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

       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.

       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.

       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.

	    #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 "The Typemap" for more info about
       handling qualifiers and unary operators in C types.

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


	 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 *".

       When you're returning an "AV *" or a "HV *", you have to make sure
       their reference count is decremented by making the AV or HV mortal:

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

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

   The MODULE Keyword
       The MODULE keyword is used to start the XS code and to specify the
       package of the functions which are being defined.  All text preceding
       the first MODULE keyword is considered C code and is passed through to
       the output with POD stripped, but otherwise untouched.  Every XS module
       will have a bootstrap function which is used to hook the XSUBs into
       Perl.  The package name of this bootstrap function will match the value
       of the last MODULE statement in the XS source files.  The value of
       MODULE should always remain constant within the same XS file, though
       this is not required.

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


   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_


   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,

	 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

   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:, PPCODE:, or OUTPUT: blocks which are present in
       the XSUB.  The code specified for the cleanup block will be added as
       the last statements in the XSUB.

   The POSTCALL: Keyword
       This keyword can be used when an XSUB requires special procedures
       executed after the C subroutine call is performed.  When the POSTCALL:
       keyword is used it must precede OUTPUT: and CLEANUP: blocks which are
       present in the XSUB.

       See examples in "The NO_OUTPUT Keyword" and "Returning Undef And Empty

       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 enabled by default.  When prototypes are
       enabled XSUBs will be given Perl prototypes.  This keyword may be used
       multiple times in an XS module to enable and disable prototypes for
       different parts of the module.

       To enable prototypes:


       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.

	   INCLUDE: cat Rpcb1.xsh |

   The CASE: Keyword
       The CASE: keyword allows an XSUB to have multiple distinct parts with
       each part acting as a virtual XSUB.  CASE: is greedy and if it is used
       then all other XS keywords must be contained within a CASE:.  This
       means nothing may precede the first CASE: in the XSUB and anything
       following the last CASE: is included in that case.

       A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS:
       variable (see "The ALIAS: Keyword"), or maybe via the "items" variable
       (see "Variable-length Parameter Lists").	 The last CASE: becomes the
       default case if it is not associated with a conditional.	 The following
       example shows CASE switched via "ix" with a function "rpcb_gettime()"
       having an alias "x_gettime()".  When the function is called as
       "rpcb_gettime()" its parameters are the usual "(char *host, time_t
       *timep)", but when the function is called as "x_gettime()" its
       parameters are reversed, "(time_t *timep, char *host)".

	     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 & 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
		    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 the typemap
       section 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.

   The Typemap
       The typemap is a collection of code fragments which are used by the
       xsubpp compiler to map C function parameters and values to Perl values.
       The typemap file may consist of three sections labelled "TYPEMAP",
       "INPUT", and "OUTPUT".  An unlabelled initial section is assumed to be
       a "TYPEMAP" section.  The INPUT section tells the compiler how to
       translate Perl values into variables of certain C types.	 The OUTPUT
       section tells the compiler how to translate the values from certain C
       types into values Perl can understand.  The TYPEMAP section tells the
       compiler which of the INPUT and OUTPUT code fragments should be used to
       map a given C type to a Perl value.  The section labels "TYPEMAP",
       "INPUT", or "OUTPUT" must begin in the first column on a line by
       themselves, and must be in uppercase.

       The default typemap in the "lib/ExtUtils" directory of the Perl source
       contains many useful types which can be used by Perl extensions.	 Some
       extensions define additional typemaps which they keep in their own
       directory.  These additional typemaps may reference INPUT and OUTPUT
       maps in the main typemap.  The xsubpp compiler will allow the
       extension's own typemap to override any mappings which are in the
       default typemap.

       Most extensions which require a custom typemap will need only the
       TYPEMAP section of the typemap file.  The custom typemap used in the
       getnetconfigent() example shown earlier demonstrates what may be the
       typical use of extension typemaps.  That typemap is used to equate a C
       structure with the T_PTROBJ typemap.  The typemap used by
       getnetconfigent() is shown here.	 Note that the C type is separated
       from the XS type with a tab and that the C unary operator "*" is
       considered to be a part of the C type name.

	       Netconfig *<tab>T_PTROBJ

       Here's a more complicated example: suppose that you wanted "struct
       netconfig" to be blessed into the class "Net::Config".  One way to do
       this is to use underscores (_) to separate package names, as follows:

	       typedef struct netconfig * Net_Config;

       And then provide a typemap entry "T_PTROBJ_SPECIAL" that maps
       underscores to double-colons (::), and declare "Net_Config" to be of
       that type:

	       Net_Config      T_PTROBJ_SPECIAL

		       if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
			       IV tmp = SvIV((SV*)SvRV($arg));
			       $var = INT2PTR($type, tmp);
			       croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")

		       sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",

       The INPUT and OUTPUT sections substitute underscores for double-colons
       on the fly, giving the desired effect.  This example demonstrates some
       of the power and versatility of the typemap facility.

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

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

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

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

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

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

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

	   /* Global Data */

	   #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION

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


	   MODULE = BlindMice		PACKAGE = BlindMice

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

	   newMouse(char * name)
	       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
	       RETVAL = MY_CXT.lives ++;
	       if (index > MY_CXT.count)
		 croak("There are only 3 blind mice.");
		 RETVAL = newSVpv([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 initialises 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.

	    #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.

	    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";

       This document covers features supported by "xsubpp" 1.935.

       Originally written by Dean Roehrich <>.

       Maintained since 1996 by The Perl Porters <>.

perl v5.12.2			  2010-09-06			     PERLXS(1)
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