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+=head1 NAME
+
+perlxs - XS language reference manual
+
+=head1 DESCRIPTION
+
+=head2 Introduction
+
+XS is an interface description file format used to create an extension
+interface between Perl and C code (or a C library) which one wishes
+to use with Perl. The XS interface is combined with the library to
+create a new library which can then be either dynamically loaded
+or statically linked into perl. The XS interface description is
+written in the XS language and is the core component of the Perl
+extension interface.
+
+An B<XSUB> forms the basic unit of the XS interface. After compilation
+by the B<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 I<description> of what should be done by
+the glue, and let the XS compiler B<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 C<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 B<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 B<typemaps> to determine how to map C function parameters
+and output values to Perl values and back. The default typemap
+(which comes with Perl) handles many common C types. A supplementary
+typemap may also be needed to handle any special structures and types
+for the library being linked.
+
+A file in XS format starts with a C language section which goes until the
+first C<MODULE =Z<>> 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. B<xsubpp> recognizes and
+skips POD (see L<perlpod>) in both the C and XS language sections, which
+allows the XS file to contain embedded documentation.
+
+See L<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.
+
+=head2 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
+C<EXTERN.h>, C<perl.h>, and C<XSUB.h>, will always be present at the
+beginning of an XS file. This approach and others will be
+expanded later in this document.
+
+ #include "EXTERN.h"
+ #include "perl.h"
+ #include "XSUB.h"
+ #include <rpc/rpc.h>
+
+ MODULE = RPC PACKAGE = RPC
+
+ bool_t
+ rpcb_gettime(host,timep)
+ char *host
+ time_t &timep
+ OUTPUT:
+ timep
+
+Any extension to Perl, including those containing XSUBs,
+should have a Perl module to serve as the bootstrap which
+pulls the extension into Perl. This module will export the
+extension's functions and variables to the Perl program and
+will cause the extension's XSUBs to be linked into Perl.
+The following module will be used for most of the examples
+in this document and should be used from Perl with the C<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.
+
+=head2 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 B<xsubpp> like this:
+
+ char * s
+ char &c
+
+Both these XS declarations correspond to the C<char*> C type, but they have
+different semantics, see L<"The & Unary Operator">.
+
+It is convenient to think that the indirection operator
+C<*> should be considered as a part of the type and the address operator C<&>
+should be considered part of the variable. See L<"The Typemap">
+for more info about handling qualifiers and unary operators in C types.
+
+The function name and the return type must be placed on
+separate lines and should be flush left-adjusted.
+
+ INCORRECT CORRECT
+
+ double sin(x) double
+ double x sin(x)
+ double x
+
+The rest of the function description may be indented or left-adjusted. The
+following example shows a function with its body left-adjusted. Most
+examples in this document will indent the body for better readability.
+
+ CORRECT
+
+ double
+ sin(x)
+ double x
+
+More complicated XSUBs may contain many other sections. Each section of
+an XSUB starts with the corresponding 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 L<The INPUT: Keyword>.)
+
+An XSUB section continues until another section-start keyword is found.
+
+=head2 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 B<ST(x)>, where I<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 B<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.
+
+=head2 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 B<xsubpp> compiler will declare this variable in each XSUB
+with non-C<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 C<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, C<void> return value should be used
+only for subroutines which do not return a value, I<even if> CODE:
+directive is used which sets ST(0) explicitly.
+
+Older versions of this document recommended to use C<void> return
+value in such cases. It was discovered that this could lead to
+segfaults in cases when XSUB was I<truly> C<void>. This practice is
+now deprecated, and may be not supported at some future version. Use
+the return value C<SV *> in such cases. (Currently C<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 C<SV *> as return value.)
+
+=head2 Returning SVs, AVs and HVs through RETVAL
+
+When you're using RETVAL to return an C<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 L<perlguts>.) To make your life
+easier, the typemap file automatically makes C<RETVAL> mortal when
+you're returning an C<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 C<SV *>, it's unfortunately not as easy
+to have C<AV *> or C<HV *> as a return value. You I<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 C<AV *>
+is not properly decremented. Thus, the above XSUB would leak memory
+whenever it is being called. The same problem exists for C<HV *>.
+
+When you're returning an C<AV *> or a C<HV *>, you have to make sure
+their reference count is decremented by making the AV or HV mortal:
+
+ AV *
+ array()
+ CODE:
+ RETVAL = newAV();
+ sv_2mortal((SV*)RETVAL);
+ /* do something with RETVAL */
+ OUTPUT:
+ RETVAL
+
+And also remember that you don't have to do this for an C<SV *>.
+
+=head2 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
+
+=head2 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.
+
+=head2 The PREFIX Keyword
+
+The PREFIX keyword designates prefixes which should be
+removed from the Perl function names. If the C function is
+C<rpcb_gettime()> and the PREFIX value is C<rpcb_> then Perl will
+see this function as C<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_
+
+=head2 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 B<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);
+
+B<xsubpp> emits an automatic C<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 C<SETMAGIC: DISABLE> line
+to disable it for the remainder of the parameters in the OUTPUT section.
+Likewise, C<SETMAGIC: ENABLE> can be used to reenable it for the
+remainder of the OUTPUT section. See L<perlguts> for more details
+about 'set' magic.
+
+=head2 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-C<void> return type, the return value of this C subroutine should not
+be returned from the generated Perl subroutine.
+
+With this keyword present L<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 C<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.
+
+=head2 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
+
+=head2 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");
+
+=head2 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 B<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
+
+=head2 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
+C<=>, C<;> or C<+> on a line in the INPUT: section. The only
+exception happens if this C<;> terminates the line, then this C<;>
+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 C<$>, C<@>, or C<\\>]
+must be protected with backslashes. The variables $var, $arg,
+and $type can be used as in typemaps.
+
+ bool_t
+ rpcb_gettime(host,timep)
+ char *host = (char *)SvPV_nolen($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 C<=>, then it is output in
+the declaration for the input variable, replacing the initialization
+supplied by the typemap. If the initialization
+begins with C<;> or C<+>, then it is performed after
+all of the input variables have been declared. In the C<;>
+case the initialization normally supplied by the typemap is not performed.
+For the C<+> case, the declaration for the variable will include the
+initialization from the typemap. A global
+variable, C<%v>, is available for the truly rare case where
+information from one initialization is needed in another
+initialization.
+
+Here's a truly obscure example:
+
+ bool_t
+ rpcb_gettime(host,timep)
+ time_t &timep; /* \$v{timep}=@{[$v{timep}=$arg]} */
+ char *host + SvOK($v{timep}) ? SvPV_nolen($arg) : NULL;
+ OUTPUT:
+ timep
+
+The construct C<\$v{timep}=@{[$v{timep}=$arg]}> used in the above
+example has a two-fold purpose: first, when this line is processed by
+B<xsubpp>, the Perl snippet C<$v{timep}=$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 C<char *host> line,
+$arg will evaluate to C<ST(0)>, and C<$v{timep}> will evaluate to
+C<ST(1)>.
+
+=head2 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 C<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
+
+=head2 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 C<;>-type or C<+>-type initialization of
+parameters is used (see L<"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 C<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 C<MyObject> in the INPUT: section and from
+MyObject when processing RETVAL will modify a global variable C<global_state>.
+After these conversions are performed, we restore the old value of
+C<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
+
+=head2 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 C</*scope*/> then scoping will
+be automatically enabled for that XSUB.
+
+To enable scoping:
+
+ SCOPE: ENABLE
+
+To disable scoping:
+
+ SCOPE: DISABLE
+
+=head2 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 C<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 C<char *> is a "simple" one,
+thus C<host> is initialized on the declaration line, and our assignment
+C<h = host> is not performed too early. Otherwise one would need to have the
+assignment C<h = host> in a CODE: or INIT: section.)
+
+=head2 The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
+
+In the list of parameters for an XSUB, one can precede parameter names
+by the C<IN>/C<OUTLIST>/C<IN_OUTLIST>/C<OUT>/C<IN_OUT> keywords.
+C<IN> keyword is the default, the other keywords indicate how the Perl
+interface should differ from the C interface.
+
+Parameters preceded by C<OUTLIST>/C<IN_OUTLIST>/C<OUT>/C<IN_OUT>
+keywords are considered to be used by the C subroutine I<via
+pointers>. C<OUTLIST>/C<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 C<OUTLIST> keyword do not appear in the usage
+signature of the generated Perl function.
+
+Parameters preceded by C<IN_OUTLIST>/C<IN_OUT>/C<OUT> I<do> appear as
+parameters to the Perl function. With the exception of
+C<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 C<void> return type or
+C<The NO_OUTPUT Keyword> was used) followed by all the C<OUTLIST>
+and C<IN_OUTLIST> parameters (in the order of appearance). On the
+return from the XSUB the C<IN_OUT>/C<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, $month) = day_month(time);
+
+The C signature of the corresponding function should be
+
+ void day_month(int *day, int unix_time, int *month);
+
+The C<IN>/C<OUTLIST>/C<IN_OUTLIST>/C<IN_OUT>/C<OUT> keywords can be
+mixed with ANSI-style declarations, as in
+
+ void
+ day_month(OUTLIST int day, int unix_time, OUTLIST int month)
+
+(here the optional C<IN> keyword is omitted).
+
+The C<IN_OUT> parameters are identical with parameters introduced with
+L<The & Unary Operator> and put into the C<OUTPUT:> section (see
+L<The OUTPUT: Keyword>). The C<IN_OUTLIST> parameters are very similar,
+the only difference being that the value C function writes through the
+pointer would not modify the Perl parameter, but is put in the output
+list.
+
+The C<OUTLIST>/C<OUT> parameter differ from C<IN_OUTLIST>/C<IN_OUT>
+parameters only by the initial value of the Perl parameter not
+being read (and not being given to the C function - which gets some
+garbage instead). For example, the same C function as above can be
+interfaced with as
+
+ void day_month(OUT int day, int unix_time, OUT int month);
+
+or
+
+ void
+ day_month(day, unix_time, month)
+ int &day = NO_INIT
+ int unix_time
+ int &month = NO_INIT
+ OUTPUT:
+ day
+ month
+
+However, the generated Perl function is called in very C-ish style:
+
+ my ($day, $month);
+ day_month($day, time, $month);
+
+=head2 The C<length(NAME)> Keyword
+
+If one of the input arguments to the C function is the length of a string
+argument C<NAME>, one can substitute the name of the length-argument by
+C<length(NAME)> in the XSUB declaration. This argument must be omitted when
+the generated Perl function is called. E.g.,
+
+ void
+ dump_chars(char *s, short l)
+ {
+ short n = 0;
+ while (n < l) {
+ printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
+ n++;
+ }
+ }
+
+ MODULE = x PACKAGE = x
+
+ void dump_chars(char *s, short length(s))
+
+should be called as C<dump_chars($string)>.
+
+This directive is supported with ANSI-type function declarations only.
+
+=head2 Variable-length Parameter Lists
+
+XSUBs can have variable-length parameter lists by specifying an ellipsis
+C<(...)> 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 C<items> variable which the
+B<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 I<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 *)SvPV_nolen(ST(1));
+ RETVAL = rpcb_gettime( host, &timep );
+ OUTPUT:
+ timep
+ RETVAL
+
+=head2 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
+C<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
+
+=head2 The PPCODE: Keyword
+
+The PPCODE: keyword is an alternate form of the CODE: keyword and is used
+to tell the B<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 C<SP> macro (which stands for the I<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 C<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 C<void>ness of the
+return value of the C function, and heuristics mentioned in
+L<"The RETVAL Variable">). The trailer generated for a PPCODE: section
+is based on the number of return values and on the number of times
+C<SP> was updated by C<[X]PUSH*()> macros.
+
+Note that macros C<ST(i)>, C<XST_m*()> and C<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 C<void> return type for this function tells the B<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
+B<xsubpp> compiler to create a stack pointer available as C<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 L<perlguts> for details about 'set' magic.
+
+=head2 Returning Undef And Empty Lists
+
+Occasionally the programmer will want to return simply
+C<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 C<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 C<return> in the above
+XSUB, rather than letting control fall through to the end. In those
+situations C<XSRETURN_EMPTY> should be used, instead. This will ensure that
+the XSUB stack is properly adjusted. Consult L<perlapi> for other
+C<XSRETURN> macros.
+
+Since C<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;
+
+=head2 The REQUIRE: Keyword
+
+The REQUIRE: keyword is used to indicate the minimum version of the
+B<xsubpp> compiler needed to compile the XS module. An XS module which
+contains the following statement will compile with only B<xsubpp> version
+1.922 or greater:
+
+ REQUIRE: 1.922
+
+=head2 The CLEANUP: Keyword
+
+This keyword can be used when an XSUB requires special cleanup procedures
+before it terminates. When the CLEANUP: keyword is used it must follow
+any CODE:, PPCODE:, or OUTPUT: blocks which are present in the XSUB. The
+code specified for the cleanup block will be added as the last statements
+in the XSUB.
+
+=head2 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 L<"The NO_OUTPUT Keyword"> and L<"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.
+
+=head2 The BOOT: Keyword
+
+The BOOT: keyword is used to add code to the extension's bootstrap
+function. The bootstrap function is generated by the B<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");
+
+=head2 The VERSIONCHECK: Keyword
+
+The VERSIONCHECK: keyword corresponds to B<xsubpp>'s C<-versioncheck> and
+C<-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.
+
+=head2 The PROTOTYPES: Keyword
+
+The PROTOTYPES: keyword corresponds to B<xsubpp>'s C<-prototypes> and
+C<-noprototypes> options. This keyword overrides the command line options.
+Prototypes are enabled by default. When prototypes are enabled XSUBs will
+be given Perl prototypes. This keyword may be used multiple times in an XS
+module to enable and disable prototypes for different parts of the module.
+
+To enable prototypes:
+
+ PROTOTYPES: ENABLE
+
+To disable prototypes:
+
+ PROTOTYPES: DISABLE
+
+=head2 The PROTOTYPE: Keyword
+
+This keyword is similar to the PROTOTYPES: keyword above but can be used to
+force B<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 L<perlsub/Prototypes> 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 *)SvPV_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
+ ...
+
+=head2 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 C<ix> which contain the
+index of the alias which was used. When the XSUB is called with its
+declared name C<ix> will be 0.
+
+The following example will create aliases C<FOO::gettime()> and
+C<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
+
+=head2 The OVERLOAD: Keyword
+
+Instead of writing an overloaded interface using pure Perl, you
+can also use the OVERLOAD keyword to define additional Perl names
+for your functions (like the ALIAS: keyword above). However, the
+overloaded functions must be defined with three parameters (except
+for the nomethod() function which needs four parameters). If any
+function has the OVERLOAD: keyword, several additional lines
+will be defined in the c file generated by xsubpp in order to
+register with the overload magic.
+
+Since blessed objects are actually stored as RV's, it is useful
+to use the typemap features to preprocess parameters and extract
+the actual SV stored within the blessed RV. See the sample for
+T_PTROBJ_SPECIAL below.
+
+To use the OVERLOAD: keyword, create an XS function which takes
+three input parameters ( or use the c style '...' definition) like
+this:
+
+ SV *
+ cmp (lobj, robj, swap)
+ My_Module_obj lobj
+ My_Module_obj robj
+ IV swap
+ OVERLOAD: cmp <=>
+ { /* function defined here */}
+
+In this case, the function will overload both of the three way
+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).
+
+=head2 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
+L<overload/Fallback> for more details.
+
+=head2 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
+C<XSINTERFACE_FUNC_SET>, see the next section.)
+
+=head2 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, C<CV*>,
+and C<XSANY.any_dptr> for this C<CV*>. The setter macro is given cv,
+and the function pointer.
+
+The default value is C<XSINTERFACE_FUNC> and C<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
+C<fp[]> with offsets being C<multiply_off>, C<divide_off>, C<add_off>,
+C<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.
+
+=head2 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 F<Rpcb1.xsh> contains our C<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 (C<|>) then
+the compiler will interpret the parameters as a command.
+
+ INCLUDE: cat Rpcb1.xsh |
+
+=head2 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 C<ix> ALIAS:
+variable (see L<"The ALIAS: Keyword">), or maybe via the C<items> variable
+(see L<"Variable-length Parameter Lists">). The last CASE: becomes the
+B<default> case if it is not associated with a conditional. The following
+example shows CASE switched via C<ix> with a function C<rpcb_gettime()>
+having an alias C<x_gettime()>. When the function is called as
+C<rpcb_gettime()> its parameters are the usual C<(char *host, time_t *timep)>,
+but when the function is called as C<x_gettime()> its parameters are
+reversed, C<(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 );
+
+=head2 The & Unary Operator
+
+The C<&> unary operator in the INPUT: section is used to tell B<xsubpp>
+that it should convert a Perl value to/from C using the C type to the left
+of C<&>, 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
+C<int> or C<long> but not an C<int*> or C<long*>).
+
+The following XSUB will generate incorrect C code. The B<xsubpp> compiler will
+turn this into code which calls C<rpcb_gettime()> with parameters C<(char
+*host, time_t timep)>, but the real C<rpcb_gettime()> wants the C<timep>
+parameter to be of type C<time_t*> rather than C<time_t>.
+
+ bool_t
+ rpcb_gettime(host,timep)
+ char *host
+ time_t timep
+ OUTPUT:
+ timep
+
+That problem is corrected by using the C<&> operator. The B<xsubpp> compiler
+will now turn this into code which calls C<rpcb_gettime()> correctly with
+parameters C<(char *host, time_t *timep)>. It does this by carrying the
+C<&> through, so the function call looks like C<rpcb_gettime(host, &timep)>.
+
+ bool_t
+ rpcb_gettime(host,timep)
+ char *host
+ time_t &timep
+ OUTPUT:
+ timep
+
+=head2 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 C<=cut> command;
+C<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 C<=>. 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 C<#> 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 C<#>.
+
+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 B<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.
+
+=head2 Using XS With C++
+
+If an XSUB name contains C<::>, 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 C<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-E<gt>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 C<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 B<DESTROY> then the C++ C<delete> function will be
+called and C<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 B<new> then the C++ 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 C<CLASS>, to be given as the first
+argument.
+
+ color *
+ color::new()
+
+The generated C++ code will call C<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;
+ }
+
+=head2 Interface Strategy
+
+When designing an interface between Perl and a C library a straight
+translation from C to XS (such as created by C<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 C<negative_is_failure> which converts negative values to C<undef>, or
+maybe croak()s. After this the return value of type C<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 C<&> unary operator,
+and, possibly, using the NO_INIT keyword.
+Some others will require handling of types like C<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 C<h2xs -x>.)
+
+If the same C type is used in several different contexts which require
+different translations, C<typedef> several new types mapped to this C type,
+and create separate F<typemap> entries for these new types. Use these
+types in declarations of return type and parameters to XSUBs.
+
+=head2 Perl Objects And C Structures
+
+When dealing with C structures one should select either
+B<T_PTROBJ> or B<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 B<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 C<typedef> will be created for C<struct netconfig>. The Perl
+object will be blessed in a class matching the name of the C
+type, with the tag C<Ptr> appended, and the name should not
+have embedded spaces if it will be a Perl package name. The
+destructor will be placed in a class corresponding to the
+class of the object and the PREFIX keyword will be used to
+trim the name to the word DESTROY as Perl will expect.
+
+ typedef struct netconfig Netconfig;
+
+ MODULE = RPC PACKAGE = RPC
+
+ Netconfig *
+ getnetconfigent(netid)
+ char *netid
+
+ MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
+
+ void
+ rpcb_DESTROY(netconf)
+ Netconfig *netconf
+ CODE:
+ printf("Now in NetconfigPtr::DESTROY\n");
+ free( netconf );
+
+This example requires the following typemap entry. Consult the typemap
+section for more information about adding new typemaps for an 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.
+
+=head2 The Typemap
+
+The typemap is a collection of code fragments which are used by the B<xsubpp>
+compiler to map C function parameters and values to Perl values. The
+typemap file may consist of three sections labelled C<TYPEMAP>, C<INPUT>, and
+C<OUTPUT>. An unlabelled initial section is assumed to be a C<TYPEMAP>
+section. The INPUT section tells
+the compiler how to translate Perl values
+into variables of certain C types. The OUTPUT section tells the compiler
+how to translate the values from certain C types into values Perl can
+understand. The TYPEMAP section tells the compiler which of the INPUT and
+OUTPUT code fragments should be used to map a given C type to a Perl value.
+The section labels C<TYPEMAP>, C<INPUT>, or C<OUTPUT> must begin
+in the first column on a line by themselves, and must be in uppercase.
+
+The default typemap in the C<lib/ExtUtils> directory of the Perl source
+contains many useful types which can be used by Perl extensions. Some
+extensions define additional typemaps which they keep in their own directory.
+These additional typemaps may reference INPUT and OUTPUT maps in the main
+typemap. The B<xsubpp> compiler will allow the extension's own typemap to
+override any mappings which are in the default typemap.
+
+Most extensions which require a custom typemap will need only the TYPEMAP
+section of the typemap file. The custom typemap used in the
+getnetconfigent() example shown earlier demonstrates what may be the typical
+use of extension typemaps. That typemap is used to equate a C structure
+with the T_PTROBJ typemap. The typemap used by getnetconfigent() is shown
+here. Note that the C type is separated from the XS type with a tab and
+that the C unary operator C<*> is considered to be a part of the C type name.
+
+ TYPEMAP
+ Netconfig *<tab>T_PTROBJ
+
+Here's a more complicated example: suppose that you wanted C<struct
+netconfig> to be blessed into the class C<Net::Config>. One way to do
+this is to use underscores (_) to separate package names, as follows:
+
+ typedef struct netconfig * Net_Config;
+
+And then provide a typemap entry C<T_PTROBJ_SPECIAL> that maps underscores to
+double-colons (::), and declare C<Net_Config> to be of that type:
+
+
+ TYPEMAP
+ Net_Config T_PTROBJ_SPECIAL
+
+ INPUT
+ T_PTROBJ_SPECIAL
+ if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
+ IV tmp = SvIV((SV*)SvRV($arg));
+ $var = INT2PTR($type, tmp);
+ }
+ else
+ croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")
+
+ OUTPUT
+ T_PTROBJ_SPECIAL
+ sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",
+ (void*)$var);
+
+The INPUT and OUTPUT sections substitute underscores for double-colons
+on the fly, giving the desired effect. This example demonstrates some
+of the power and versatility of the typemap facility.
+
+The INT2PTR macro (defined in perl.h) casts an integer to a pointer,
+of a given type, taking care of the possible different size of integers
+and pointers. There are also PTR2IV, PTR2UV, PTR2NV macros,
+to map the other way, which may be useful in OUTPUT sections.
+
+=head2 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 C<-g> (C<--global>) option with h2xs (see L<h2xs>).
+
+Below is an example module that makes use of the macros.
+
+ #include "EXTERN.h"
+ #include "perl.h"
+ #include "XSUB.h"
+
+ /* Global Data */
+
+ #define MY_CXT_KEY "BlindMice::_guts" XS_VERSION
+
+ typedef struct {
+ int count;
+ char name[3][100];
+ } my_cxt_t;
+
+ START_MY_CXT
+
+ MODULE = BlindMice PACKAGE = BlindMice
+
+ BOOT:
+ {
+ MY_CXT_INIT;
+ MY_CXT.count = 0;
+ strcpy(MY_CXT.name[0], "None");
+ strcpy(MY_CXT.name[1], "None");
+ strcpy(MY_CXT.name[2], "None");
+ }
+
+ int
+ newMouse(char * name)
+ char * name;
+ PREINIT:
+ dMY_CXT;
+ CODE:
+ if (MY_CXT.count >= 3) {
+ warn("Already have 3 blind mice");
+ RETVAL = 0;
+ }
+ else {
+ RETVAL = ++ MY_CXT.count;
+ strcpy(MY_CXT.name[MY_CXT.count - 1], name);
+ }
+
+ char *
+ get_mouse_name(index)
+ int index
+ CODE:
+ dMY_CXT;
+ RETVAL = MY_CXT.lives ++;
+ if (index > MY_CXT.count)
+ croak("There are only 3 blind mice.");
+ else
+ RETVAL = newSVpv(MY_CXT.name[index - 1]);
+
+ void
+ CLONE(...)
+ CODE:
+ MY_CXT_CLONE;
+
+B<REFERENCE>
+
+=over 5
+
+=item 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
+
+=item typedef my_cxt_t
+
+This struct typedef I<must> always be called C<my_cxt_t>. The other
+C<CXT*> macros assume the existence of the C<my_cxt_t> typedef name.
+
+Declare a typedef named C<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;
+
+=item START_MY_CXT
+
+Always place the START_MY_CXT macro directly after the declaration
+of C<my_cxt_t>.
+
+=item MY_CXT_INIT
+
+The MY_CXT_INIT macro initialises storage for the C<my_cxt_t> struct.
+
+It I<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 C<MY_CXT_CLONE> below.)
+
+=item dMY_CXT
+
+Use the dMY_CXT macro (a declaration) in all the functions that access
+MY_CXT.
+
+=item MY_CXT
+
+Use the MY_CXT macro to access members of the C<my_cxt_t> struct. For
+example, if C<my_cxt_t> is
+
+ typedef struct {
+ int index;
+ } my_cxt_t;
+
+then use this to access the C<index> member
+
+ dMY_CXT;
+ MY_CXT.index = 2;
+
+=item aMY_CXT/pMY_CXT
+
+C<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 C<aMY_CXT>/C<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 C<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. C<_aMY_CXT>, C<aMY_CXT_>, C<_pMY_CXT> and C<pMY_CXT_>.
+
+=item MY_CXT_CLONE
+
+By default, when a new interpreter is created as a copy of an existing one
+(eg via C<< threads->create() >>), both interpreters share the same physical
+my_cxt_t structure. Calling C<MY_CXT_CLONE> (typically via the package's
+C<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.
+
+=item MY_CXT_INIT_INTERP(my_perl)
+
+=item dMY_CXT_INTERP(my_perl)
+
+These are versions of the macros which take an explicit interpreter as an
+argument.
+
+=back
+
+Note that these macros will only work together within the I<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.
+
+=head2 Thread-aware system interfaces
+
+Starting from Perl 5.8, in C/C++ level Perl knows how to wrap
+system/library interfaces that have thread-aware versions
+(e.g. getpwent_r()) into frontend macros (e.g. getpwent()) that
+correctly handle the multithreaded interaction with the Perl
+interpreter. This will happen transparently, the only thing
+you need to do is to instantiate a Perl interpreter.
+
+This wrapping happens always when compiling Perl core source
+(PERL_CORE is defined) or the Perl core extensions (PERL_EXT is
+defined). When compiling XS code outside of Perl core the wrapping
+does not take place. Note, however, that intermixing the _r-forms
+(as Perl compiled for multithreaded operation will do) and the _r-less
+forms is neither well-defined (inconsistent results, data corruption,
+or even crashes become more likely), nor is it very portable.
+
+=head1 EXAMPLES
+
+File C<RPC.xs>: Interface to some ONC+ RPC bind library functions.
+
+ #include "EXTERN.h"
+ #include "perl.h"
+ #include "XSUB.h"
+
+ #include <rpc/rpc.h>
+
+ typedef struct netconfig Netconfig;
+
+ MODULE = RPC PACKAGE = RPC
+
+ SV *
+ rpcb_gettime(host="localhost")
+ char *host
+ PREINIT:
+ time_t timep;
+ CODE:
+ ST(0) = sv_newmortal();
+ if( rpcb_gettime( host, &timep ) )
+ sv_setnv( ST(0), (double)timep );
+
+ Netconfig *
+ getnetconfigent(netid="udp")
+ char *netid
+
+ MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
+
+ void
+ rpcb_DESTROY(netconf)
+ Netconfig *netconf
+ CODE:
+ printf("NetconfigPtr::DESTROY\n");
+ free( netconf );
+
+File C<typemap>: Custom typemap for RPC.xs.
+
+ TYPEMAP
+ Netconfig * T_PTROBJ
+
+File C<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 C<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";
+
+
+=head1 XS VERSION
+
+This document covers features supported by C<xsubpp> 1.935.
+
+=head1 AUTHOR
+
+Originally written by Dean Roehrich <F<roehrich@cray.com>>.
+
+Maintained since 1996 by The Perl Porters <F<perlbug@perl.org>>.