perlxs



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




NAME

       perlxs - XS language reference manual


DESCRIPTION

       Introduction

       XS is an interface description file format used to create an extension
       interface between Perl and C code (or a C library) which one wishes to
       use with Perl.  The XS interface is combined with the library to create
       a new library which can then be either dynamically loaded or statically
       linked into perl.  The XS interface description is written in the XS
       language and is the core component of the Perl extension interface.

       An XSUB forms the basic unit of the XS interface.  After compilation by
       the xsubpp compiler, each XSUB amounts to a C function definition which
       will provide the glue between Perl calling conventions and C calling
       conventions.

       The glue code pulls the arguments from the Perl stack, converts these
       Perl values to the formats expected by a C function, call this C func-
       tion, transfers the return values of the C function back to Perl.
       Return values here may be a conventional C return value or any C func-
       tion arguments that may serve as output parameters.  These return val-
       ues 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-ori-
       ented 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 mul-
       tiple 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 defini-
       tions may follow this line.  The "language" used in this part of the
       file is usually referred to as the XS language.  xsubpp recognizes and
       skips POD (see perlpod) in both the C and XS language sections, which
       allows the XS file to contain embedded documentation.

       See perlxstut for a tutorial on the whole extension creation process.

       Note: For some extensions, Dave Beazley’s SWIG system may provide a
       significantly more convenient mechanism for creating the extension glue
       code.  See http://www.swig.org/ for more information.

       On The Road

       Many of the examples which follow will concentrate on creating an
       interface between Perl and the ONC+ RPC bind library functions.  The
       rpcb_gettime() function is used to demonstrate many features of the XS
       language.  This function has two parameters; the first is an input
       parameter and the second is an output parameter.  The function also
       returns a status value.

               bool_t rpcb_gettime(const char *host, time_t *timep);

       From C this function will be called with the following statements.

            #include <rpc/rpc.h>
            bool_t status;
            time_t timep;
            status = rpcb_gettime( "localhost", &timep );

       If an XSUB is created to offer a direct translation between this func-
       tion 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 demon-
       strates one possible interface to the rpcb_gettime() function.  This
       XSUB represents a direct translation between C and Perl and so pre-
       serves the interface even from Perl.  This XSUB will be invoked from
       Perl with the usage shown above.  Note that the first three #include
       statements, for "EXTERN.h", "perl.h", and "XSUB.h", will always be
       present at the beginning of an XS file.  This approach and others will
       be expanded later in this document.

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

            MODULE = RPC  PACKAGE = RPC

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
               OUTPUT:
                 timep

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

            package RPC;

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

            bootstrap RPC;
            1;

       Throughout this document a variety of interfaces to the rpcb_gettime()
       XSUB will be explored.  The XSUBs will take their parameters in differ-
       ent 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 sin-
       gle argument and returns a single value.

            double
            sin(x)
              double x

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

            double
            sin(double x)

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

            double
            sin(double x);

       Parameters with C pointer types can have different semantic: C func-
       tions 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 func-
       tions 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.

         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 read-
       ability.

         CORRECT

         double
         sin(x)
         double x

       More complicated XSUBs may contain many other sections.  Each section
       of an XSUB starts with the corresponding keyword, such as INIT: or
       CLEANUP:.  However, the first two lines of an XSUB always contain the
       same data: descriptions of the return type and the names of the func-
       tion and its parameters.  Whatever immediately follows these is consid-
       ered to be an INPUT: section unless explicitly marked with another key-
       word.  (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 posi-
       tions 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 automati-
       cally 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: sec-
       tion 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: direc-
       tive 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-prac-
       tice-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 auto-
       matically makes "RETVAL" mortal when you’re returning an "SV *". Thus,
       the following two XSUBs are more or less equivalent:

         void
         alpha()
             PPCODE:
                 ST(0) = newSVpv("Hello World",0);
                 sv_2mortal(ST(0));
                 XSRETURN(1);

         SV *
         beta()
             CODE:
                 RETVAL = newSVpv("Hello World",0);
             OUTPUT:
                 RETVAL

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

         AV *
         array()
             CODE:
                 RETVAL = newAV();
                 /* do something with RETVAL */
             OUTPUT:
                 RETVAL

       But due to an unfixable bug (fixing it would break lots of existing
       CPAN modules) in the typemap file, the reference count of the "AV *" is
       not properly decremented. Thus, the above XSUB would leak memory when-
       ever it is being called. The same problem exists for "HV *".

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

         AV *
         array()
             CODE:
                 RETVAL = newAV();
                 sv_2mortal((SV*)RETVAL);
                 /* do something with RETVAL */
             OUTPUT:
                 RETVAL

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

       The MODULE Keyword

       The MODULE keyword is used to start the XS code and to specify the
       package of the functions which are being defined.  All text preceding
       the first MODULE keyword is considered C code and is passed through to
       the output with POD stripped, but otherwise untouched.  Every XS module
       will have a bootstrap function which is used to hook the XSUBs into
       Perl.  The package name of this bootstrap function will match the value
       of the last MODULE statement in the XS source files.  The value of MOD-
       ULE 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 func-
       tions 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 MOD-
       ULE keyword and must follow immediately after it when used.

            MODULE = RPC  PACKAGE = RPC

            [ XS code in package RPC ]

            MODULE = RPC  PACKAGE = RPCB

            [ XS code in package RPCB ]

            MODULE = RPC  PACKAGE = RPC

            [ XS code in package RPC ]

       The same package name can be used more than once, allowing for non-con-
       tiguous 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 "get-
       time()".

       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 desig-
       nated as an output value.  For more complex functions the xsubpp com-
       piler will need help to determine which variables are output variables.

       This keyword will normally be used to complement the CODE:  keyword.
       The RETVAL variable is not recognized as an output variable when the
       CODE: keyword is present.  The OUTPUT:  keyword is used in this situa-
       tion to tell the compiler that RETVAL really is an output variable.

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

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
               OUTPUT:
                 timep

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

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
               OUTPUT:
                 timep sv_setnv(ST(1), (double)timep);

       xsubpp emits an automatic "SvSETMAGIC()" for all parameters in the OUT-
       PUT 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 sec-
       tion.  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 key-
       word indicates that while the C subroutine we provide an interface to
       has a non-"void" return type, the return value of this C subroutine
       should not be returned from the generated Perl subroutine.

       With this keyword present "The RETVAL Variable" is created, and in the
       generated call to the subroutine this variable is assigned to, but the
       value of this variable is not going to be used in the auto-generated
       code.

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

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

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

       The CODE: Keyword

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

       The following XSUB is for a C function which requires special handling
       of its parameters.  The Perl usage is given first.

            $status = rpcb_gettime( "localhost", $timep );

       The XSUB follows.

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

       The INIT: Keyword

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

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

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

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

       The NO_INIT Keyword

       The NO_INIT keyword is used to indicate that a function parameter is
       being used only as an output value.  The xsubpp compiler will normally
       generate code to read the values of all function parameters from the
       argument stack and assign them to C variables upon entry to the func-
       tion.  NO_INIT will tell the compiler that some parameters will be used
       for output rather than for input and that they will be handled before
       the function terminates.

       The following example shows a variation of the rpcb_gettime() function.
       This function uses the timep variable only as an output variable and
       does not care about its initial contents.

            bool_t
            rpcb_gettime(host,timep)
                 char *host
                 time_t &timep = NO_INIT
               OUTPUT:
                 timep

       Initializing Function Parameters

       C function parameters are normally initialized with their values from
       the argument stack (which in turn contains the parameters that were
       passed to the XSUB from Perl).  The typemaps contain the code segments
       which are used to translate the Perl values to the C parameters.  The
       programmer, however, is allowed to override the typemaps and supply
       alternate (or additional) initialization code.  Initialization code
       starts with the first "=", ";" or "+" on a line in the INPUT: section.
       The only exception happens if this ";" terminates the line, then this
       ";" is quietly ignored.

       The following code demonstrates how to supply initialization code for
       function parameters.  The initialization code is eval’d within double
       quotes by the compiler before it is added to the output so anything
       which should be interpreted literally [mainly "$", "@", or "\\"] must
       be protected with backslashes.  The variables $var, $arg, and $type can
       be used as in typemaps.

            bool_t
            rpcb_gettime(host,timep)
                 char *host = (char *)SvPV($arg,PL_na);
                 time_t &timep = 0;
               OUTPUT:
                 timep

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

       If the initialization begins with "=", then it is output in the decla-
       ration 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 initial-
       ization is needed in another initialization.

       Here’s a truly obscure example:

            bool_t
            rpcb_gettime(host,timep)
                 time_t &timep ; /* \$v{timep}=@{[$v{timep}=$arg]} */
                 char *host + SvOK($v{timep}) ? SvPV($arg,PL_na) : NULL;
               OUTPUT:
                 timep

       The construct "\$v{timep}=@{[$v{timep}=$arg]}" used in the above exam-
       ple has a two-fold purpose: first, when this line is processed by
       xsubpp, the Perl snippet "$v{timep}=$arg" is evaluated.  Second, the
       text of the evaluated snippet is output into the generated C file
       (inside a C comment)!  During the processing of "char *host" line, $arg
       will evaluate to ST(0), and $v{timep} will evaluate to ST(1).

       Default Parameter Values

       Default values for XSUB arguments can be specified by placing an
       assignment statement in the parameter list.  The default value may be a
       number, a string or the special string "NO_INIT".  Defaults should
       always be used on the right-most parameters only.

       To allow the XSUB for rpcb_gettime() to have a default host value the
       parameters to the XSUB could be rearranged.  The XSUB will then call
       the real rpcb_gettime() function with the parameters in the correct
       order.  This XSUB can be called from Perl with either of the following
       statements:

            $status = rpcb_gettime( $timep, $host );

            $status = rpcb_gettime( $timep );

       The XSUB will look like the code  which  follows.   A  CODE: block  is
       used to call the real rpcb_gettime() function with the parameters in
       the correct order for that function.

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

       The PREINIT: Keyword

       The PREINIT: keyword allows extra variables to be declared immediately
       before or after the declarations of the parameters from the INPUT: sec-
       tion 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 ini-
       tialization of parameters is used (see "Initializing Function Parame-
       ters").  Declaring these variables in an INIT: section will not help.

       In such cases, to force an additional variable to be declared together
       with declarations of other variables, place the declaration into a
       PREINIT: section.  The PREINIT: keyword may be used one or more times
       within an XSUB.

       The following examples are equivalent, but if the code is using complex
       typemaps then the first example is safer.

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

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

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

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

            bool_t
            rpcb_gettime(timep)
                 time_t timep = NO_INIT
               CODE:
                 {
                   char *host = "localhost";
                   RETVAL = rpcb_gettime( host, &timep );
                 }
               OUTPUT:
                 timep
                 RETVAL

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

           MyObject
           mutate(o)
               PREINIT:
                   MyState st = global_state;
               INPUT:
                   MyObject o;
               CLEANUP:
                   reset_to(global_state, st);

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

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

           MyObject
           mutate(o)
                 MyState st = global_state;
                 MyObject o;
               CLEANUP:
                 reset_to(global_state, st);

       and the code for rpcb_gettime() can be rewritten as

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

       The SCOPE: Keyword

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

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

       To enable scoping:

           SCOPE: ENABLE

       To disable scoping:

           SCOPE: DISABLE

       The INPUT: Keyword

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

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

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

       The next example shows each input parameter evaluated late.

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

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

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

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

       The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords

       In the list of parameters for an XSUB, one can precede parameter names
       by the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords.  "IN" key-
       word 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.  "OUT-
       LIST"/"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 parame-
       ters to the Perl function.  With the exception of "OUT"-parameters,
       these parameters are converted to the corresponding C type, then point-
       ers to these data are given as arguments to the C function.  It is
       expected that the C function will write through these pointers.

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

       For example, an XSUB

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

       should be used from Perl as

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

       The C signature of the corresponding function should be

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

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

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

       (here the optional "IN" keyword is omitted).

       The "IN_OUT" parameters are identical with parameters introduced with
       "The & Unary Operator" and put into the "OUTPUT:" section (see "The
       OUTPUT: Keyword").  The "IN_OUTLIST" parameters are very similar, the
       only difference being that the value C function writes through the
       pointer would not modify the Perl parameter, but is put in the output
       list.

       The "OUTLIST"/"OUT" parameter differ from "IN_OUTLIST"/"IN_OUT" parame-
       ters only by the initial value of the Perl parameter not being read
       (and not being given to the C function - which gets some garbage
       instead).  For example, the same C function as above can be interfaced
       with as

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

       or

         void
         day_month(day, unix_time, month)
             int &day = NO_INIT
             int  unix_time
             int &month = NO_INIT
           OUTPUT:
             day
             month

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

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

       The "length(NAME)" Keyword

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

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

         MODULE = x            PACKAGE = x

         void dump_chars(char *s, short length(s))

       should be called as "dump_chars($string)".

       This directive is supported with ANSI-type function declarations only.

       Variable-length Parameter Lists

       XSUBs can have variable-length parameter lists by specifying an ellip-
       sis "(...)" in the parameter list.  This use of the ellipsis is similar
       to that found in ANSI C.  The programmer is able to determine the num-
       ber of arguments passed to the XSUB by examining the "items" variable
       which the xsubpp compiler supplies for all XSUBs.  By using this mecha-
       nism one can create an XSUB which accepts a list of parameters of
       unknown length.

       The host parameter for the rpcb_gettime() XSUB can be optional so the
       ellipsis can be used to indicate that the XSUB will take a variable
       number of parameters.  Perl should be able to call this XSUB with
       either of the following statements.

            $status = rpcb_gettime( $timep, $host );

            $status = rpcb_gettime( $timep );

       The XS code, with ellipsis, follows.

            bool_t
            rpcb_gettime(timep, ...)
                 time_t timep = NO_INIT
               PREINIT:
                 char *host = "localhost";
                 STRLEN n_a;
               CODE:
                 if( items > 1 )
                      host = (char *)SvPV(ST(1), n_a);
                 RETVAL = rpcb_gettime( host, &timep );
               OUTPUT:
                 timep
                 RETVAL

       The C_ARGS: Keyword

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

       For example, suppose that a C function is declared as

           symbolic nth_derivative(int n, symbolic function, int flags);

       and that the default flags are kept in a global C variable
       "default_flags".  Suppose that you want to create an interface which is
       called as

           $second_deriv = $function->nth_derivative(2);

       To do this, declare the XSUB as

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

       The PPCODE: Keyword

       The PPCODE: keyword is an alternate form of the CODE: keyword and is
       used to tell the xsubpp compiler that the programmer is supplying the
       code to control the argument stack for the XSUBs return values.  Occa-
       sionally 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 ini-
       tialization 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 men-
       tioned in "The RETVAL Variable").  The trailer generated for a PPCODE:
       section is based on the number of return values and on the number of
       times "SP" was updated by "[X]PUSH*()" macros.

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

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

            void
            rpcb_gettime(host)
                 char *host
               PREINIT:
                 time_t  timep;
                 bool_t  status;
               PPCODE:
                 status = rpcb_gettime( host, &timep );
                 EXTEND(SP, 2);
                 PUSHs(sv_2mortal(newSViv(status)));
                 PUSHs(sv_2mortal(newSViv(timep)));

       Notice that the programmer must supply the C code necessary to have the
       real rpcb_gettime() function called and to have the return values prop-
       erly 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 follow-
       ing statement.

            ($status, $timep) = rpcb_gettime("localhost");

       When handling output parameters with a PPCODE section, be sure to han-
       dle ’set’ magic properly.  See perlguts for details about ’set’ magic.

       Returning Undef And Empty Lists

       Occasionally the programmer will want to return simply "undef" or an
       empty list if a function fails rather than a separate status value.
       The rpcb_gettime() function offers just this situation.  If the func-
       tion 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 initial-
       ize the return value to undef, making that the default return value.

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

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

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

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

            void
            rpcb_gettime(host)
                 char *host
               PREINIT:
                 time_t  timep;
               PPCODE:
                 if( rpcb_gettime( host, &timep ) )
                      PUSHs(sv_2mortal(newSViv(timep)));
                 else{
                     /* Nothing pushed on stack, so an empty
                      * list is implicitly returned. */
                 }

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

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

            int
            rpcb_gettime(host)
                 char *host
               PREINIT:
                 time_t  timep;
               CODE:
                 RETVAL = rpcb_gettime( host, &timep );
                 if (RETVAL == 0)
                       XSRETURN_UNDEF;
               OUTPUT:
                 RETVAL

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

            int
            rpcb_gettime(host)
                 char *host
                 time_t  timep;
               POSTCALL:
                 if (RETVAL == 0)
                       XSRETURN_UNDEF;

       The REQUIRE: Keyword

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

               REQUIRE: 1.922

       The CLEANUP: Keyword

       This keyword can be used when an XSUB requires special cleanup proce-
       dures 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 exe-
       cuted after the C subroutine call is performed.  When the POSTCALL:
       keyword is used it must precede OUTPUT: and CLEANUP: blocks which are
       present in the XSUB.

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

       The POSTCALL: block does not make a lot of sense when the C subroutine
       call is supplied by user by providing either CODE: or PPCODE: section.

       The BOOT: Keyword

       The BOOT: keyword is used to add code to the extension’s bootstrap
       function.  The bootstrap function is generated by the xsubpp compiler
       and normally holds the statements necessary to register any XSUBs with
       Perl.  With the BOOT: keyword the programmer can tell the compiler to
       add extra statements to the bootstrap function.

       This keyword may be used any time after the first MODULE keyword and
       should appear on a line by itself.  The first blank line after the key-
       word will terminate the code block.

            BOOT:
            # The following message will be printed when the
            # bootstrap function executes.
            printf("Hello from the bootstrap!\n");

       The VERSIONCHECK: Keyword

       The VERSIONCHECK: keyword corresponds to xsubpp’s "-versioncheck" and
       "-noversioncheck" options.  This keyword overrides the command line
       options.  Version checking is enabled by default.  When version check-
       ing is enabled the XS module will attempt to verify that its version
       matches the version of the PM module.

       To enable version checking:

           VERSIONCHECK: ENABLE

       To disable version checking:

           VERSIONCHECK: DISABLE

       The PROTOTYPES: Keyword

       The PROTOTYPES: keyword corresponds to xsubpp’s "-prototypes" and
       "-noprototypes" options.  This keyword overrides the command line
       options.  Prototypes are enabled by default.  When prototypes are
       enabled XSUBs will be given Perl prototypes.  This keyword may be used
       multiple times in an XS module to enable and disable prototypes for
       different parts of the module.

       To enable prototypes:

           PROTOTYPES: ENABLE

       To disable prototypes:

           PROTOTYPES: DISABLE

       The PROTOTYPE: Keyword

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

           bool_t
           rpcb_gettime(timep, ...)
                 time_t timep = NO_INIT
               PROTOTYPE: $;$
               PREINIT:
                 char *host = "localhost";
                 STRLEN n_a;
               CODE:
                         if( items > 1 )
                              host = (char *)SvPV(ST(1), n_a);
                         RETVAL = rpcb_gettime( host, &timep );
               OUTPUT:
                 timep
                 RETVAL

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

           void
           rpcb_gettime_noproto()
               PROTOTYPE: DISABLE
           ...

       The ALIAS: Keyword

       The ALIAS: keyword allows an XSUB to have two or more unique Perl names
       and to know which of those names was used when it was invoked.  The
       Perl names may be fully-qualified with package names.  Each alias is
       given an index.  The compiler will setup a variable called "ix" which
       contain the index of the alias which was used.  When the XSUB is called
       with its declared name "ix" will be 0.

       The following example will create aliases "FOO::gettime()" and
       "BAR::getit()" for this function.

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

       The OVERLOAD: Keyword

       Instead of writing an overloaded interface using pure Perl, you can
       also use the OVERLOAD keyword to define additional Perl names for your
       functions (like the ALIAS: keyword above).  However, the overloaded
       functions must be defined with three parameters (except for the
       nomethod() function which needs four parameters).  If any function has
       the OVERLOAD: keyword, several additional lines will be defined in the
       c file generated by xsubpp in order to register with the overload
       magic.

       Since blessed objects are actually stored as RV’s, it is useful to use
       the typemap features to preprocess parameters and extract the actual SV
       stored within the blessed RV. See the sample for T_PTROBJ_SPECIAL
       below.

       To use the OVERLOAD: keyword, create an XS function which takes three
       input parameters ( or use the c style ’...’ definition) like this:

           SV *
           cmp (lobj, robj, swap)
           My_Module_obj    lobj
           My_Module_obj    robj
           IV               swap
           OVERLOAD: cmp <=>
           { /* function defined here */}

       In this case, the function will overload both of the three way compari-
       son operators.  For all overload operations using non-alpha characters,
       you must type the parameter without quoting, seperating multiple over-
       loads with whitespace.  Note that "" (the stringify overload) should be
       entered as \"\" (i.e. escaped).

       The FALLBACK: Keyword

       In addition to the OVERLOAD keyword, if you need to control how Perl
       autogenerates missing overloaded operators, you can set the FALLBACK
       keyword in the module header section, like this:

           MODULE = RPC  PACKAGE = RPC

           FALLBACK: TRUE
           ...

       where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF.
       If you do not set any FALLBACK value when using OVERLOAD, it defaults
       to UNDEF.  FALLBACK is not used except when one or more functions using
       OVERLOAD have been defined.  Please see "Fallback" in overload for more
       details.

       The INTERFACE: Keyword

       This keyword declares the current XSUB as a keeper of the given calling
       signature.  If some text follows this keyword, it is considered as a
       list of functions which have this signature, and should be attached to
       the current XSUB.

       For example, if you have 4 C functions multiply(), divide(), add(),
       subtract() all having the signature:

           symbolic f(symbolic, symbolic);

       you can make them all to use the same XSUB using this:

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

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

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

           CV *mycv = newXSproto("Symbolic::remainder",
                                 XS_Symbolic_interface_s_ss, __FILE__, "$$");
           XSINTERFACE_FUNC_SET(mycv, remainder);

       say, from another XSUB.  (This example supposes that there was no
       INTERFACE_MACRO: section, otherwise one needs to use something else
       instead of "XSINTERFACE_FUNC_SET", see the next section.)

       The INTERFACE_MACRO: Keyword

       This keyword allows one to define an INTERFACE using a different way to
       extract a function pointer from an XSUB.  The text which follows this
       keyword should give the name of macros which would extract/set a func-
       tion pointer.  The extractor macro is given return type, "CV*", and
       "XSANY.any_dptr" for this "CV*".  The setter macro is given cv, and the
       function pointer.

       The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET".  An
       INTERFACE keyword with an empty list of functions can be omitted if
       INTERFACE_MACRO keyword is used.

       Suppose that in the previous example functions pointers for multiply(),
       divide(), add(), subtract() are kept in a global C array "fp[]" with
       offsets being "multiply_off", "divide_off", "add_off", "subtract_off".
       Then one can use

           #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
               ((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32])
           #define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
               CvXSUBANY(cv).any_i32 = CAT2( f, _off )

       in C section,

           symbolic
           interface_s_ss(arg1, arg2)
               symbolic        arg1
               symbolic        arg2
             INTERFACE_MACRO:
               XSINTERFACE_FUNC_BYOFFSET
               XSINTERFACE_FUNC_BYOFFSET_set
             INTERFACE:
               multiply divide
               add subtract

       in XSUB section.

       The INCLUDE: Keyword

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

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

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
               OUTPUT:
                 timep

       The XS module can use INCLUDE: to pull that file into it.

           INCLUDE: Rpcb1.xsh

       If the parameters to the INCLUDE: keyword are followed by a pipe ("│")
       then the compiler will interpret the parameters as a command.

           INCLUDE: cat Rpcb1.xsh │

       The CASE: Keyword

       The CASE: keyword allows an XSUB to have multiple distinct parts with
       each part acting as a virtual XSUB.  CASE: is greedy and if it is used
       then all other XS keywords must be contained within a CASE:.  This
       means nothing may precede the first CASE: in the XSUB and anything fol-
       lowing 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 parame-
       ters are reversed, "(time_t *timep, char *host)".

           long
           rpcb_gettime(a,b)
             CASE: ix == 1
               ALIAS:
                 x_gettime = 1
               INPUT:
                 # ’a’ is timep, ’b’ is host
                 char *b
                 time_t a = NO_INIT
               CODE:
                      RETVAL = rpcb_gettime( b, &a );
               OUTPUT:
                 a
                 RETVAL
             CASE:
                 # ’a’ is host, ’b’ is timep
                 char *a
                 time_t &b = NO_INIT
               OUTPUT:
                 b
                 RETVAL

       That function can be called with either of the following statements.
       Note the different argument lists.

               $status = rpcb_gettime( $host, $timep );

               $status = x_gettime( $timep, $host );

       The & Unary Operator

       The "&" unary operator in the INPUT: section is used to tell xsubpp
       that it should convert a Perl value to/from C using the C type to the
       left of "&", but provide a pointer to this value when the C function is
       called.

       This is useful to avoid a CODE: block for a C function which takes a
       parameter by reference.  Typically, the parameter should be not a
       pointer type (an "int" or "long" but not an "int*" or "long*").

       The following XSUB will generate incorrect C code.  The xsubpp compiler
       will turn this into code which calls "rpcb_gettime()" with parameters
       "(char *host, time_t timep)", but the real "rpcb_gettime()" wants the
       "timep" parameter to be of type "time_t*" rather than "time_t".

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t timep
               OUTPUT:
                 timep

       That problem is corrected by using the "&" operator.  The xsubpp com-
       piler will now turn this into code which calls "rpcb_gettime()" cor-
       rectly with parameters "(char *host, time_t *timep)".  It does this by
       carrying the "&" through, so the function call looks like "rpcb_get-
       time(host, &timep)".

           bool_t
           rpcb_gettime(host,timep)
                 char *host
                 time_t &timep
               OUTPUT:
                 timep

       Inserting POD, Comments and C Preprocessor Directives

       C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
       CODE:, PPCODE:, POSTCALL:, and CLEANUP: blocks, as well as outside the
       functions.  Comments are allowed anywhere after the MODULE keyword.
       The compiler will pass the preprocessor directives through untouched
       and will remove the commented lines. POD documentation is allowed at
       any point, both in the C and XS language sections. POD must be termi-
       nated 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 mis-
       taken for POD, as most indenting styles result in whitespace in front
       of any line starting with "=". Machine generated XS files may fall into
       this trap unless care is taken to ensure that a space breaks the
       sequence "\n=".

       Comments can be added to XSUBs by placing a "#" as the first non-
       whitespace of a line.  Care should be taken to avoid making the comment
       look like a C preprocessor directive, lest it be interpreted as such.
       The simplest way to prevent this is to put whitespace in front of the
       "#".

       If you use preprocessor directives to choose one of two versions of a
       function, use

           #if ... version1
           #else /* ... version2  */
           #endif

       and not

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

       because otherwise xsubpp will believe that you made a duplicate defini-
       tion 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() func-
       tion 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 consid-
       ered to be a static method.  It will call the C++ function using the
       class::method() syntax.  If the method is not static the function will
       be called using the THIS->method() syntax.

       The next examples will use the following C++ class.

            class color {
                 public:
                 color();
                 ~color();
                 int blue();
                 void set_blue( int );

                 private:
                 int c_blue;
            };

       The XSUBs for the blue() and set_blue() methods are defined with the
       class name but the parameter for the object (THIS, or "self") is
       implicit and is not listed.

            int
            color::blue()

            void
            color::set_blue( val )
                 int val

       Both Perl functions will expect an object as the first parameter.  In
       the generated C++ code the object is called "THIS", and the method call
       will be performed on this object.  So in the C++ code the blue() and
       set_blue() methods will be called as this:

            RETVAL = THIS->blue();

            THIS->set_blue( val );

       You could also write a single get/set method using an optional argu-
       ment:

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

       If the function’s name is DESTROY then the C++ "delete" function will
       be called and "THIS" will be given as its parameter.  The generated C++
       code for

            void
            color::DESTROY()

       will look like this:

            color *THIS = ...; // Initialized as in typemap

            delete THIS;

       If the function’s name is new then the C++ "new" function will be
       called to create a dynamic C++ object.  The XSUB will expect the class
       name, which will be kept in a variable called "CLASS", to be given as
       the first argument.

            color *
            color::new()

       The generated C++ code will call "new".

            RETVAL = new color();

       The following is an example of a typemap that could be used for this
       C++ example.

           TYPEMAP
           color *             O_OBJECT

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

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

       Interface Strategy

       When designing an interface between Perl and a C library a straight
       translation from C to XS (such as created by "h2xs -x") is often suffi-
       cient.  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 situa-
       tion.  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_fail-
       ure" will create more Perl-like interface.

       Identify which values are used by only the C and XSUB functions them-
       selves, 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 val-
       ues.  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 automati-
       cally by "h2xs -x".)

       If the same C type is used in several different contexts which require
       different translations, "typedef" several new types mapped to this C
       type, and create separate typemap entries for these new types.  Use
       these types in declarations of return type and parameters to XSUBs.

       Perl Objects And C Structures

       When dealing with C structures one should select either T_PTROBJ or
       T_PTRREF for the XS type.  Both types are designed to handle pointers
       to complex objects.  The T_PTRREF type will allow the Perl object to be
       unblessed while the T_PTROBJ type requires that the object be blessed.
       By using T_PTROBJ one can achieve a form of type-checking because the
       XSUB will attempt to verify that the Perl object is of the expected
       type.

       The following XS code shows the getnetconfigent() function which is
       used with ONC+ TIRPC.  The getnetconfigent() function will return a
       pointer to a C structure and has the C prototype shown below.  The
       example will demonstrate how the C pointer will become a Perl refer-
       ence.  Perl will consider this reference to be a pointer to a blessed
       object and will attempt to call a destructor for the object.  A
       destructor will be provided in the XS source to free the memory used by
       getnetconfigent().  Destructors in XS can be created by specifying an
       XSUB function whose name ends with the word DESTROY.  XS destructors
       can be used to free memory which may have been malloc’d by another
       XSUB.

            struct netconfig *getnetconfigent(const char *netid);

       A "typedef" will be created for "struct netconfig".  The Perl object
       will be blessed in a class matching the name of the C type, with the
       tag "Ptr" appended, and the name should not have embedded spaces if it
       will be a Perl package name.  The destructor will be placed in a class
       corresponding to the class of the object and the PREFIX keyword will be
       used to trim the name to the word DESTROY as Perl will expect.

            typedef struct netconfig Netconfig;

            MODULE = RPC  PACKAGE = RPC

            Netconfig *
            getnetconfigent(netid)
                 char *netid

            MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

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

       This example requires the following typemap entry.  Consult the typemap
       section for more information about adding new typemaps for an exten-
       sion.

            TYPEMAP
            Netconfig *  T_PTROBJ

       This example will be used with the following Perl statements.

            use RPC;
            $netconf = getnetconfigent("udp");

       When Perl destroys the object referenced by $netconf it will send the
       object to the supplied XSUB DESTROY function.  Perl cannot determine,
       and does not care, that this object is a C struct and not a Perl
       object.  In this sense, there is no difference between the object cre-
       ated 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 them-
       selves, 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 exten-
       sion’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 getnetconfi-
       gent() is shown here.  Note that the C type is separated from the XS
       type with a tab and that the C unary operator "*" is considered to be a
       part of the C type name.

               TYPEMAP
               Netconfig *<tab>T_PTROBJ

       Here’s a more complicated example: suppose that you wanted "struct net-
       config" 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 under-
       scores to double-colons (::), and declare "Net_Config" to be of that
       type:

               TYPEMAP
               Net_Config      T_PTROBJ_SPECIAL

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

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

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

       Safely Storing Static Data in XS

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

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

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

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

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

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

           /* Global Data */

           #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION

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

           START_MY_CXT

           MODULE = BlindMice           PACKAGE = BlindMice

           BOOT:
           {
               MY_CXT_INIT;
               MY_CXT.count = 0;
               strcpy(MY_CXT.name[0], "None");
               strcpy(MY_CXT.name[1], "None");
               strcpy(MY_CXT.name[2], "None");
           }

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

           char *
           get_mouse_name(index)
             int index
             CODE:
               dMY_CXT;
               RETVAL = MY_CXT.lives ++;
               if (index > MY_CXT.count)
                 croak("There are only 3 blind mice.");
               else
                 RETVAL = newSVpv(MY_CXT.name[index - 1]);

       REFERENCE

       MY_CXT_KEY
            This macro is used to define a unique key to refer to the static
            data for an XS module. The suggested naming scheme, as used by
            h2xs, is to use a string that consists of the module name, the
            string "::_guts" and the module version number.

                #define MY_CXT_KEY "MyModule::_guts" XS_VERSION

       typedef my_cxt_t
            This struct typedef must always be called "my_cxt_t" -- the other
            "CXT*" macros assume the existence of the "my_cxt_t" typedef name.

            Declare a typedef named "my_cxt_t" that is a structure that con-
            tains all the data that needs to be interpreter-local.

                typedef struct {
                    int some_value;
                } my_cxt_t;

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

       MY_CXT_INIT
            The MY_CXT_INIT macro initialises storage for the "my_cxt_t"
            struct.

            It must be called exactly once -- typically in a BOOT: section.

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

       MY_CXT
            Use the MY_CXT macro to access members of the "my_cxt_t" struct.
            For example, if "my_cxt_t" is

                typedef struct {
                    int index;
                } my_cxt_t;

            then use this to access the "index" member

                dMY_CXT;
                MY_CXT.index = 2;


EXAMPLES

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

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

            #include <rpc/rpc.h>

            typedef struct netconfig Netconfig;

            MODULE = RPC  PACKAGE = RPC

            SV *
            rpcb_gettime(host="localhost")
                 char *host
               PREINIT:
                 time_t  timep;
               CODE:
                 ST(0) = sv_newmortal();
                 if( rpcb_gettime( host, &timep ) )
                      sv_setnv( ST(0), (double)timep );

            Netconfig *
            getnetconfigent(netid="udp")
                 char *netid

            MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

            void
            rpcb_DESTROY(netconf)
                 Netconfig *netconf
               CODE:
                 printf("NetconfigPtr::DESTROY\n");
                 free( netconf );

       File "typemap": Custom typemap for RPC.xs.

            TYPEMAP
            Netconfig *  T_PTROBJ

       File "RPC.pm": Perl module for the RPC extension.

            package RPC;

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

            bootstrap RPC;
            1;

       File "rpctest.pl": Perl test program for the RPC extension.

            use RPC;

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

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


XS VERSION

       This document covers features supported by "xsubpp" 1.935.


AUTHOR

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

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



perl v5.8.6                       2004-11-05                         PERLXS(1)

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