perlxs


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
       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

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

            bootstrap RPC;
            1;

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

         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

       this is not required.

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

            MODULE = RPC

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

            MODULE = RPC  PACKAGE = RPC

            [ XS code in package RPC ]

            MODULE = RPC  PACKAGE = RPCB

            [ XS code in package RPCB ]

            MODULE = RPC  PACKAGE = RPC

            [ XS code in package RPC ]

       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, 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.
            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.

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

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

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

       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

       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'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).

   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.

       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 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 );
                   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);

       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
       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.

           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;

       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

         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

         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.,

         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 );
                      host = (char *)SvPV_nolen(ST(1));
                 RETVAL = rpcb_gettime( host, &timep );
               OUTPUT:
                 timep
                 RETVAL

   The C_ARGS: Keyword
       The C_ARGS: keyword allows creating of XSUBS which have different
       calling sequence from Perl than from C, without a need to write CODE:
       or 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
                 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 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:
                 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.

            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

               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

       To disable version checking:

           VERSIONCHECK: DISABLE

       Note that if the version of the PM module is an NV (a floating point

       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:

           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:

       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, 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);


       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,

           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
       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 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 );

       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".

           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

           #if ... version1

       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 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();

            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
           # 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.
       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 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 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

            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 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

       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}\")

               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 */

           {
               MY_CXT_INIT;
               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 structure that
            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;
                }

            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)
       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>

            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.

            use RPC;

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

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

XS VERSION
       This document covers features supported by "xsubpp" 1.935.

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

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



perl v5.10.1                      2009-07-27                         PERLXS(1)
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