PERLXS(1) PERLXS(1)
perlxs - XS language reference manual
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.
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
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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 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 [Toc] [Back]
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.
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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 [Toc] [Back]
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 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
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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 truely 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 "truely-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 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 [Toc] [Back]
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
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[ 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 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.
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bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep sv_setnv(ST(1), (double)timep);
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
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The NO_INIT Keyword [Toc] [Back]
The NO_INIT keyword is used to indicate that a function parameter is
being used only as an output value. The xsubpp compiler will normally
generate code to read the values of all function parameters from the
argument stack and assign them to C variables upon entry to the function.
NO_INIT will tell the compiler that some parameters will be used for
output rather than for input and that they will be handled before the
function terminates.
The following example shows a variation of the rpcb_gettime() function.
This function uses the timep variable only as an output variable and does
not care about its initial contents.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep = NO_INIT
OUTPUT:
timep
Initializing Function Parameters [Toc] [Back]
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.
Default Parameter Values [Toc] [Back]
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
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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.
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.
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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 [Toc] [Back]
XSUBs can have variable-length parameter lists by specifying an ellipsis
(...) in the parameter list. This use of the ellipsis is similar to that
found in ANSI C. The programmer is able to determine the number of
arguments passed to the XSUB by examining the items variable which the
xsubpp compiler supplies for all XSUBs. By using this mechanism one can
create an XSUB which accepts a list of parameters of unknown length.
The host parameter for the rpcb_gettime() XSUB can be optional so the
ellipsis can be used to indicate that the XSUB will take a variable
number of parameters. Perl should be able to call this XSUB with either
of the following statements.
$status = rpcb_gettime( $timep, $host );
$status = rpcb_gettime( $timep );
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The XS code, with ellipsis, follows.
bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
CODE:
if( items > 1 )
host = (char *)SvPV(ST(1), na);
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The PPCODE: Keyword
The PPCODE: keyword is an alternate form of the CODE: keyword and is used
to tell the xsubpp compiler that the programmer is supplying the code to
control the argument stack for the XSUBs return values. Occasionally one
will want an XSUB to return a list of values rather than a single value.
In these cases one must use PPCODE: and then explicitly push the list of
values on the stack. The PPCODE: and CODE: keywords are not used
together within the same XSUB.
The following XSUB will call the C rpcb_gettime() function and will
return its two output values, timep and status, to Perl as a single list.
void
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
bool_t status;
PPCODE:
status = rpcb_gettime( host, &timep );
EXTEND(sp, 2);
PUSHs(sv_2mortal(newSViv(status)));
PUSHs(sv_2mortal(newSViv(timep)));
Notice that the programmer must supply the C code necessary to have the
real rpcb_gettime() function called and to have the return values
properly placed on the argument stack.
The void return type for this function tells the xsubpp compiler that the
RETVAL variable is not needed or used and that it should not be created.
In most scenarios the void return type should be used with the PPCODE:
directive.
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The EXTEND() macro is used to make room on the argument stack for 2
return values. The PPCODE: directive causes the xsubpp compiler to
create a stack pointer called sp, and it is this pointer which is being
used in the EXTEND() macro. The values are then pushed onto the stack
with the PUSHs() macro.
Now the rpcb_gettime() function can be used from Perl with the following
statement.
($status, $timep) = rpcb_gettime("localhost");
Returning Undef And Empty Lists [Toc] [Back]
Occasionally the programmer will want to return simply undef or an empty
list if a function fails rather than a separate status value. The
rpcb_gettime() function offers just this situation. If the function
succeeds we would like to have it return the time and if it fails we
would like to have undef returned. In the following Perl code the value
of $timep will either be undef or it will be a valid time.
$timep = rpcb_gettime( "localhost" );
The following XSUB uses the SV * return type as a 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.
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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 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
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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.
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:
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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.
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
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 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
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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
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).
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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.
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 &
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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 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.
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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 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.
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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 [Toc] [Back]
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.
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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 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.
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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 [Toc] [Back]
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. 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 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:
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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}\")
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.
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
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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.
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";
This document covers features supported by xsubpp 1.935.
Dean Roehrich <roehrich@cray.com> Jul 8, 1996
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