path: root/pod/perlxs.pod

=head1 NAME

perlxs - XS language reference manual

=head1 DESCRIPTION

=head2 Introduction

XS is a language used to create an extension interface
between Perl and some C library which one wishes to use with
Perl.  The XS interface is combined with the library to
create a new library which can be linked to Perl.  An B<XSUB>
is a function in the XS language and is the core component
of the Perl application interface.

The XS compiler is called B<xsubpp>.  This compiler will embed
the constructs necessary to let an XSUB, which is really a C
function in disguise, manipulate Perl values and creates the
glue necessary to let Perl access the XSUB.  The compiler
uses B<typemaps> to determine how to map C function parameters
and variables to Perl values.  The default typemap handles
many common C types.  A supplement typemap must be created
to handle special structures and types for the library being
linked.

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

=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 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

When using C pointers the indirection operator C<*> should be considered
part of the type and the address operator C<&> should be considered part of
the variable, as is demonstrated in the rpcb_gettime() function above.  See
the section on typemaps for more about handling qualifiers and unary
operators in C types.

The function name and the return type must be placed on
separate lines.

  INCORRECT                        CORRECT

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

The function body 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.

  CORRECT

  double
  sin(x)
  double x

=head2 The Argument Stack

The argument stack is used to store the values which are
sent as parameters to the XSUB and to store the XSUB's
return value.  In reality all Perl functions keep their
values on this stack at 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 magic variable which always matches
the return type of the C library function.  The B<xsubpp> compiler will
supply this variable in each XSUB and by default will use it 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 supply a RETVAL variable for that function.  When using
the PPCODE: directive the RETVAL variable is not needed, unless used
explicitly.

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<truely> 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 "truely-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 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
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 ]

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, such as the sin() function above, the RETVAL variable is
automatically designated as an output value.  In 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);

=head2 The CODE: Keyword

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

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

=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

Function parameters are normally initialized with their
values from the argument stack.  The typemaps contain the
code segments which are used to transfer the Perl values to
the C parameters.  The programmer, however, is allowed to
override the typemaps and supply alternate initialization
code.

The following code demonstrates how to supply initialization code for
function parameters.  The initialization code is eval'd by the compiler
before it is added to the output so anything which should be interpreted
literally, such as double quotes, must be protected with backslashes.

     bool_t
     rpcb_gettime(host,timep)
          char *host = (char *)SvPV(ST(0),na);
          time_t &timep = 0;
          OUTPUT:
          timep

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

=head2 Default Parameter Values

Default values can be specified for function parameters by
placing an assignment statement in the parameter list.  The
default value may be a number or a string.  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.  Perl will call this
XSUB 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 before the
typemaps are expanded.  If a variable is declared in a CODE: block then that
variable will follow any typemap code.  This may result in a C syntax
error.  To force the variable to be declared before the typemap code, place
it into a PREINIT: block.  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

A 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

=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 this XSUB contains a comment like C</*scope*/> then scoping will
automatically be 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

=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(ST(1), na);
		  RETVAL = rpcb_gettime( host, &timep );
          OUTPUT:
          timep
          RETVAL

=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 are not used
together within the same XSUB.

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 called 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");

=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 mneumonic 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:
          ST(0) = sv_newmortal();
          if( rpcb_gettime( host, &timep ) ){
               sv_setnv( ST(0), (double)timep);
          }
          else{
               ST(0) = &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<perlguts/"API LISTING"> for
other C<XSRETURN> macros.

=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 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

=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(ST(1), na);
		  RETVAL = rpcb_gettime( host, &timep );
          OUTPUT:
          timep
          RETVAL

=head2 The ALIAS: Keyword

The ALIAS: keyword allows an XSUB to have two 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 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 & unary operator is used to tell the compiler that it should dereference
the object when it calls the C function.  This is used when a CODE: block is
not used and the object is a not a pointer type (the object is an C<int> or
C<long> but not a C<int*> or C<long*>).

The following XSUB will generate incorrect C code.  The 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 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 Comments and C Preprocessor Directives

C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
CODE:, PPCODE:, 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.

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 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 a function is defined as a C++ method then it will assume
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 method is defined as static 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 functions will expect an object as the first parameter.  The xsubpp
compiler will call that object C<THIS> and will use it to call the specified
method.  So in the C++ code the blue() and set_blue() methods will be called
in the following manner.

     RETVAL = THIS->blue();

     THIS->set_blue( val );

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.

     void
     color::DESTROY()

The C++ code will call C<delete>.

     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 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 is often sufficient.  The interface will often be
very C-like and occasionally nonintuitive, especially when the C function
modifies one of its parameters.  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 which modify their parameters.  The XSUBs for
these functions may be able to return lists to Perl, or may be
candidates to return undef or an empty list in case of failure.

Identify which values are used by only the C and XSUB functions
themselves.  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 can be handled in XS with the & unary operator on
the variable name while others will require the use of the * operator on
the type name.  In general it is easier to work with the & operator.

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.

=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 labeled C<TYPEMAP>, C<INPUT>, and
C<OUTPUT>.  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.
Each of the sections of the typemap must be preceded by one of the TYPEMAP,
INPUT, or OUTPUT keywords.

The default typemap in the C<ext> 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

=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

Dean Roehrich <F<roehrich@cray.com>>
Jul 8, 1996