MirOS Manual: perlxs(1)


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 com-
     bined 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, con-
     verts these Perl values to the formats expected by a C func-
     tion, 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 hap-
     pens.  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 writ-
     ing this glue C code in long-hand, one can write a more con-
     cise 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

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     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 func-
     tion 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 direc-
     tives 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 creat-
     ing the extension glue code.  See http://www.swig.org/ for
     more information.

     On The Road

     Many of the examples which follow will concentrate on creat-
     ing an interface between Perl and the ONC+ RPC bind library
     functions.  The rpcb_gettime() function is used to demon-
     strate 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.

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          #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 vari-
     ables will contain the output of the function.

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

     The following XS file shows an XS subroutine, or XSUB, which
     demonstrates one possible interface to the rpcb_gettime()
     function.  This XSUB represents a direct translation between
     C and Perl and so preserves the interface even from Perl.
     This XSUB will be invoked from Perl with the usage shown
     above.  Note that the first three #include statements, for
     "EXTERN.h", "perl.h", and "XSUB.h", will always be present
     at the beginning of an XS file.  This approach and others
     will be expanded later in this document.

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

          MODULE = RPC  PACKAGE = RPC

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

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

          package RPC;

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

          bootstrap RPC;
          1;

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

     The Anatomy of an XSUB

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

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

          double
          sin(x)
            double x

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

          double
          sin(double x)

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

          double
          sin(double x);

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

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

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     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 Opera-
     tor".

     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 readability.

       CORRECT

       double
       sin(x)
       double x

     More complicated XSUBs may contain many other sections.
     Each section of an XSUB starts with the corresponding key-
     word, 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 con-
     sidered to be an INPUT: section unless explicitly marked
     with another keyword. (See "The INPUT: Keyword".)

     An XSUB section continues until another section-start key-
     word 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

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     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 han-
     dle 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 vari-
     able is required, the section may use direct stack manipula-
     tion 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.)

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     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 atten-
     tion to reference counts. (For more about reference counts,
     see perlguts.) To make your life easier, the typemap file
     automatically makes "RETVAL" mortal when you're returning an
     "SV *". Thus, the following two XSUBs are more or less
     equivalent:

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

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

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

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

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

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

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

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

     The MODULE Keyword

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

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

          MODULE = RPC

     The PACKAGE Keyword

     When functions within an XS source file must be separated
     into packages the PACKAGE keyword should be used.  This key-
     word is used with the MODULE keyword and must follow immedi-
     ately after it when used.

          MODULE = RPC  PACKAGE = RPC

          [ XS code in package RPC ]

          MODULE = RPC  PACKAGE = RPCB

          [ XS code in package RPCB ]

          MODULE = RPC  PACKAGE = RPC

          [ XS code in package RPC ]

     The same package name can be used more than once, allowing
     for non-contiguous code. This is useful if you have a
     stronger ordering principle than package names.

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     Although this keyword is optional and in some cases provides
     redundant information it should always be used.  This key-
     word will ensure that the XSUBs appear in the desired pack-
     age.

     The PREFIX Keyword

     The PREFIX keyword designates prefixes which should be
     removed from the Perl function names.  If the C function is
     "rpcb_gettime()" and the PREFIX value is "rpcb_" then Perl
     will see this function as "gettime()".

     This keyword should follow the PACKAGE keyword when used. If
     PACKAGE is not used then PREFIX should follow the MODULE
     keyword.

          MODULE = RPC  PREFIX = rpc_

          MODULE = RPC  PACKAGE = RPCB  PREFIX = rpcb_

     The OUTPUT: Keyword

     The OUTPUT: keyword indicates that certain function parame-
     ters 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 compiler will need help to determine which variables
     are output variables.

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

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

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

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

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     typemap.

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

     xsubpp emits an automatic "SvSETMAGIC()" for all parameters
     in the OUTPUT section of the XSUB, except RETVAL.  This is
     the usually desired behavior, as it takes care of properly
     invoking 'set' magic on output parameters (needed for hash
     or array element parameters that must be created if they
     didn't exist).  If for some reason, this behavior is not
     desired, the OUTPUT section may contain a "SETMAGIC: DIS-
     ABLE" 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 pro-
     vide 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.

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     The CODE: Keyword

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

     The following XSUB is for a C function which requires spe-
     cial 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 precondi-
     tions before making a call to the C function:

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

     The NO_INIT Keyword

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

     The following example shows a variation of the
     rpcb_gettime() function. This function uses the timep vari-
     able 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 program-
     mer, however, is allowed to override the typemaps and supply
     alternate (or additional) initialization code.  Initializa-
     tion 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

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     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 parame-
     ters.  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 sec-
     tion.

     If the initialization begins with "=", then it is output in
     the declaration for the input variable, replacing the ini-
     tialization 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 per-
     formed. For the "+" case, the declaration for the variable
     will include the initialization from the typemap.  A global
     variable, %v, is available for the truly rare case where
     information from one initialization is needed in another
     initialization.

     Here's a truly obscure example:

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

     The construct "\$v{timep}=@{[$v{timep}=$arg]}" used in the
     above example has a two-fold purpose: first, when this line
     is processed by xsubpp, the Perl snippet "$v{timep}=$arg" is
     evaluated.  Second, the text of the evaluated snippet is
     output into the generated C file (inside a C comment)!  Dur-
     ing 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 plac-
     ing 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.

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     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 parame-
     ters 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 param-
     eters.  This may result in the declaration ending up after C
     code, which is C syntax error.  Similar errors may happen
     with an explicit ";"-type or "+"-type initialization of
     parameters is used (see "Initializing Function Parameters").
     Declaring these variables in an INIT: section will not help.

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

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

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          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);

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

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

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

     and the code for rpcb_gettime() can be rewritten as

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

     The SCOPE: Keyword

     The SCOPE: keyword allows scoping to be enabled for a par-
     ticular 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

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     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 key-
     word 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:

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         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 declara-
     tion line, and our assignment "h = host" is not performed
     too early.  Otherwise one would need to have the assignment
     "h = host" in a CODE: or INIT: section.)

     The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords

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

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

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

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

     The return list of the generated Perl function consists of
     the C return value from the function (unless the XSUB is of
     "void" return type or "The NO_OUTPUT Keyword" was used) fol-
     lowed 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.

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

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

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

     or

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

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     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 gen-
     erated 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 declara-
     tions 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 com-
     piler 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 );

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          $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 dif-
     ferent 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 inter-
     face 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: key-
     word and is used to tell the xsubpp compiler that the pro-
     grammer 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.

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     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 Vari-
     able").  The trailer generated for a PPCODE: section is
     based on the number of return values and on the number of
     times "SP" was updated by "[X]PUSH*()" macros.

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

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

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

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

     The "void" return type for this function tells the xsubpp
     compiler that the RETVAL variable is not needed or used and
     that it should not be created. In most scenarios the void
     return type should be used with the PPCODE: directive.

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     The EXTEND() macro is used to make room on the argument
     stack for 2 return values.  The PPCODE: directive causes the
     xsubpp compiler to create a stack pointer available as "SP",
     and it is this pointer which is being used in the EXTEND()
     macro. The values are then pushed onto the stack with the
     PUSHs() macro.

     Now the rpcb_gettime() function can be used from Perl with
     the following statement.

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

     When handling output parameters with a PPCODE section, be
     sure to handle 'set' magic properly.  See perlguts for
     details about 'set' magic.

     Returning Undef And Empty Lists

     Occasionally the programmer will want to return simply
     "undef" or an empty list if a function fails rather than a
     separate status value.  The rpcb_gettime() function offers
     just this situation.  If the function succeeds we would like
     to have it return the time and if it fails we would like to
     have undef returned.  In the following Perl code the value
     of $timep will either be undef or it will be a valid time.

          $timep = rpcb_gettime( "localhost" );

     The following XSUB uses the "SV *" return type as a mnemonic
     only, and uses a CODE: block to indicate to the compiler
     that the programmer has supplied all the necessary code.
     The sv_newmortal() call will initialize the return value to
     undef, making that the default return value.

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

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

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          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

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     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 com-
     pile 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 OUT-
     PUT: blocks which are present in the XSUB.  The code speci-
     fied for the cleanup block will be added as the last state-
     ments in the XSUB.

     The POSTCALL: Keyword

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

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

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

     The BOOT: Keyword

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

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

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

     The VERSIONCHECK: Keyword

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

     To enable version checking:

         VERSIONCHECK: ENABLE

     To disable version checking:

         VERSIONCHECK: DISABLE

     The PROTOTYPES: Keyword

     The PROTOTYPES: keyword corresponds to xsubpp's "-proto-
     types" and "-noprototypes" options.  This keyword overrides
     the command line options. Prototypes are enabled by default.
     When prototypes are enabled XSUBs will be given Perl proto-
     types.  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.

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         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

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     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 OVER-
     LOAD: keyword, several additional lines will be defined in
     the c file generated by xsubpp in order to register with the
     overload magic.

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

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

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

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

     The FALLBACK: Keyword

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

         MODULE = RPC  PACKAGE = RPC

         FALLBACK: TRUE
         ...

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

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     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 sig-
     nature, and should be attached to the current XSUB.

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

         symbolic f(symbolic, symbolic);

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

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

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

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

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

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

     The INTERFACE_MACRO: Keyword

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

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     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 command to generate the XS code to be
     pulled into the module.

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

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

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

         INCLUDE: Rpcb1.xsh

     If the parameters to the INCLUDE: keyword are followed by a
     pipe ("|") then the compiler will interpret the parameters

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     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 func-
     tion is called as "x_gettime()" its parameters are reversed,
     "(time_t *timep, char *host)".

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

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

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

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

     The & Unary Operator

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

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

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

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

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

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

     Inserting POD, Comments and C Preprocessor Directives

     C preprocessor directives are allowed within BOOT:, PREINIT:
     INIT:, CODE:, PPCODE:, POSTCALL:, and CLEANUP: blocks, as
     well as outside the functions. Comments are allowed anywhere
     after the MODULE keyword.  The compiler will pass the
     preprocessor directives through untouched and will remove
     the commented lines. POD documentation is allowed at any
     point, both in the C and XS language sections. POD must be
     terminated with a "=cut" command; "xsubpp" will exit with an
     error if it does not. It is very unlikely that human

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     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 ver-
     sions 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 dupli-
     cate 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.

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

               private:
               int c_blue;
          };

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

          int
          color::blue()

          void
          color::set_blue( val )
               int val

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

          RETVAL = THIS->blue();

          THIS->set_blue( val );

     You could also write a single get/set method using an
     optional argument:

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

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

          void
          color::DESTROY()

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     will look like this:

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

          delete THIS;

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

          color *
          color::new()

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

          RETVAL = new color();

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

         TYPEMAP
         color *             O_OBJECT

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

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

     Interface Strategy

     When designing an interface between Perl and a C library a
     straight translation from C to XS (such as created by "h2xs
     -x") is often sufficient. However, sometimes the interface
     will look very C-like and occasionally nonintuitive, espe-
     cially 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.

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     Identify the C functions with input/output or output parame-
     ters.  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 func-
     tions 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

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

          struct netconfig *getnetconfigent(const char *netid);

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

          typedef struct netconfig Netconfig;

          MODULE = RPC  PACKAGE = RPC

          Netconfig *
          getnetconfigent(netid)
               char *netid

          MODULE = RPC  PACKAGE = NetconfigPtr  PREFIX = rpcb_

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

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

          TYPEMAP
          Netconfig *  T_PTROBJ

     This example will be used with the following Perl state-
     ments.

          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 getnetconfi-
     gent() XSUB and an object created by a normal Perl subrou-
     tine.

     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

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     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 con-
     sidered to be a part of the C type name.

             TYPEMAP
             Netconfig *<tab>T_PTROBJ

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

             typedef struct netconfig * Net_Config;

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

             TYPEMAP
             Net_Config      T_PTROBJ_SPECIAL

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

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

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

     The INT2PTR macro (defined in perl.h) casts an integer to a
     pointer, of a given type, taking care of the possible dif-
     ferent 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.

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     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");
         }

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         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 con-
          sists of the module name, the string "::_guts" and the
          module version number.

              #define MY_CXT_KEY "MyModule::_guts" XS_VERSION

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

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

              typedef struct {
                  int some_value;
              } my_cxt_t;

     START_MY_CXT
          Always place the START_MY_CXT macro directly after the

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          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 func-
          tions 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 func-
     tions.

          #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 );

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          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>.

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