PERLXS(1) Perl Programmers Reference Guide PERLXS(1)
NAME
perlxs - XS language reference manual
DESCRIPTION
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
XSUB is a function in the XS language and is the core
component of the Perl application interface.
The XS compiler is called 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 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 the perlxstut manpage for a tutorial on the whole
extension creation process.
Note: For many extensions, Dave Beazley's SWIG system
provides a significantly more convenient mechanism for
creating the XS glue code. See the section on
/www.cs.utah.edu/~beazley/SWIG in the http: manpage for
more information.
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
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used from Perl with the following code. The $status and
$timep variables will contain the output of the function.
use RPC;
$status = rpcb_gettime( "localhost", $timep );
The following XS file shows an XS subroutine, or XSUB,
which demonstrates one possible interface to the
rpcb_gettime() function. This XSUB represents a direct
translation between C and Perl and so preserves the
interface even from Perl. This XSUB will be invoked from
Perl with the usage shown above. Note that the first
three #include statements, for EXTERN.h, perl.h, and
XSUB.h, will always be present at the beginning of an XS
file. This approach and others will be expanded later in
this document.
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <rpc/rpc.h>
MODULE = RPC PACKAGE = RPC
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
Any extension to Perl, including those containing XSUBs,
should have a Perl module to serve as the bootstrap which
pulls the extension into Perl. This module will export
the extension's functions and variables to the Perl
program and will cause the extension's XSUBs to be linked
into Perl. The following module will be used for most of
the examples in this document and should be used from Perl
with the use command as shown earlier. Perl modules are
explained in more detail later in this document.
package RPC;
require Exporter;
require DynaLoader;
@ISA = qw(Exporter DynaLoader);
@EXPORT = qw( rpcb_gettime );
bootstrap RPC;
1;
Throughout this document a variety of interfaces to the
rpcb_gettime() XSUB will be explored. The XSUBs will take
their parameters in different orders or will take
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different numbers of parameters. In each case the XSUB is
an abstraction between Perl and the real C rpcb_gettime()
function, and the XSUB must always ensure that the real
rpcb_gettime() function is called with the correct
parameters. This abstraction will allow the programmer to
create a more Perl-like interface to the C function.
The Anatomy of an XSUB
The 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 * should be
considered part of the type and the address operator &
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
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
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function will be referred to as position 0 for that
function.
XSUBs refer to their stack arguments with the macro ST(x),
where x refers to a position in this XSUB's part of the
stack. Position 0 for that function would be known to the
XSUB as ST(0). The XSUB's incoming parameters and
outgoing return values always begin at ST(0). For many
simple cases the xsubpp compiler will generate the code
necessary to handle the argument stack by embedding code
fragments found in the typemaps. In more complex cases
the programmer must supply the code.
The RETVAL Variable
The RETVAL variable is a magic variable which always
matches the return type of the C library function. The
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 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, void return value should
be used only for subroutines which do not return a value,
even if CODE: directive is used which sets ST(0)
explicitly.
Older versions of this document recommended to use void
return value in such cases. It was discovered that this
could lead to segfaults in cases when XSUB was truly void.
This practice is now deprecated, and may be not supported
at some future version. Use the return value SV * in such
cases. (Currently xsubpp contains some heuristic code
which tries to disambiguate between "truly-void" and "old-
practice-declared-as-void" functions. Hence your code is
at mercy of this heuristics unless you use SV * as return
value.)
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
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the last MODULE statement in the XS source files. The
value of MODULE should always remain constant within the
same XS file, though this is not required.
The following example will start the XS code and will
place all functions in a package named RPC.
MODULE = RPC
The PACKAGE Keyword
When functions within an XS source file must be separated
into packages the PACKAGE keyword should be used. This
keyword is used with the MODULE keyword and must follow
immediately after it when used.
MODULE = RPC PACKAGE = RPC
[ XS code in package RPC ]
MODULE = RPC PACKAGE = RPCB
[ XS code in package RPCB ]
MODULE = RPC PACKAGE = RPC
[ XS code in package RPC ]
Although this keyword is optional and in some cases
provides redundant information it should always be used.
This keyword will ensure that the XSUBs appear in the
desired package.
The PREFIX Keyword
The PREFIX keyword designates prefixes which should be
removed from the Perl function names. If the C function
is rpcb_gettime() and the PREFIX value is rpcb_ then Perl
will see this function as gettime().
This keyword should follow the PACKAGE keyword when used.
If PACKAGE is not used then PREFIX should follow the
MODULE keyword.
MODULE = RPC PREFIX = rpc_
MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_
The OUTPUT: Keyword
The OUTPUT: keyword indicates that certain function
parameters should be updated (new values made visible to
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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 xsubpp compiler will
need help to determine which variables are output
variables.
This keyword will normally be used to complement the CODE:
keyword. The RETVAL variable is not recognized as an
output variable when the CODE: keyword is present. The
OUTPUT: keyword is used in this situation to tell the
compiler that RETVAL really is an output variable.
The OUTPUT: keyword can also be used to indicate that
function parameters are output variables. This may be
necessary when a parameter has been modified within the
function and the programmer would like the update to be
seen by Perl.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
The OUTPUT: keyword will also allow an output parameter to
be mapped to a matching piece of code rather than to a
typemap.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep sv_setnv(ST(1), (double)timep);
xsubpp emits an automatic SvSETMAGIC() for all parameters
in the OUTPUT section of the XSUB, except RETVAL. This is
the usually desired behavior, as it takes care of properly
invoking 'set' magic on output parameters (needed for hash
or array element parameters that must be created if they
didn't exist). If for some reason, this behavior is not
desired, the OUTPUT section may contain a SETMAGIC:
DISABLE line to disable it for the remainder of the
parameters in the OUTPUT section. Likewise, SETMAGIC:
ENABLE can be used to reenable it for the remainder of the
OUTPUT section. See the perlguts manpage for more details
about 'set' magic.
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The CODE: Keyword
This keyword is used in more complicated XSUBs which
require special handling for the C function. The RETVAL
variable is 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
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
The NO_INIT Keyword
The NO_INIT keyword is used to indicate that a function
parameter is being used only as an output value. The
xsubpp compiler will normally generate code to read the
values of all function parameters from the argument stack
and assign them to C variables upon entry to the function.
NO_INIT will tell the compiler that some parameters will
be used for output rather than for input and that they
will be handled before the function terminates.
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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
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 (or
additional) initialization code.
The following code demonstrates how to supply
initialization code for function parameters. The
initialization code is eval'd within double quotes by the
compiler before it is added to the output so anything
which should be interpreted literally [mainly $, @, or \\]
must be protected with backslashes. The variables $var,
$arg, and $type can be used as in typemaps.
bool_t
rpcb_gettime(host,timep)
char *host = (char *)SvPV($arg,PL_na);
time_t &timep = 0;
OUTPUT:
timep
This should not be used to supply default values for
parameters. One would normally use this when a function
parameter must be processed by another library function
before it can be used. Default parameters are covered in
the next section.
If the initialization begins with =, then it is output on
the same line where the input variable is declared. If
the initialization begins with ; or +, then it is output
after all of the input variables have been declared. The
= and ; cases replace the initialization normally supplied
from the typemap. For the + case, the initialization from
the typemap will precede the initialization code included
after the +. A global variable, %v, is available for the
truly rare case where information from one initialization
is needed in another initialization.
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bool_t
rpcb_gettime(host,timep)
time_t &timep ; /*\$v{time}=@{[$v{time}=$arg]}*/
char *host + SvOK($v{time}) ? SvPV($arg,PL_na) : NULL;
OUTPUT:
timep
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
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.
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bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
CODE:
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
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
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 /*scope*/
then scoping will automatically be enabled for that XSUB.
To enable scoping:
SCOPE: ENABLE
To disable scoping:
SCOPE: DISABLE
The INPUT: Keyword
The XSUB's parameters are usually evaluated immediately
after entering the XSUB. The INPUT: keyword can be used
to force those parameters to be evaluated a little later.
The INPUT: keyword can be used multiple times within an
XSUB and can be used to list one or more input variables.
This keyword is used with the PREINIT: keyword.
The following example shows how the input parameter timep
can be evaluated late, after a PREINIT.
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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
Variable-length Parameter Lists
XSUBs can have variable-length parameter lists by
specifying an ellipsis (...) in the parameter list. This
use of the ellipsis is similar to that found in ANSI C.
The programmer is able to determine the number of
arguments passed to the XSUB by examining the items
variable which the xsubpp compiler supplies for all XSUBs.
By using this mechanism one can create an XSUB which
accepts a list of parameters of unknown length.
The host parameter for the rpcb_gettime() XSUB can be
optional so the ellipsis can be used to indicate that the
XSUB will take a variable number of parameters. Perl
should be able to call this XSUB with either of the
following statements.
$status = rpcb_gettime( $timep, $host );
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$status = rpcb_gettime( $timep );
The XS code, with ellipsis, follows.
bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
STRLEN n_a;
CODE:
if( items > 1 )
host = (char *)SvPV(ST(1), n_a);
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The C_ARGS: Keyword
The C_ARGS: keyword allows creating of XSUBS which have
different calling sequence from Perl than from C, without
a need to write CODE: or CPPCODE: 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 C function is declared as
symbolic nth_derivative(int n, symbolic function, int flags);
and that the default flags are kept in a global C variable
default_flags. Suppose that you want to create an
interface which is called as
$second_deriv = $function->nth_derivative(2);
To do this, declare the XSUB as
symbolic
nth_derivative(function, n)
symbolic function
int n
C_ARGS:
n, function, default_flags
The PPCODE: Keyword
The PPCODE: keyword is an alternate form of the CODE:
keyword and is used to tell the xsubpp compiler that the
programmer is supplying the code to control the argument
stack for the XSUBs return values. Occasionally one will
want an XSUB to return a list of values rather than a
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single value. In these cases one must use PPCODE: and
then explicitly push the list of values on the stack. The
PPCODE: and CODE: keywords are not used together within
the same XSUB.
The following XSUB will call the C rpcb_gettime() function
and will return its two output values, timep and status,
to Perl as a single list.
void
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
bool_t status;
PPCODE:
status = rpcb_gettime( host, &timep );
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(status)));
PUSHs(sv_2mortal(newSViv(timep)));
Notice that the programmer must supply the C code
necessary to have the real rpcb_gettime() function called
and to have the return values properly placed on the
argument stack.
The void return type for this function tells the xsubpp
compiler that the RETVAL variable is not needed or used
and that it should not be created. In most scenarios the
void return type should be used with the PPCODE:
directive.
The EXTEND() macro is used to make room on the argument
stack for 2 return values. The PPCODE: directive causes
the xsubpp compiler to create a stack pointer available as
SP, and it is this pointer which is being used in the
EXTEND() macro. The values are then pushed onto the stack
with the PUSHs() macro.
Now the rpcb_gettime() function can be used from Perl with
the following statement.
($status, $timep) = rpcb_gettime("localhost");
When handling output parameters with a PPCODE section, be
sure to handle 'set' magic properly. See the perlguts
manpage for details about 'set' magic.
Returning Undef And Empty Lists
Occasionally the programmer will want to return simply
undef or an empty list if a function fails rather than a
separate status value. The rpcb_gettime() function offers
just this situation. If the function succeeds we would
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like to have it return the time and if it fails we would
like to have undef returned. In the following Perl code
the value of $timep will either be undef or it will be a
valid time.
$timep = rpcb_gettime( "localhost" );
The following XSUB uses the SV * return type as a mnemonic
only, and uses a CODE: block to indicate to the compiler
that the programmer has supplied all the necessary code.
The sv_newmortal() call will initialize the return value
to undef, making that the default return value.
SV *
rpcb_gettime(host)
char * host
PREINIT:
time_t timep;
bool_t x;
CODE:
ST(0) = sv_newmortal();
if( rpcb_gettime( host, &timep ) )
sv_setnv( ST(0), (double)timep);
The next example demonstrates how one would place an
explicit undef in the return value, should the need arise.
SV *
rpcb_gettime(host)
char * host
PREINIT:
time_t timep;
bool_t x;
CODE:
ST(0) = sv_newmortal();
if( rpcb_gettime( host, &timep ) ){
sv_setnv( ST(0), (double)timep);
}
else{
ST(0) = &PL_sv_undef;
}
To return an empty list one must use a PPCODE: block and
then not push return values on the stack.
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void
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
PPCODE:
if( rpcb_gettime( host, &timep ) )
PUSHs(sv_2mortal(newSViv(timep)));
else{
/* Nothing pushed on stack, so an empty */
/* list is implicitly returned. */
}
Some people may be inclined to include an explicit return
in the above XSUB, rather than letting control fall
through to the end. In those situations XSRETURN_EMPTY
should be used, instead. This will ensure that the XSUB
stack is properly adjusted. Consult the section on API
LISTING in the perlguts manpage for other XSRETURN macros.
The REQUIRE: Keyword
The REQUIRE: keyword is used to indicate the minimum
version of the xsubpp compiler needed to compile the XS
module. An XS module which contains the following
statement will compile with only xsubpp version 1.922 or
greater:
REQUIRE: 1.922
The CLEANUP: Keyword
This keyword can be used when an XSUB requires special
cleanup procedures before it terminates. When the
CLEANUP: keyword is used it must follow any CODE:,
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.
The BOOT: Keyword
The BOOT: keyword is used to add code to the extension's
bootstrap function. The bootstrap function is generated
by the xsubpp compiler and normally holds the statements
necessary to register any XSUBs with Perl. With the BOOT:
keyword the programmer can tell the compiler to add extra
statements to the bootstrap function.
This keyword may be used any time after the first MODULE
keyword and should appear on a line by itself. The first
blank line after the keyword will terminate the code
block.
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BOOT:
# The following message will be printed when the
# bootstrap function executes.
printf("Hello from the bootstrap!\n");
The VERSIONCHECK: Keyword
The VERSIONCHECK: keyword corresponds to xsubpp's
-versioncheck and -noversioncheck options. This keyword
overrides the command line options. Version checking is
enabled by default. When version checking is enabled the
XS module will attempt to verify that its version matches
the version of the PM module.
To enable version checking:
VERSIONCHECK: ENABLE
To disable version checking:
VERSIONCHECK: DISABLE
The PROTOTYPES: Keyword
The PROTOTYPES: keyword corresponds to xsubpp's
-prototypes and -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
The PROTOTYPE: Keyword
This keyword is similar to the PROTOTYPES: keyword above
but can be used to force xsubpp to use a specific
prototype for the XSUB. This keyword overrides all other
prototype options and keywords but affects only the
current XSUB. Consult the Prototypes entry in the perlsub
manpage for information about Perl prototypes.
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bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PROTOTYPE: $;$
PREINIT:
char *host = "localhost";
STRLEN n_a;
CODE:
if( items > 1 )
host = (char *)SvPV(ST(1), n_a);
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The ALIAS: Keyword
The ALIAS: keyword allows an XSUB to have two or more
unique Perl names and to know which of those names was
used when it was invoked. The Perl names may be fully-
qualified with package names. Each alias is given an
index. The compiler will setup a variable called ix which
contain the index of the alias which was used. When the
XSUB is called with its declared name ix will be 0.
The following example will create aliases FOO::gettime()
and BAR::getit() for this function.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
ALIAS:
FOO::gettime = 1
BAR::getit = 2
INIT:
printf("# ix = %d\n", ix );
OUTPUT:
timep
The 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 XSUBs.
Say, if you have 4 functions multiply(), divide(), add(),
subtract() all having the signature
symbolic f(symbolic, symbolic);
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you code them all by using XSUB
symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE:
multiply divide
add subtract
The advantage of this approach comparing to ALIAS: keyword
is that 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);
(This example supposes that there was no INTERFACE_MACRO:
section, otherwise one needs to use something else instead
of XSINTERFACE_FUNC_SET.)
The INTERFACE_MACRO: Keyword
This keyword allows one to define an INTERFACE using a
different way to extract a function pointer from an XSUB.
The text which follows this keyword should give the name
of macros which would extract/set a function pointer. The
extractor macro is given return type, CV*, and
XSANY.any_dptr for this CV*. The setter macro is given
cv, and the function pointer.
The default value is XSINTERFACE_FUNC and
XSINTERFACE_FUNC_SET. An INTERFACE keyword with an empty
list of functions can be omitted if INTERFACE_MACRO
keyword is used.
Suppose that in the previous example functions pointers
for multiply(), divide(), add(), subtract() are kept in a
global C array fp[] with offsets being multiply_off,
divide_off, add_off, subtract_off. Then one can use
#define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32])
#define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
CvXSUBANY(cv).any_i32 = CAT2( f, _off )
in C section,
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symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE_MACRO:
XSINTERFACE_FUNC_BYOFFSET
XSINTERFACE_FUNC_BYOFFSET_set
INTERFACE:
multiply divide
add subtract
in XSUB section.
The INCLUDE: Keyword
This keyword can be used to pull other files into the XS
module. The other files may have XS code. INCLUDE: can
also be used to run a command to generate the XS code to
be pulled into the module.
The file Rpcb1.xsh contains our rpcb_gettime() function:
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
The XS module can use INCLUDE: to pull that file into it.
INCLUDE: Rpcb1.xsh
If the parameters to the INCLUDE: keyword are followed by
a pipe (|) then the compiler will interpret the parameters
as a command.
INCLUDE: cat Rpcb1.xsh |
The CASE: Keyword
The CASE: keyword allows an XSUB to have multiple distinct
parts with each part acting as a virtual XSUB. CASE: is
greedy and if it is used then all other XS keywords must
be contained within a CASE:. This means nothing may
precede the first CASE: in the XSUB and anything following
the last CASE: is included in that case.
A CASE: might switch via a parameter of the XSUB, via the
ix ALIAS: variable (see the section on The ALIAS:
Keyword), or maybe via the items variable (see the section
on Variable-length Parameter Lists). The last CASE:
becomes the default case if it is not associated with a
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conditional. The following example shows CASE switched
via ix with a function rpcb_gettime() having an alias
x_gettime(). When the function is called as
rpcb_gettime() its parameters are the usual (char *host,
time_t *timep), but when the function is called as
x_gettime() its parameters are reversed, (time_t *timep,
char *host).
long
rpcb_gettime(a,b)
CASE: ix == 1
ALIAS:
x_gettime = 1
INPUT:
# 'a' is timep, 'b' is host
char *b
time_t a = NO_INIT
CODE:
RETVAL = rpcb_gettime( b, &a );
OUTPUT:
a
RETVAL
CASE:
# 'a' is host, 'b' is timep
char *a
time_t &b = NO_INIT
OUTPUT:
b
RETVAL
That function can be called with either of the following
statements. Note the different argument lists.
$status = rpcb_gettime( $host, $timep );
$status = x_gettime( $timep, $host );
The & 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 int
or long but not a int* or long*).
The following XSUB will generate incorrect C code. The
xsubpp compiler will turn this into code which calls
rpcb_gettime() with parameters (char *host, time_t timep),
but the real rpcb_gettime() wants the timep parameter to
be of type time_t* rather than time_t.
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bool_t
rpcb_gettime(host,timep)
char *host
time_t timep
OUTPUT:
timep
That problem is corrected by using the & operator. The
xsubpp compiler will now turn this into code which calls
rpcb_gettime() correctly with parameters (char *host,
time_t *timep). It does this by carrying the & through,
so the function call looks like rpcb_gettime(host,
&timep).
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
Inserting 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 # as the first
non-whitespace of a line. Care should be taken to avoid
making the comment look like a C preprocessor directive,
lest it be interpreted as such. The simplest way to
prevent this is to put whitespace in front of the #.
If you use preprocessor directives to choose one of two
versions of a function, use
#if ... version1
#else /* ... version2 */
#endif
and not
#if ... version1
#endif
#if ... version2
#endif
because otherwise xsubpp will believe that you made a
duplicate definition of the function. Also, put a blank
line before the #else/#endif so it will not be seen as
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part of the function body.
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->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 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 DESTROY then the C++ delete
function will be called and THIS will be given as its
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parameter.
void
color::DESTROY()
The C++ code will call delete.
delete THIS;
If the function's name is new then the C++ new function
will be called to create a dynamic C++ object. The XSUB
will expect the class name, which will be kept in a
variable called CLASS, to be given as the first argument.
color *
color::new()
The C++ code will call new.
RETVAL = new color();
The following is an example of a typemap that could be
used for this C++ example.
TYPEMAP
color * O_OBJECT
OUTPUT
# The Perl object is blessed into 'CLASS', which should be a
# char* having the name of the package for the blessing.
O_OBJECT
sv_setref_pv( $arg, CLASS, (void*)$var );
INPUT
O_OBJECT
if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
$var = ($type)SvIV((SV*)SvRV( $arg ));
else{
warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
XSRETURN_UNDEF;
}
Interface Strategy
When designing an interface between Perl and a C library a
straight translation from C to XS 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.
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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.
Perl Objects And C Structures
When dealing with C structures one should select either
T_PTROBJ or T_PTRREF for the XS type. Both types are
designed to handle pointers to complex objects. The
T_PTRREF type will allow the Perl object to be unblessed
while the T_PTROBJ type requires that the object be
blessed. By using T_PTROBJ one can achieve a form of
type-checking because the XSUB will attempt to verify that
the Perl object is of the expected type.
The following XS code shows the getnetconfigent() function
which is used with ONC+ TIRPC. The getnetconfigent()
function will return a pointer to a C structure and has
the C prototype shown below. The example will demonstrate
how the C pointer will become a Perl reference. Perl will
consider this reference to be a pointer to a blessed
object and will attempt to call a destructor for the
object. A destructor will be provided in the XS source to
free the memory used by getnetconfigent(). Destructors in
XS can be created by specifying an XSUB function whose
name ends with the word DESTROY. XS destructors can be
used to free memory which may have been malloc'd by
another XSUB.
struct netconfig *getnetconfigent(const char *netid);
A typedef will be created for struct netconfig. The Perl
object will be blessed in a class matching the name of the
C type, with the tag Ptr appended, and the name should not
have embedded spaces if it will be a Perl package name.
The destructor will be placed in a class corresponding to
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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.
The Typemap
The typemap is a collection of code fragments which are
used by the xsubpp compiler to map C function parameters
and values to Perl values. The typemap file may consist
of three sections labeled TYPEMAP, INPUT, and OUTPUT. Any
unlabelled initial section is assumed to be a TYPEMAP
section if a name is not explicitly specified. 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
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TYPEMAP section tells the compiler which of the INPUT and
OUTPUT code fragments should be used to map a given C type
to a Perl value. The section labels TYPEMAP, INPUT, or
OUTPUT must begin in the first column on a line by
themselves, and must be in uppercase.
The default typemap in the 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 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 * 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
struct netconfig to be blessed into the class Net::Config.
One way to do this is to use underscores (_) to separate
package names, as follows:
typedef struct netconfig * Net_Config;
And then provide a typemap entry T_PTROBJ_SPECIAL that
maps underscores to double-colons (::), and declare
Net_Config to be of that type:
TYPEMAP
Net_Config T_PTROBJ_SPECIAL
INPUT
T_PTROBJ_SPECIAL
if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
IV tmp = SvIV((SV*)SvRV($arg));
$var = ($type) tmp;
}
else
croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")
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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.
EXAMPLES
File 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 typemap: Custom typemap for RPC.xs.
TYPEMAP
Netconfig * T_PTROBJ
File RPC.pm: Perl module for the RPC extension.
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package RPC;
require Exporter;
require DynaLoader;
@ISA = qw(Exporter DynaLoader);
@EXPORT = qw(rpcb_gettime getnetconfigent);
bootstrap RPC;
1;
File rpctest.pl: Perl test program for the RPC extension.
use RPC;
$netconf = getnetconfigent();
$a = rpcb_gettime();
print "time = $a\n";
print "netconf = $netconf\n";
$netconf = getnetconfigent("tcp");
$a = rpcb_gettime("poplar");
print "time = $a\n";
print "netconf = $netconf\n";
XS VERSION
This document covers features supported by xsubpp 1.935.
AUTHOR
Dean Roehrich <roehrich@cray.com> Jul 8, 1996
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Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others. |
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