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


       perlxs - XS language reference manual


       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.

       Before writing XS, read the "CAVEATS" section below.

       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

       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.
       For more information on typemaps, see perlxstypemap.

       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 <> for more information.

       For simple bindings to C libraries as well as other machine code
       libraries, consider instead using the much simpler libffi
       <> interface via CPAN modules like
       FFI::Platypus or FFI::Raw.

   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.  A #define for
       "PERL_NO_GET_CONTEXT" should be present to fetch the interpreter
       context more efficiently, see perlguts for details.

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

            MODULE = RPC  PACKAGE = RPC

                 char *host
                 time_t &timep

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

            package RPC;

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

            bootstrap RPC;

       Throughout this document a variety of interfaces to the rpcb_gettime()
       XSUB will be explored.  The XSUBs will take their parameters in
       different orders or will take different numbers of parameters.  In each
       case the XSUB is an abstraction between Perl and the real C
       rpcb_gettime() function, and the XSUB must always ensure that the real
       rpcb_gettime() function is called with the correct parameters.  This
       abstraction will allow the programmer to create a more Perl-like
       interface to the C function.

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

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

              double x

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

            sin(double x)

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

            sin(double x);

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

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

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

            char *  s
            char    &c

       Both these XS declarations correspond to the "char*" C type, but they
       have different semantics, see "The & Unary 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 perlxstypemap for more info about
       handling qualifiers and unary operators in C types.

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

         INCORRECT                        CORRECT

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

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


         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" and "old-practice-
       declared-as-void" functions. Hence your code is at mercy of this
       heuristics unless you use "SV *" as return value.)

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

                 ST(0) = newSVpv("Hello World",0);

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

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

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

       But due to an unfixable bug (fixing it would break lots of existing
       CPAN modules) in the typemap file, the reference count of the "AV *" is
       not properly decremented. Thus, the above XSUB would leak memory
       whenever it is being called. The same problem exists for "HV *", "CV
       *", and "SVREF" (which indicates a scalar reference, not a general "SV
       *").  In XS code on perls starting with perl 5.16, you can override the
       typemaps for any of these types with a version that has proper handling
       of refcounts. In your "TYPEMAP" section, do


       to get the repaired variant. For backward compatibility with older
       versions of perl, you can instead decrement the reference count
       manually when you're returning one of the aforementioned types using

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

       Remember that you don't have to do this for an "SV *". The reference
       documentation for all core typemaps can be found in perlxstypemap.

   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

       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

       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

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

                 char *host
                 time_t &timep

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

                 char *host
                 time_t &timep
                 timep sv_setnv(ST(1), (double)timep);

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

   The NO_OUTPUT Keyword
       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

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

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

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

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

       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.

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

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

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

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

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

   The NO_INIT Keyword
       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.

                 char *host
                 time_t &timep = NO_INIT

   The TYPEMAP: Keyword
       Starting with Perl 5.16, you can embed typemaps into your XS code
       instead of or in addition to typemaps in a separate file.  Multiple
       such embedded typemaps will be processed in order of appearance in the
       XS code and like local typemap files take precedence over the default
       typemap, the embedded typemaps may overwrite previous definitions of
       TYPEMAP, INPUT, and OUTPUT stanzas.  The syntax for embedded typemaps

             TYPEMAP: <<HERE
             ... your typemap code here ...

       where the "TYPEMAP" keyword must appear in the first column of a new

       Refer to perlxstypemap for details on writing typemaps.

   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.

                 char *host = (char *)SvPVbyte_nolen($arg);
                 time_t &timep = 0;

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

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

       Here's a truly obscure example:

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

       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

            $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.

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

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

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

       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.

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

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

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

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

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

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

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

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

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

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

       and the code for rpcb_gettime() can be rewritten as

                 time_t timep = NO_INIT
                 char *host = "localhost";
                 host, &timep

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

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

       To enable scoping:

           SCOPE: ENABLE

       To disable scoping:

           SCOPE: DISABLE

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

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

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

       The next example shows each input parameter evaluated late.

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

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

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

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

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

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

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

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

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

       For example, an XSUB

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

       should be used from Perl as

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

       The C signature of the corresponding function should be

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

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

         day_month(OUTLIST int day, int unix_time, OUTLIST int 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

       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);


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

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

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

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

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

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

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

       For example, suppose that a C function is declared as

           symbolic nth_derivative(int n, symbolic function, int 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

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

   The PPCODE: Keyword
       The PPCODE: keyword is an alternate form of the CODE: keyword and is
       used to tell the xsubpp compiler that the programmer is supplying the
       code to control the argument stack for the XSUBs return values.
       Occasionally one will want an XSUB to return a list of values rather
       than a single value.  In these cases one must use PPCODE: and then
       explicitly push the list of values on the stack.  The PPCODE: and CODE:
       keywords should not be used together within the same XSUB.

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

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

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

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

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

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

   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

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

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

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

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

                 char *host
                 time_t  timep;
                 if( rpcb_gettime( host, &timep ) )
                     /* Nothing pushed on stack, so an empty
                      * list is implicitly returned. */

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

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

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

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

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

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

               REQUIRE: 1.922

   The CLEANUP: Keyword
       This keyword can be used when an XSUB requires special cleanup
       procedures before it terminates.  When the CLEANUP: keyword is used it
       must follow any CODE:, 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

       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.

            # 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:


       To disable version checking:


       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 disabled 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.  Note that xsubpp will nag you if you
       don't explicitly enable or disable prototypes, with:

           Please specify prototyping behavior for Foo.xs (see perlxs manual)

       To enable prototypes:


       To disable prototypes:


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

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

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

               PROTOTYPE: DISABLE

   The ALIAS: Keyword
       The ALIAS: keyword allows an XSUB to have two or more unique Perl names
       and to know which of those names was used when it was invoked.  The
       Perl names may be 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.

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

   The OVERLOAD: Keyword
       Instead of writing an overloaded interface using pure Perl, you can
       also use the OVERLOAD keyword to define additional Perl names for your
       functions (like the ALIAS: keyword above).  However, the overloaded
       functions must be defined in such a way as to accept the number of
       parameters supplied by perl's overload system.  For most overload
       methods, it will be three parameters; for the "nomethod" function it
       will be four.  However, the bitwise operators "&", "|", "^", and "~"
       may be called with three or five arguments (see overload).

       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

       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).

       Since, as mentioned above, bitwise operators may take extra arguments,
       you may want to use something like "(lobj, robj, swap, ...)" (with
       literal "...") as your parameter list.

   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

   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:

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

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

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

           CV *mycv = newXSproto("Symbolic::remainder",
                                 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.)

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

       in C section,

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

       in XSUB section.

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

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

                 char *host
                 time_t &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. This
       feature is mildly deprecated in favour of the "INCLUDE_COMMAND:"
       directive, as documented below.

           INCLUDE: cat Rpcb1.xsh |

       Do not use this to run perl: "INCLUDE: perl |" will run the perl that
       happens to be the first in your path and not necessarily the same perl
       that is used to run "xsubpp". See "The INCLUDE_COMMAND: Keyword".

       Runs the supplied command and includes its output into the current XS
       document. "INCLUDE_COMMAND" assigns special meaning to the $^X token in
       that it runs the same perl interpreter that is running "xsubpp":

           INCLUDE_COMMAND: cat Rpcb1.xsh

           INCLUDE_COMMAND: $^X -e ...

   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)".

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

       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 EXPORT_XSUB_SYMBOLS: keyword is likely something you will never
       need.  In perl versions earlier than 5.16.0, this keyword does nothing.
       Starting with 5.16, XSUB symbols are no longer exported by default.
       That is, they are "static" functions. If you include


       in your XS code, the XSUBs following this line will not be declared
       "static".  You can later disable this with


       which, again, is the default that you should probably never change.
       You cannot use this keyword on versions of perl before 5.16 to make
       XSUBs "static".

   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

       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".

                 char *host
                 time_t 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)".

                 char *host
                 time_t &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  */

       and not

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

       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 {
                 int blue();
                 void set_blue( int );

                 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.


            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

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

       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


       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 *

       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.

           color *  O_OBJECT

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

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

   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

       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

            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 *
                 char *netid

            MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

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

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

            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.

   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.

           #define PERL_NO_GET_CONTEXT
           #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;


           MODULE = BlindMice           PACKAGE = BlindMice

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

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

           char *
                 int index
                 if (index > MY_CXT.count)
                   croak("There are only 3 blind mice.");
                   RETVAL =[index - 1];



            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;

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

            The MY_CXT_INIT macro initializes storage for the "my_cxt_t"

            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.)

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

                MY_CXT.index = 2;

            "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() {
                    MY_CXT.index = 1;

                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_".

            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.

            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 the Perl core, the wrapping does not take
       place before Perl 5.28.  Starting in that release you can

        #define PERL_REENTRANT

       in your code to enable the wrapping.  It is advisable to do so if you
       are using such functions, as 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.  Unfortunately,
       not all systems have all the "_r" forms, but using this "#define" gives
       you whatever protection that Perl is aware is available on each system.


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

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

            /* Note: On glibc 2.13 and earlier, this needs be <rpc/rpc.h> */
            #include <tirpc/rpc.h>

            typedef struct netconfig Netconfig;

            MODULE = RPC  PACKAGE = RPC

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

            Netconfig *
                 char *netid

            MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

                 Netconfig *netconf
                 free( netconf );

       File "typemap": Custom typemap for RPC.xs. (cf. perlxstypemap)

            Netconfig *  T_PTROBJ

       File "": Perl module for the RPC extension.

            package RPC;

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

            bootstrap RPC;

       File "": 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";

       In Makefile.PL add -ltirpc and -I/usr/include/tirpc.


       XS code has full access to system calls including C library functions.
       It thus has the capability of interfering with things that the Perl
       core or other modules have set up, such as signal handlers or file
       handles.  It could mess with the memory, or any number of harmful
       things.  Don't.

       Some modules have an event loop, waiting for user-input.  It is highly
       unlikely that two such modules would work adequately together in a
       single Perl application.

       In general, the perl interpreter views itself as the center of the
       universe as far as the Perl program goes.  XS code is viewed as a help-
       mate, to accomplish things that perl doesn't do, or doesn't do fast
       enough, but always subservient to perl.  The closer XS code adheres to
       this model, the less likely conflicts will occur.

       One area where there has been conflict is in regards to C locales.
       (See perllocale.)  perl, with one exception and unless told otherwise,
       sets up the underlying locale the program is running in to the locale
       passed into it from the environment.  This is an important difference
       from a generic C language program, where the underlying locale is the
       "C" locale unless the program changes it.  As of v5.20, this underlying
       locale is completely hidden from pure Perl code outside the lexical
       scope of "use locale" except for a couple of function calls in the
       POSIX module which of necessity use it.  But the underlying locale,
       with that one exception is exposed to XS code, affecting all C library
       routines whose behavior is locale-dependent.  Your XS code better not
       assume that the underlying locale is "C".  The exception is the
       "LC_NUMERIC" locale category, and the reason it is an exception is that
       experience has shown that it can be problematic for XS code, whereas we
       have not had reports of problems with the other locale categories.  And
       the reason for this one category being problematic is that the
       character used as a decimal point can vary.  Many European languages
       use a comma, whereas English, and hence Perl are expecting a dot
       (U+002E: FULL STOP).  Many modules can handle only the radix character
       being a dot, and so perl attempts to make it so.  Up through Perl
       v5.20, the attempt was merely to set "LC_NUMERIC" upon startup to the
       "C" locale.  Any setlocale() otherwise would change it; this caused
       some failures.  Therefore, starting in v5.22, perl tries to keep
       "LC_NUMERIC" always set to "C" for XS code.

       To summarize, here's what to expect and how to handle locales in XS

       Non-locale-aware XS code
           Keep in mind that even if you think your code is not locale-aware,
           it may call a library function that is.  Hopefully the man page for
           such a function will indicate that dependency, but the
           documentation is imperfect.

           The current locale is exposed to XS code except possibly
           "LC_NUMERIC" (explained in the next paragraph).  There have not
           been reports of problems with the other categories.  Perl
           initializes things on start-up so that the current locale is the
           one which is indicated by the user's environment in effect at that
           time.  See "ENVIRONMENT" in perllocale.

           However, up through v5.20, Perl initialized things on start-up so
           that "LC_NUMERIC" was set to the "C" locale.  But if any code
           anywhere changed it, it would stay changed.  This means that your
           module can't count on "LC_NUMERIC" being something in particular,
           and you can't expect floating point numbers (including version
           strings) to have dots in them.  If you don't allow for a non-dot,
           your code could break if anyone anywhere changed the locale.  For
           this reason, v5.22 changed the behavior so that Perl tries to keep
           "LC_NUMERIC" in the "C" locale except around the operations
           internally where it should be something else.  Misbehaving XS code
           will always be able to change the locale anyway, but the most
           common instance of this is checked for and handled.

       Locale-aware XS code
           If the locale from the user's environment is desired, there should
           be no need for XS code to set the locale except for "LC_NUMERIC",
           as perl has already set the others up.  XS code should avoid
           changing the locale, as it can adversely affect other, unrelated,
           code and may not be thread-safe.  To minimize problems, the macros
           "STORE_LC_NUMERIC_SET_TO_NEEDED" in perlapi,
           "STORE_LC_NUMERIC_FORCE_TO_UNDERLYING" in perlapi, and
           "RESTORE_LC_NUMERIC" in perlapi should be used to affect any needed

           But, starting with Perl v5.28, locales are thread-safe on platforms
           that support this functionality.  Windows has this starting with
           Visual Studio 2005.  Many other modern platforms support the
           thread-safe POSIX 2008 functions.  The C "#define"
           "USE_THREAD_SAFE_LOCALE" will be defined iff this build is using
           these.  From Perl-space, the read-only variable "${SAFE_LOCALES}"
           is 1 if either the build is not threaded, or if
           "USE_THREAD_SAFE_LOCALE" is defined; otherwise it is 0.

           The way this works under-the-hood is that every thread has a choice
           of using a locale specific to it (this is the Windows and POSIX
           2008 functionality), or the global locale that is accessible to all
           threads (this is the functionality that has always been there).
           The implementations for Windows and POSIX are completely different.
           On Windows, the runtime can be set up so that the standard
           setlocale(3) function either only knows about the global locale or
           the locale for this thread.  On POSIX, "setlocale" always deals
           with the global locale, and other functions have been created to
           handle per-thread locales.  Perl makes this transparent to perl-
           space code.  It continues to use "POSIX::setlocale()", and the
           interpreter translates that into the per-thread functions.

           All other locale-sensitive functions automatically use the per-
           thread locale, if that is turned on, and failing that, the global
           locale.  Thus calls to "setlocale" are ineffective on POSIX systems
           for the current thread if that thread is using a per-thread locale.
           If perl is compiled for single-thread operation, it does not use
           the per-thread functions, so "setlocale" does work as expected.

           If you have loaded the "POSIX" module you can use the methods given
           in perlcall to call "POSIX::setlocale" to safely change or query
           the locale (on systems where it is safe to do so), or you can use
           the new 5.28 function "Perl_setlocale" in perlapi instead, which is
           a drop-in replacement for the system setlocale(3), and handles
           single-threaded and multi-threaded applications transparently.

           There are some locale-related library calls that still aren't
           thread-safe because they return data in a buffer global to all
           threads.  In the past, these didn't matter as locales weren't
           thread-safe at all.  But now you have to be aware of them in case
           your module is called in a multi-threaded application.  The known
           ones are

            gcvt() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
            wcrtomb() if its final argument is NULL
            wcsrtombs() if its final argument is NULL

           Some of these shouldn't really be called in a Perl application, and
           for others there are thread-safe versions of these already


           The "_r" forms are automatically used, starting in Perl 5.28, if
           you compile your code, with

            #define PERL_REENTRANT

           See also "Perl_langinfo" in perlapi.  You can use the methods given
           in perlcall, to get the best available locale-safe versions of


           And note, that some items returned by "Localeconv" are available
           through "Perl_langinfo" in perlapi.

           The others shouldn't be used in a threaded application.

           Some modules may call a non-perl library that is locale-aware.
           This is fine as long as it doesn't try to query or change the
           locale using the system "setlocale".  But if these do call the
           system "setlocale", those calls may be ineffective.  Instead,
           "Perl_setlocale" works in all circumstances.  Plain setlocale is
           ineffective on multi-threaded POSIX 2008 systems.  It operates only
           on the global locale, whereas each thread has its own locale,
           paying no attention to the global one.  Since converting these non-
           Perl libraries to "Perl_setlocale" is out of the question, there is
           a new function in v5.28 "switch_to_global_locale" that will switch
           the thread it is called from so that any system "setlocale" calls
           will have their desired effect.  The function "sync_locale" must be
           called before returning to perl.

           This thread can change the locale all it wants and it won't affect
           any other thread, except any that also have been switched to the
           global locale.  This means that a multi-threaded application can
           have a single thread using an alien library without a problem; but
           no more than a single thread can be so-occupied.  Bad results
           likely will happen.

           In perls without multi-thread locale support, some alien libraries,
           such as "Gtk" change locales.  This can cause problems for the Perl
           core and other modules.  For these, before control is returned to
           perl, starting in v5.20.1, calling the function sync_locale() from
           XS should be sufficient to avoid most of these problems.  Prior to
           this, you need a pure Perl statement that does this:

            POSIX::setlocale(LC_ALL, POSIX::setlocale(LC_ALL));

           or use the methods given in perlcall.


       This document covers features supported by "ExtUtils::ParseXS" (also
       known as "xsubpp") 3.13_01.


       Originally written by Dean Roehrich <>.

       Maintained since 1996 by The Perl Porters <>.

perl v5.34.0                      2021-05-04                       PERLXS(1pm)

perl 5.34.0 - Generated Sun Feb 27 16:08:53 CST 2022
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