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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 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 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 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 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. leftadjusted.
       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
       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" "trulyvoid"
       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);

         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 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.

       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.

       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.

           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 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.

       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 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);

       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.

       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

       (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

         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.,

         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

   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 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 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.

            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:

               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 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:

           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:

           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

       (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,

           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 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".

           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
           #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 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
           # 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 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 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 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}\")

               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;
               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 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;
                }

            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>

            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";

            $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 v5.12.5                                     2012-11-03                                        PERLXS(1)

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