my_cxt_t".

       MY_CXT_INIT
            The MY_CXT_INIT macro initialises storage for the
            "my_cxt_t" struct.

            It must be called exactly once -- typically in a
            BOOT: section. If you are maintaining multiple
            interpreters, it should be called once in each
            interpreter instance, except for interpreters cloned
            from existing ones.  (But see "MY_CXT_CLONE" below.)

       dMY_CXT
            Use the dMY_CXT macro (a declaration) in all the
            functions that access MY_CXT.

       MY_CXT
            Use the MY_CXT macro to access members of the
            "my_cxt_t" struct. For example, if "my_cxt_t" is

                typedef struct {
                    int index;
                } my_cxt_t;

            then use this to access the "index" member

                dMY_CXT;
                MY_CXT.index = 2;

       aMY_CXT/pMY_CXT
            "dMY_CXT" may be quite expensive to calculate, and to
            avoid the overhead of invoking it in each function it
            is possible to pass the declaration onto other
            functions using the "aMY_CXT"/"pMY_CXT" macros, eg

                void sub1() {
                    dMY_CXT;
                    MY_CXT.index = 1;
                    sub2(aMY_CXT);
                }

                void sub2(pMY_CXT) {
                    MY_CXT.index = 2;
                }




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            Analogously to "pTHX", there are equivalent forms for
            when the macro is the first or last in multiple
            arguments, where an underscore represents a comma,
            i.e.  "_aMY_CXT", "aMY_CXT_", "_pMY_CXT" and
            "pMY_CXT_".

       MY_CXT_CLONE
            By default, when a new interpreter is created as a
            copy of an existing one (eg via "threads->create()"),
            both interpreters share the same physical my_cxt_t
            structure. Calling "MY_CXT_CLONE" (typically via the
            package's "CLONE()" function), causes a byte-for-byte
            copy of the structure to be taken, and any future
            dMY_CXT will cause the copy to be accessed instead.

       MY_CXT_INIT_INTERP(my_perl)
       dMY_CXT_INTERP(my_perl)
            These are versions of the macros which take an
            explicit interpreter as an argument.

       Note that these macros will only work together within the
       same source file; that is, a dMY_CTX in one source file
       will access a different structure than a dMY_CTX in
       another source file.

   Thread-aware system interfaces
       Starting from Perl 5.8, in C/C++ level Perl knows how to
       wrap system/library interfaces that have thread-aware
       versions (e.g. getpwent_r()) into frontend macros (e.g.
       getpwent()) that correctly handle the multithreaded
       interaction with the Perl interpreter.  This will happen
       transparently, the only thing you need to do is to
       instantiate a Perl interpreter.

       This wrapping happens always when compiling Perl core
       source (PERL_CORE is defined) or the Perl core extensions
       (PERL_EXT is defined).  When compiling XS code outside of
       Perl core the wrapping does not take place.  Note,
       however, that intermixing the _r-forms (as Perl compiled
       for multithreaded operation will do) and the _r-less forms
       is neither well-defined (inconsistent results, data
       corruption, or even crashes become more likely), nor is it
       very portable.

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>




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            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.

            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";




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


            $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
       Originally written by Dean Roehrich <roehrich@cray.com>.

       Maintained since 1996 by The Perl Porters
       <perlbug@perl.org>.












































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