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

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

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

NNAAMMEE #

 perlxs - XS language reference manual

DDEESSCCRRIIPPTTIIOONN #

IInnttrroodduuccttiioonn 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 XXSSUUBB forms the basic unit of the XS interface.  After compilation by
 the xxssuubbpppp 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, calls this C
 function, and then 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 _d_e_s_c_r_i_p_t_i_o_n of what should be done by the glue, and
 let the XS compiler xxssuubbpppp 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 xxssuubbpppp.  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 ttyyppeemmaappss 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.  xxssuubbpppp 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.

OOnn TThhee RRooaadd Many of the examples which follow will concentrate on creating an interface between Perl and the ONC+ RPC bind library functions. The rrppccbb__ggeettttiimmee(()) 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 rrppccbb__ggeettttiimmee(()) 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 rrppccbb__ggeettttiimmee(())
 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 rrppccbb__ggeettttiimmee(())
 function, and the XSUB must always ensure that the real rrppccbb__ggeettttiimmee(())
 function is called with the correct parameters.  This abstraction will
 allow the programmer to create a more Perl-like interface to the C
 function.

TThhee AAnnaattoommyy ooff aann XXSSUUBB 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 ssiinn(()).  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 to xxssuubbpppp 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.

TThhee AArrgguummeenntt SSttaacckk 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 SSTT((xx)), 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 SSTT(0).  The XSUB's incoming
 parameters and outgoing return values always begin at SSTT(0).  For many
 simple cases the xxssuubbpppp 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.

TThhee RREETTVVAALL VVaarriiaabbllee 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 xxssuubbpppp 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 SSTT(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, _e_v_e_n _i_f CODE: directive is
 used which sets SSTT(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 _t_r_u_l_y "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.)

RReettuurrnniinngg SSVVss,, AAVVss aanndd HHVVss tthhrroouugghh RREETTVVAALL 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 _s_h_o_u_l_d 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.

TThhee MMOODDUULLEE KKeeyywwoorrdd 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 #

TThhee PPAACCKKAAGGEE KKeeyywwoorrdd 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.

TThhee PPRREEFFIIXX KKeeyywwoorrdd 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_

TThhee OOUUTTPPUUTT:: KKeeyywwoorrdd 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 ssiinn(()) function above, the RETVAL variable is automatically designated as an output value. For more complex functions the xxssuubbpppp 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);

 xxssuubbpppp 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.

TThhee NNOO__OOUUTTPPUUTT KKeeyywwoorrdd 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 ddiiee(())
 with a meaningful error message on error.

TThhee CCOODDEE:: KKeeyywwoorrdd 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 #

TThhee IINNIITT:: KKeeyywwoorrdd 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");

TThhee NNOO__IINNIITT KKeeyywwoorrdd The NO_INIT keyword is used to indicate that a function parameter is being used only as an output value. The xxssuubbpppp 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 rrppccbb__ggeettttiimmee(()) 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

TThhee TTYYPPEEMMAAPP:: KKeeyywwoorrdd 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.

IInniittiiaalliizziinngg FFuunnccttiioonn PPaarraammeetteerrss 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 xxssuubbpppp, 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).

DDeeffaauulltt PPaarraammeetteerr VVaalluueess 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 rrppccbb__ggeettttiimmee(()) to have a default host value the
 parameters to the XSUB could be rearranged.  The XSUB will then call the
 real rrppccbb__ggeettttiimmee(()) 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 rrppccbb__ggeettttiimmee(()) 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 #

TThhee PPRREEIINNIITT:: KKeeyywwoorrdd 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 mmuuttaattee(()) can be rewritten
 as

     MyObject
     mutate(o)
           MyState st = global_state;
           MyObject o;

CLEANUP: #

           reset_to(global_state, st);

 and the code for rrppccbb__ggeettttiimmee(()) can be rewritten as

      bool_t
      rpcb_gettime(timep)
           time_t timep = NO_INIT
           char *host = "localhost";

C_ARGS: #

           host, &timep

OUTPUT: #

           timep

RETVAL #

TThhee SSCCOOPPEE:: KKeeyywwoorrdd 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 #

TThhee IINNPPUUTT:: KKeeyywwoorrdd 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.)

TThhee IINN//OOUUTTLLIISSTT//IINN__OOUUTTLLIISSTT//OOUUTT//IINN__OOUUTT KKeeyywwoorrddss 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 _v_i_a _p_o_i_n_t_e_r_s.  "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" _d_o 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);

TThhee “"lleennggtthh((NNAAMMEE))“” KKeeyywwoorrdd 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.

VVaarriiaabbllee--lleennggtthh PPaarraammeetteerr LLiissttss 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 xxssuubbpppp compiler supplies for all XSUBs. By using this mechanism one can create an XSUB which accepts a list of parameters of unknown length.

 The _h_o_s_t parameter for the rrppccbb__ggeettttiimmee(()) 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 #

TThhee CC__AARRGGSS:: KKeeyywwoorrdd 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

TThhee PPPPCCOODDEE:: KKeeyywwoorrdd The PPCODE: keyword is an alternate form of the CODE: keyword and is used to tell the xxssuubbpppp 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 _c_u_r_r_e_n_t 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 rrppccbb__ggeettttiimmee(()) 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 rrppccbb__ggeettttiimmee(()) function called and to have the return values
 properly placed on the argument stack.

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

 Now the rrppccbb__ggeettttiimmee(()) 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.

RReettuurrnniinngg UUnnddeeff AAnndd EEmmppttyy LLiissttss Occasionally the programmer will want to return simply “undef” or an empty list if a function fails rather than a separate status value. The rrppccbb__ggeettttiimmee(()) 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 ssvv__nneewwmmoorrttaall(()) 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; #

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

REQUIRE: 1.922 #

TThhee CCLLEEAANNUUPP:: KKeeyywwoorrdd 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.

TThhee PPOOSSTTCCAALLLL:: KKeeyywwoorrdd 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.

TThhee BBOOOOTT:: KKeeyywwoorrdd The BOOT: keyword is used to add code to the extension’s bootstrap function. The bootstrap function is generated by the xxssuubbpppp 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");

TThhee VVEERRSSIIOONNCCHHEECCKK:: KKeeyywwoorrdd The VERSIONCHECK: keyword corresponds to xxssuubbpppp’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.

TThhee PPRROOTTOOTTYYPPEESS:: KKeeyywwoorrdd The PROTOTYPES: keyword corresponds to xxssuubbpppp’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 xxssuubbpppp 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 #

TThhee PPRROOTTOOTTYYPPEE:: KKeeyywwoorrdd This keyword is similar to the PROTOTYPES: keyword above but can be used to force xxssuubbpppp 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 #

     ...

TThhee AALLIIAASS:: KKeeyywwoorrdd 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

TThhee OOVVEERRLLOOAADD:: KKeeyywwoorrdd 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 _o_r 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.

TThhee FFAALLLLBBAACCKK:: KKeeyywwoorrdd 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.

TThhee IINNTTEERRFFAACCEE:: KKeeyywwoorrdd 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 mmuullttiippllyy(()), ddiivviiddee(()), aadddd(()),
 ssuubbttrraacctt(()) 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 rreemmaaiinnddeerr(()) 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.)

TThhee IINNTTEERRFFAACCEE__MMAACCRROO:: KKeeyywwoorrdd 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 mmuullttiippllyy(()),
 ddiivviiddee(()), aadddd(()), ssuubbttrraacctt(()) 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.

TThhee IINNCCLLUUDDEE:: KKeeyywwoorrdd 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 _R_p_c_b_1_._x_s_h 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".

TThhee IINNCCLLUUDDEE__CCOOMMMMAANNDD:: KKeeyywwoorrdd 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 ...

TThhee CCAASSEE:: KKeeyywwoorrdd 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
 ddeeffaauulltt 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 );

TThhee EEXXPPOORRTT__XXSSUUBB__SSYYMMBBOOLLSS:: KKeeyywwoorrdd 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".

TThhee && UUnnaarryy OOppeerraattoorr The “&” unary operator in the INPUT: section is used to tell xxssuubbpppp 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 xxssuubbpppp 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 xxssuubbpppp 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

IInnsseerrttiinngg PPOODD,, CCoommmmeennttss aanndd CC PPrreepprroocceessssoorr DDiirreeccttiivveess 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 xxssuubbpppp 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.

UUssiinngg XXSS WWiitthh CC++++ 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 nneeww(()) function and should be blessed by Perl with the ssvv__sseettrreeff__ppvv(()) 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 ccllaassss::::mmeetthhoodd(()) syntax.  If the method is not static the function
 will be called using the THIS->mmeetthhoodd(()) 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 bblluuee(()) and sseett__bblluuee(()) 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 bblluuee(()) and
 sseett__bblluuee(()) 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 DDEESSTTRROOYY 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 nneeww 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; #

         }

IInntteerrffaaccee SSttrraatteeggyy 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
 ccrrooaakk(())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 _t_y_p_e_m_a_p entries for these new types.  Use these
 types in declarations of return type and parameters to XSUBs.

PPeerrll OObbjjeeccttss AAnndd CC SSttrruuccttuurreess When dealing with C structures one should select either TT__PPTTRROOBBJJ or TT__PPTTRRRREEFF 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 ggeettnneettccoonnffiiggeenntt(()) function which is used
 with ONC+ TIRPC.  The ggeettnneettccoonnffiiggeenntt(()) 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 ggeettnneettccoonnffiiggeenntt(()).
 Destructors in XS can be created by specifying an XSUB function whose
 name ends with the word DDEESSTTRROOYY.  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
 ggeettnneettccoonnffiiggeenntt(()) XSUB and an object created by a normal Perl subroutine.

SSaaffeellyy SSttoorriinngg SSttaattiicc DDaattaa iinn XXSS 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; #

_M_Y___C_X_T _R_E_F_E_R_E_N_C_E #

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 _m_u_s_t 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 _m_u_s_t 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 _s_a_m_e source
 file; that is, a dMY_CTX in one source file will access a different
 structure than a dMY_CTX in another source file.

TThhrreeaadd--aawwaarree ssyysstteemm iinntteerrffaacceess Starting from Perl 5.8, in C/C++ level Perl knows how to wrap system/library interfaces that have thread-aware versions (e.g. ggeettppwweenntt__rr(())) into frontend macros (e.g. ggeettppwweenntt(())) 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.

EEXXAAMMPPLLEESS #

 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.

CCAAVVEEAATTSS #

 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 sseettllooccaallee(()) 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 ssyynncc__llooccaallee(()) 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.

XXSS VVEERRSSIIOONN #

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

AAUUTTHHOORR #

 Originally written by Dean Roehrich <_r_o_e_h_r_i_c_h_@_c_r_a_y_._c_o_m>.

 Maintained since 1996 by The Perl Porters <_p_e_r_l_5_-_p_o_r_t_e_r_s_@_p_e_r_l_._o_r_g>.

perl v5.36.3 2023-02-15 PERLXS(1)