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.
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
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.
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
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. The XSUB's incoming parameters and outgoing return values always begin at ST. 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.
ST 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
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 following example will start the XS code and will place all functions in a package named RPC.
MODULE = RPC
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.
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_
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);
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
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
INIT:
printf("# Host is %s\n", host );
OUTPUT:
timep
The following example shows a variation of the rpcb_gettime
function. This function uses the timep variable as only 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
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.
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 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
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 following example shows how the input parameter timep can be evaluated late, after a
PREINIT.
bool_t
rpcb_gettime(host,timep)
char *host
PREINIT:
time_t tt;
INPUT:
time_t timep
CODE:
RETVAL = rpcb_gettime( host, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
The next example shows each input parameter evaluated late.
bool_t
rpcb_gettime(host,timep)
PREINIT:
time_t tt;
INPUT:
char *host
PREINIT:
char *h;
INPUT:
time_t timep
CODE:
h = host;
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
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( $timep );
The XS code, with ellipsis, follows.
bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
CODE:
if( items > 1 )
host = (char *)SvPV(ST(1), na);
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The following
XSUB will call the
C rpcb_gettime function and will return its two output values, timep and status, to Perl as a single list.
void
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
bool_t status;
PPCODE:
status = rpcb_gettime( host, &timep );
EXTEND(sp, 2);
PUSHs(sv_2mortal(newSViv(status)));
PUSHs(sv_2mortal(newSViv(timep)));
Notice that the programmer must supply the
C code necessary to have the real
rpcb_gettime function called and to have the return values
properly placed on the argument stack.
The void return type for this function tells the xsubpp compiler that the
RETVAL variable is not needed or used and that it should not be created. In most scenarios the void return type should be used with the
PPCODE: directive.
The EXTEND macro is used to make room on the argument stack for 2 return values. The
PPCODE: directive causes the
xsubpp compiler to create a stack pointer called sp, and it is this pointer which is being used in the EXTEND
macro. The values are then pushed onto the stack with the
PUSHs macro.
Now the rpcb_gettime function can be used from Perl with the
following statement.
($status, $timep) = rpcb_gettime("localhost");
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.
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 API LISTING for other XSRETURN macros.
REQUIRE: 1.922
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");
-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
-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
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 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 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 |
A
CASE: might switch via a parameter of the
XSUB, via the
ix
ALIAS: variable (see The ALIAS: Keyword), or maybe via the items variable (see Variable-length Parameter Lists). The last
CASE: becomes the
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 , but when the function is called as x_gettime its parameters are reversed, .
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 );
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 , 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 . It does this by carrying the
& through, so the function call looks like rpcb_gettime.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
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.
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 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;
}
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.
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.
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.
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
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.
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";
xsubpp 1.935.