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.

   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>


             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.


        [ XS code in package RPC ]


        [ XS code in package RPCB ]


        [ 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 *)SvPV_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($v{timep}) ? SvPV_nolen($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:


   To disable scoping:


   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 *)SvPV_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 *)SvPV_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:


   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 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

   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).

   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:



   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;


        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 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.


   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"

        #include <rpc/rpc.h>

        typedef struct netconfig Netconfig;


        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";


   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 "uselocale" 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 C 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 it up.  XS code should avoid changing the
       locale, as it can adversely affect other, unrelated, code and may
       not be thread safe.  However, some alien libraries that may be
       called do set it, such as "Gtk".  This can cause problems for the
       perl core and other modules.  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));

       In the event that your XS code may need the underlying "LC_NUMERIC"
       locale, there are macros available to access this; see "Locale-
       related functions and macros" in perlapi.


   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 <>.


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