phpman > perldoc > 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.

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

    An XSUB forms the basic unit of the XS interface. After compilation by the xsubpp compiler, each
    XSUB amounts to a C function definition which will provide the glue between Perl calling
    conventions and C calling 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 function, transfers the return values of the C function
    back to Perl. Return values here may be a conventional C return value or any C function
    arguments that may serve as output parameters. These return values may be passed back to Perl
    either by putting them on the Perl stack, or by modifying the arguments supplied from the Perl
    side.

    The above is a somewhat simplified view of what really happens. Since Perl allows more flexible
    calling conventions than C, XSUBs may do much more in practice, such as checking input
    parameters for validity, throwing exceptions (or returning undef/empty list) if the return value
    from the C function indicates failure, calling different C functions based on numbers and types
    of the arguments, providing an object-oriented interface, etc.

    Of course, one could write such glue code directly in C. However, this would be a tedious task,
    especially if one needs to write glue for multiple C functions, and/or one is not familiar
    enough with the Perl stack discipline and other such arcana. XS comes to the rescue here:
    instead of writing this glue C code in long-hand, one can write a more concise short-hand
    *description* of what should be done by the glue, and let the XS compiler xsubpp handle the
    rest.

    The XS language allows one to describe the mapping between how the C routine is used, and how
    the corresponding Perl routine is used. It also allows creation of Perl routines which are
    directly translated to C code and which are not related to a pre-existing C function. In cases
    when the C interface coincides with the Perl interface, the XSUB declaration is almost identical
    to a declaration of a C function (in K&R style). In such circumstances, there is another tool
    called "h2xs" that is able to translate an entire C header file into a corresponding XS file
    that will provide glue to the functions/macros described in the header file.

    The XS compiler is called xsubpp. This compiler creates the constructs necessary to let an XSUB
    manipulate Perl values, and creates the glue necessary to let Perl call the XSUB. The compiler
    uses typemaps to determine how to map C function parameters and output values to Perl values and
    back. The default typemap (which comes with Perl) handles many common C types. A supplementary
    typemap may also be needed to handle any special structures and types for the library being
    linked. For more information on typemaps, see perlxstypemap.

    A file in XS format starts with a C language section which goes until the first "MODULE ="
    directive. Other XS directives and XSUB definitions may follow this line. The "language" used in
    this part of the file is usually referred to as the XS language. xsubpp recognizes and skips POD
    (see perlpod) in both the C and XS language sections, which allows the XS file to contain
    embedded documentation.

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

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

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

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

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

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

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

    If an XSUB is created to offer a direct translation between this function and Perl, then this
    XSUB will be used from Perl with the following code. The $status and $timep variables will
    contain the output of the function.

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

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

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

         MODULE = RPC  PACKAGE = RPC

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

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

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

         double
         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);

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

         char *  s
         char    &c

    Both these XS declarations correspond to the "char*" C type, but they have different semantics,
    see "The & Unary Operator".

    It is convenient to think that the indirection operator "*" should be considered as a part of
    the type and the address operator "&" should be considered part of the variable. See
    perlxstypemap for more info about handling qualifiers and unary operators in C types.

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

      INCORRECT                        CORRECT

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

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

      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 function and its
    parameters. Whatever immediately follows these is considered to be an INPUT: section unless
    explicitly marked with another keyword. (See "The INPUT: Keyword".)

    An XSUB section continues until another section-start keyword is found.

  The Argument Stack
    The Perl argument stack is used to store the values which are sent as parameters to the XSUB and
    to store the XSUB's return value(s). In reality all Perl functions (including non-XSUB ones)
    keep their values on this stack all the same time, each limited to its own range of positions on
    the stack. In this document the first position on that stack which belongs to the active
    function will be referred to as position 0 for that function.

    XSUBs refer to their stack arguments with the macro ST(x), where *x* refers to a position in
    this XSUB's part of the stack. Position 0 for that function would be known to the XSUB as ST(0).
    The XSUB's incoming parameters and outgoing return values always begin at ST(0). For many simple
    cases the xsubpp compiler will generate the code necessary to handle the argument stack by
    embedding code fragments found in the typemaps. In more complex cases the programmer must supply
    the code.

  The RETVAL Variable
    The RETVAL variable is a special C variable that is declared automatically for you. The C type
    of RETVAL matches the return type of the C library function. The xsubpp compiler will declare
    this variable in each XSUB with non-"void" return type. By default the generated C function will
    use RETVAL to hold the return value of the C library function being called. In simple cases the
    value of RETVAL will be placed in ST(0) of the argument stack where it can be received by Perl
    as the return value of the XSUB.

    If the XSUB has a return type of "void" then the compiler will not declare a RETVAL variable for
    that function. When using a PPCODE: section no manipulation of the RETVAL variable is required,
    the section may use direct stack manipulation to place output values on the stack.

    If PPCODE: directive is not used, "void" return value should be used only for subroutines which
    do not return a value, *even if* CODE: directive is used which sets ST(0) explicitly.

    Older versions of this document recommended to use "void" return value in such cases. It was
    discovered that this could lead to segfaults in cases when XSUB was *truly* "void". This
    practice is now deprecated, and may be not supported at some future version. Use the return
    value "SV *" in such cases. (Currently "xsubpp" contains some heuristic code which tries to
    disambiguate between "truly-void" and "old-practice-declared-as-void" functions. Hence your code
    is at mercy of this heuristics unless you use "SV *" as return value.)

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

      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 *", "CV *", and "SVREF"
    (which indicates a scalar reference, not a general "SV *"). In XS code on perls starting with
    perl 5.16, you can override the typemaps for any of these types with a version that has proper
    handling of refcounts. In your "TYPEMAP" section, do

      AV*   T_AVREF_REFCOUNT_FIXED

    to get the repaired variant. For backward compatibility with older versions of perl, you can
    instead decrement the reference count manually when you're returning one of the aforementioned
    types using "sv_2mortal":

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

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

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

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

         MODULE = RPC

  The PACKAGE Keyword
    When functions within an XS source file must be separated into packages the PACKAGE keyword
    should be used. This keyword is used with the MODULE keyword and must follow immediately after
    it when used.

         MODULE = RPC  PACKAGE = RPC

         [ XS code in package RPC ]

         MODULE = RPC  PACKAGE = RPCB

         [ XS code in package RPCB ]

         MODULE = RPC  PACKAGE = RPC

         [ XS code in package RPC ]

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

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

  The PREFIX Keyword
    The PREFIX keyword designates prefixes which should be removed from the Perl function names. If
    the C function is "rpcb_gettime()" and the PREFIX value is "rpcb_" then Perl will see this
    function as "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 parameters should be updated (new values
    made visible to Perl) when the XSUB terminates or that certain values should be returned to the
    calling Perl function. For simple functions which have no CODE: or PPCODE: section, such as the
    sin() function above, the RETVAL variable is automatically designated as an output value. For
    more complex functions the xsubpp compiler will need help to determine which variables are
    output 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 function parameters are output variables.
    This may be necessary when a parameter has been modified within the function and the programmer
    would like the update to be seen by Perl.

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

  The NO_OUTPUT Keyword
    The NO_OUTPUT can be placed as the first token of the XSUB. This keyword indicates that while
    the C subroutine we provide an interface to has a non-"void" return type, the return value of
    this C subroutine should not be returned from the generated Perl subroutine.

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

    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 function.
    NO_INIT will tell the compiler that some parameters will be used for output rather than for
    input and that they will be handled before the function terminates.

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

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

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

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

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

    Refer to perlxstypemap for details on writing typemaps.

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

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

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

    Here's a truly obscure example:

         bool_t
         rpcb_gettime(host,timep)
              time_t &timep; /* \$v{timep}=@{[$v{timep}=$arg]} */
              char *host + SvOK($v{timep}) ? SvPVbyte_nolen($arg) : 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)! 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: section are emitted.

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

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

    To enable scoping:

        SCOPE: ENABLE

    To disable scoping:

        SCOPE: DISABLE

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

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

        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" keyword is the default, the other
    keywords indicate how the Perl interface should differ from the C interface.

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

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

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

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

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

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

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

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

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

         $status = rpcb_gettime( $timep );

    The XS code, with ellipsis, follows.

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

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

    For example, suppose that a C function is declared as

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

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

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

    To do this, declare the XSUB as

        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. Occasionally one will want an XSUB to return a list of values rather than a
    single value. In these cases one must use PPCODE: and then explicitly push the list of values on
    the stack. The PPCODE: and CODE: keywords should not be used together within the same XSUB.

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

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

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

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

    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 explicit undef in the return value, should
    the need arise.

         SV *
         rpcb_gettime(host)
              char *  host
            PREINIT:
              time_t  timep;
              bool_t x;
            CODE:
              if( rpcb_gettime( host, &timep ) ){
                   ST(0) = sv_newmortal();
                   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 procedures before it terminates.
    When the CLEANUP: keyword is used it must follow any CODE:, or OUTPUT: blocks which are present
    in the XSUB. The code specified for the cleanup block will be added as the last statements in
    the XSUB.

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

    See examples in "The NO_OUTPUT Keyword" and "Returning Undef And Empty 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 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 "-versioncheck" and "-noversioncheck" options.
    This keyword overrides the command line options. Version checking is enabled by default. When
    version checking is enabled the XS module will attempt to verify that its version matches the
    version of the PM module.

    To enable version checking:

        VERSIONCHECK: ENABLE

    To disable version checking:

        VERSIONCHECK: DISABLE

    Note that if the version of the PM module is an NV (a floating point number), it will be
    stringified with a possible loss of precision (currently chopping to nine decimal places) so
    that it may not match the version of the XS module anymore. Quoting the $VERSION declaration to
    make it a string is recommended if long version numbers are used.

  The PROTOTYPES: Keyword
    The PROTOTYPES: keyword corresponds to xsubpp's "-prototypes" and "-noprototypes" options. This
    keyword overrides the command line options. Prototypes are disabled by default. When prototypes
    are enabled, XSUBs will be given Perl prototypes. This keyword may be used multiple times in an
    XS module to enable and disable prototypes for different parts of the module. Note that xsubpp
    will nag you if you don't explicitly enable or disable prototypes, with:

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

    To enable prototypes:

        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";
            CODE:
                      if( items > 1 )
                           host = (char *)SvPVbyte_nolen(ST(1));
                      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 in such a way as to accept the number of
    parameters supplied by perl's overload system. For most overload methods, it will be three
    parameters; for the "nomethod" function it will be four. However, the bitwise operators "&",
    "|", "^", and "~" may be called with three *or* five arguments (see overload).

    If any function has the OVERLOAD: keyword, several additional lines will be defined in the c
    file generated by xsubpp in order to register with the overload magic.

    Since blessed objects are actually stored as RV's, it is useful to use the typemap features to
    preprocess parameters and extract the actual SV stored within the blessed RV. See the sample for
    T_PTROBJ_SPECIAL 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,
    separating multiple overloads with whitespace. Note that "" (the stringify overload) should be
    entered as \"\" (i.e. escaped).

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

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

        MODULE = RPC  PACKAGE = RPC

        FALLBACK: TRUE
        ...

    where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF. If you do not set any
    FALLBACK value when using OVERLOAD, it defaults to UNDEF. FALLBACK is not used except when one
    or more functions using OVERLOAD have been defined. Please see "fallback" in overload for more
    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. 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 different way to extract a function
    pointer from an XSUB. The text which follows this keyword should give the name of macros which
    would extract/set a function pointer. The extractor macro is given return type, "CV*", and
    "XSANY.any_dptr" for this "CV*". The setter macro is given cv, and the function pointer.

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

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

        #define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
            ((XSINTERFACE_CVT_ANON(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 as a command. This feature is mildly deprecated in favour of the
    "INCLUDE_COMMAND:" directive, as documented below.

        INCLUDE: cat Rpcb1.xsh |

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

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

        INCLUDE_COMMAND: cat Rpcb1.xsh

        INCLUDE_COMMAND: $^X -e ...

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

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

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

      EXPORT_XSUB_SYMBOLS: ENABLE

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

      EXPORT_XSUB_SYMBOLS: DISABLE

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

  The & Unary Operator
    The "&" unary operator in the INPUT: section is used to tell xsubpp that it should convert a
    Perl value to/from C using the C type to the left of "&", but provide a pointer to this value
    when the C function is 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 generated C code will be mistaken for POD,
    as most indenting styles result in whitespace in front of any line starting with "=". Machine
    generated XS files may fall into this trap unless care is taken to ensure that a space breaks
    the sequence "\n=".

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

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

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

    and not

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

    because otherwise xsubpp will believe that you made a duplicate definition of the function.
    Also, put a blank line before the #else/#endif so it will not be seen as part of the function
    body.

  Using XS With C++
    If an XSUB name contains "::", it is considered to be a C++ method. The generated Perl function
    will assume that its first argument is an object pointer. The object pointer will be stored in a
    variable called THIS. The object should have been created by C++ with the new() function and
    should be blessed by Perl with the sv_setref_pv() macro. The blessing of the object by Perl can
    be handled by a typemap. An example typemap is shown at the end of this section.

    If the return type of the XSUB includes "static", the method is considered to be a static
    method. It will call the C++ function using the class::method() syntax. If the method is not
    static the function will be called using the THIS->method() syntax.

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

         class color {
              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()

    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, especially when the C function modifies one of its
    parameters, or returns failure inband (as in "negative return values mean failure"). In cases
    where the programmer wishes to create a more Perl-like interface the following strategy may help
    to identify the more critical parts of the interface.

    Identify the C functions with input/output or output parameters. The XSUBs for these functions
    may be able to return lists to Perl.

    Identify the C functions which use some inband info as an indication of failure. They may be
    candidates to return undef or an empty list in case of failure. If the failure may be detected
    without a call to the C function, you may want to use an INIT: section to report the failure.
    For failures detectable after the C function returns one may want to use a POSTCALL: section to
    process the failure. In more complicated cases use CODE: or PPCODE: sections.

    If many functions use the same failure indication based on the return value, you may want to
    create a special typedef to handle this situation. Put

      typedef int negative_is_failure;

    near the beginning of XS file, and create an OUTPUT typemap entry for "negative_is_failure"
    which converts negative values to "undef", or maybe croak()s. After this the return value of
    type "negative_is_failure" will create more Perl-like interface.

    Identify which values are used by only the C and XSUB functions themselves, say, when a
    parameter to a function should be a contents of a global variable. If Perl does not need to
    access the contents of the value then it may not be necessary to provide a translation for that
    value from C to Perl.

    Identify the pointers in the C function parameter lists and return values. Some pointers may be
    used to implement input/output or output parameters, they can be handled in XS with the "&"
    unary operator, and, possibly, using the NO_INIT keyword. Some others will require handling of
    types like "int *", and one needs to decide what a useful Perl translation will do in such a
    case. When the semantic is clear, it is advisable to put the translation into a typemap file.

    Identify the structures used by the C functions. In many cases it may be helpful to use the
    T_PTROBJ typemap for these structures so they can be manipulated by Perl as blessed objects.
    (This is handled automatically by "h2xs -x".)

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

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

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

         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 created by the
    getnetconfigent() XSUB and an object created by a normal Perl subroutine.

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

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

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

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

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

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

        /* Global Data */

        #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION

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

        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)
            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);
              }
            OUTPUT:
              RETVAL

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

        void
        CLONE(...)
            CODE:
              MY_CXT_CLONE;

   MY_CXT 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 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 declaration of "my_cxt_t".

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

         It *must* be called exactly once, typically in a BOOT: section. If you are maintaining
         multiple interpreters, it should be called once in each interpreter instance, except for
         interpreters cloned from existing ones. (But see "MY_CXT_CLONE" below.)

    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;

    aMY_CXT/pMY_CXT
         "dMY_CXT" may be quite expensive to calculate, and to avoid the overhead of invoking it in
         each function it is possible to pass the declaration onto other functions using the
         "aMY_CXT"/"pMY_CXT" macros, eg

             void sub1() {
                 dMY_CXT;
                 MY_CXT.index = 1;
                 sub2(aMY_CXT);
             }

             void sub2(pMY_CXT) {
                 MY_CXT.index = 2;
             }

         Analogously to "pTHX", there are equivalent forms for when the macro is the first or last
         in multiple arguments, where an underscore represents a comma, i.e. "_aMY_CXT", "aMY_CXT_",
         "_pMY_CXT" and "pMY_CXT_".

    MY_CXT_CLONE
         By default, when a new interpreter is created as a copy of an existing one (eg via
         "threads->create()"), both interpreters share the same physical my_cxt_t structure. Calling
         "MY_CXT_CLONE" (typically via the package's "CLONE()" function), causes a byte-for-byte
         copy of the structure to be taken, and any future dMY_CXT will cause the copy to be
         accessed instead.

    MY_CXT_INIT_INTERP(my_perl)
    dMY_CXT_INTERP(my_perl)
         These are versions of the macros which take an explicit interpreter as an argument.

    Note that these macros will only work together within the *same* source file; that is, a dMY_CTX
    in one source file will access a different structure than a dMY_CTX in another source file.

  Thread-aware system interfaces
    Starting from Perl 5.8, in C/C++ level Perl knows how to wrap system/library interfaces that
    have thread-aware versions (e.g. getpwent_r()) into frontend macros (e.g. getpwent()) that
    correctly handle the multithreaded interaction with the Perl interpreter. This will happen
    transparently, the only thing you need to do is to instantiate a Perl interpreter.

    This wrapping happens always when compiling Perl core source (PERL_CORE is defined) or the Perl
    core extensions (PERL_EXT is defined). When compiling XS code outside of the Perl core, the
    wrapping does not take place before Perl 5.28. Starting in that release you can

     #define PERL_REENTRANT

    in your code to enable the wrapping. It is advisable to do so if you are using such functions,
    as intermixing the "_r"-forms (as Perl compiled for multithreaded operation will do) and the
    "_r"-less forms is neither well-defined (inconsistent results, data corruption, or even crashes
    become more likely), nor is it very portable. Unfortunately, not all systems have all the "_r"
    forms, but using this "#define" gives you whatever protection that Perl is aware is available on
    each system.

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

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

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

         typedef struct netconfig Netconfig;

         MODULE = RPC  PACKAGE = RPC

         SV *
         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. (cf. perlxstypemap)

         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";

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

         asctime_r()
         ctime_r()
         Perl_langinfo()

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

         #define PERL_REENTRANT

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

         POSIX::localeconv()
         POSIX::wcstombs()
         POSIX::wctomb()

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

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

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

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

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

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

        or use the methods given in perlcall.

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

AUTHOR
    Originally written by Dean Roehrich <roehrich AT cray.com>.

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