perlxs - XS language reference manual
Introduction
XS is an interface description file format used to create
an extension interface between Perl and C code (or a C
library) which one wishes to use with Perl. The XS interface
is combined with the library to create a new library
which can then be either dynamically loaded or statically
linked into perl. The XS interface description is written
in the XS language and is the core component of the Perl
extension interface.
An XSUB forms the basic unit of the XS interface. After
compilation by the xsubpp compiler, each XSUB amounts to a
C function definition which will provide the glue between
Perl calling conventions and C calling conventions.
The glue code pulls the arguments from the Perl stack,
converts these Perl values to the formats expected by a C
function, call this C function, transfers the return values
of the C function back to Perl. Return values here
may be a conventional C return value or any C function
arguments that may serve as output parameters. These
return values may be passed back to Perl either by putting
them on the Perl stack, or by modifying the arguments supplied
from the Perl side.
The above is a somewhat simplified view of what really
happens. Since Perl allows more flexible calling conventions
than C, XSUBs may do much more in practice, such as
checking input parameters for validity, throwing exceptions
(or returning undef/empty list) if the return value
from the C function indicates failure, calling different C
functions based on numbers and types of the arguments,
providing an object-oriented interface, etc.
Of course, one could write such glue code directly in C.
However, this would be a tedious task, especially if one
needs to write glue for multiple C functions, and/or one
is not familiar enough with the Perl stack discipline and
other such arcana. XS comes to the rescue here: instead
of writing this glue C code in long-hand, one can write a
more concise short-hand description of what should be done
by the glue, and let the XS compiler xsubpp handle the
rest.
The XS language allows one to describe the mapping between
how the C routine is used, and how the corresponding Perl
routine is used. It also allows creation of Perl routines
which are directly translated to C code and which are not
related to a pre-existing C function. In cases when the C
interface coincides with the Perl interface, the XSUB declaration
is almost identical to a declaration of a C function
(in K&R style). In such circumstances, there is
another tool called "h2xs" that is able to translate an
entire C header file into a corresponding XS file that
will provide glue to the functions/macros described in the
header file.
The XS compiler is called xsubpp. This compiler creates
the constructs necessary to let an XSUB manipulate Perl
values, and creates the glue necessary to let Perl call
the XSUB. The compiler uses typemaps to determine how to
map C function parameters and output values to Perl values
and back. The default typemap (which comes with Perl)
handles many common C types. A supplementary typemap may
also be needed to handle any special structures and types
for the library being linked.
A file in XS format starts with a C language section which
goes until the first "MODULE =" directive. Other XS
directives and XSUB definitions may follow this line. The
"language" used in this part of the file is usually
referred to as the XS language. xsubpp recognizes and
skips POD (see perlpod) in both the C and XS language sections,
which allows the XS file to contain embedded documentation.
See perlxstut for a tutorial on the whole extension creation
process.
Note: For some extensions, Dave Beazley's SWIG system may
provide a significantly more convenient mechanism for creating
the extension glue code. See http://www.swig.org/
for more information.
On The Road [Toc] [Back]
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_get-
time() 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.
The Anatomy of an XSUB [Toc] [Back]
The simplest XSUBs consist of 3 parts: a description of
the return value, the name of the XSUB routine and the
names of its arguments, and a description of types or formats
of the arguments.
The following XSUB allows a Perl program to access a C
library function called sin(). The XSUB will imitate the
C function which takes a single argument and returns a
single value.
double
sin(x)
double x
Optionally, one can merge the description of types and the
list of argument names, rewriting this as
double
sin(double x)
This makes this XSUB look similar to an ANSI C declaration.
An optional semicolon is allowed after the argument
list, as in
double
sin(double x);
Parameters with C pointer types can have different semantic:
C functions with similar declarations
bool string_looks_as_a_number(char *s);
bool make_char_uppercase(char *c);
are used in absolutely incompatible manner. Parameters to
these functions could be described xsubpp like this:
char * s
char &c
Both these XS declarations correspond to the "char*" C
type, but they have different semantics, see "The & Unary
Operator".
It is convenient to think that the indirection operator
"*" should be considered as a part of the type and the
address operator "&" should be considered part of the
variable. See "The Typemap" for more info about handling
qualifiers and unary operators in C types.
The function name and the return type must be placed on
separate lines and should be flush left-adjusted.
INCORRECT CORRECT
double sin(x) double
double x sin(x)
double x
The rest of the function description may be indented or
left-adjusted. The following example shows a function with
its body left-adjusted. Most examples in this document
will indent the body for better readability.
CORRECT
double
sin(x)
double x
More complicated XSUBs may contain many other sections.
Each section of an XSUB starts with the corresponding keyword,
such as INIT: or CLEANUP:. However, the first two
lines of an XSUB always contain the same data: descriptions
of the return type and the names of the function and
its parameters. Whatever immediately follows these is
considered to be an INPUT: section unless explicitly
marked with another keyword. (See "The INPUT: Keyword".)
An XSUB section continues until another section-start keyword
is found.
The Argument Stack [Toc] [Back]
The Perl argument stack is used to store the values which
are sent as parameters to the XSUB and to store the XSUB's
return value(s). In reality all Perl functions (including
non-XSUB ones) keep their values on this stack all the
same time, each limited to its own range of positions on
the stack. In this document the first position on that
stack which belongs to the active function will be
referred to as position 0 for that function.
XSUBs refer to their stack arguments with the macro 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 [Toc] [Back]
The RETVAL variable is a special C variable that is
declared automatically for you. The C type of RETVAL
matches the return type of the C library function. The
xsubpp compiler will declare this variable in each XSUB
with non-"void" return type. By default the generated C
function will use RETVAL to hold the return value of the C
library function being called. In simple cases the value
of RETVAL will be placed in ST(0) of the argument stack
where it can be received by Perl as the return value of
the XSUB.
If the XSUB has a return type of "void" then the compiler
will not declare a RETVAL variable for that function.
When using a PPCODE: section no manipulation of the RETVAL
variable is required, the section may use direct stack
manipulation to place output values on the stack.
If PPCODE: directive is not used, "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.)
Returning SVs, AVs and HVs through RETVAL
When you're using RETVAL to return an "SV *", there's some
magic going on behind the scenes that should be mentioned.
When you're manipulating the argument stack using the
ST(x) macro, for example, you usually have to pay special
attention to reference counts. (For more about reference
counts, see perlguts.) To make your life easier, the
typemap file automatically makes "RETVAL" mortal when
you're returning an "SV *". Thus, the following two XSUBs
are more or less equivalent:
void
alpha()
PPCODE:
ST(0) = newSVpv("Hello World",0);
sv_2mortal(ST(0));
XSRETURN(1);
SV *
beta()
CODE:
RETVAL = newSVpv("Hello World",0);
OUTPUT:
RETVAL
This is quite useful as it usually improves readability.
While this works fine for an "SV *", it's unfortunately
not as easy to have "AV *" or "HV *" as a return value.
You should be able to write:
AV *
array()
CODE:
RETVAL = newAV();
/* do something with RETVAL */
OUTPUT:
RETVAL
But due to an unfixable bug (fixing it would break lots of
existing CPAN modules) in the typemap file, the reference
count of the "AV *" is not properly decremented. Thus, the
above XSUB would leak memory whenever it is being called.
The same problem exists for "HV *".
When you're returning an "AV *" or a "HV *", you have make
sure their reference count is decremented by making the AV
or HV mortal:
AV *
array()
CODE:
RETVAL = newAV();
sv_2mortal((SV*)RETVAL);
/* do something with RETVAL */
OUTPUT:
RETVAL
And also remember that you don't have to do this for an
"SV *".
The MODULE Keyword [Toc] [Back]
The MODULE keyword is used to start the XS code and to
specify the package of the functions which are being
defined. All text preceding the first MODULE keyword is
considered C code and is passed through to the output with
POD stripped, but otherwise untouched. Every XS module
will have a bootstrap function which is used to hook the
XSUBs into Perl. The package name of this bootstrap function
will match the value of the last MODULE statement in
the XS source files. The value of MODULE should always
remain constant within the same XS file, though this is
not required.
The following example will start the XS code and will
place all functions in a package named RPC.
MODULE = RPC
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
[ XS code in package RPC ]
The same package name can be used more than once, allowing
for non-contiguous code. This is useful if you have a
stronger ordering principle than package names.
Although this keyword is optional and in some cases provides
redundant information it should always be used.
This keyword will ensure that the XSUBs appear in the
desired package.
The PREFIX Keyword [Toc] [Back]
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
which have no CODE: or PPCODE: section, such as the
sin() function above, the RETVAL variable is automatically
designated as an output value. For more complex functions
the 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 perlguts for more
details about 'set' magic.
The NO_OUTPUT Keyword [Toc] [Back]
The NO_OUTPUT can be placed as the first token of the
XSUB. This keyword indicates that while the C subroutine
we provide an interface to has a non-"void" return type,
the return value of this C subroutine should not be
returned from the generated Perl subroutine.
With this keyword present "The RETVAL Variable" is created,
and in the generated call to the subroutine this
variable is assigned to, but the value of this variable is
not going to be used in the auto-generated code.
This keyword makes sense only if "RETVAL" is going to be
accessed by the user-supplied code. It is especially useful
to make a function interface more Perl-like, especially
when the C return value is just an error condition
indicator. For example,
NO_OUTPUT int
delete_file(char *name)
POSTCALL:
if (RETVAL != 0)
croak("Error %d while deleting file '%s'", RETVAL, name);
Here the generated XS function returns nothing on success,
and will die() with a meaningful error message on error.
The CODE: Keyword
This keyword is used in more complicated XSUBs which
require special handling for the C function. The RETVAL
variable is still declared, but it will not be returned
unless it is specified in the OUTPUT: section.
The following XSUB is for a C function which requires special
handling of its parameters. The Perl usage is given
first.
$status = rpcb_gettime( "localhost", $timep );
The XSUB follows.
bool_t
rpcb_gettime(host,timep)
char *host
time_t timep
CODE:
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
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 %s0, host );
OUTPUT:
timep
Another use for the INIT: section is to check for preconditions
before making a call to the C function:
long long
lldiv(a,b)
long long a
long long b
INIT:
if (a == 0 && b == 0)
XSRETURN_UNDEF;
if (b == 0)
croak("lldiv: cannot divide by 0");
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_get-
time() 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]
C function parameters are normally initialized with their
values from the argument stack (which in turn contains the
parameters that were passed to the XSUB from Perl). The
typemaps contain the code segments which are used to
translate the Perl values to the C parameters. The programmer,
however, is allowed to override the typemaps and
supply alternate (or additional) initialization code.
Initialization code starts with the first "=", ";" or "+"
on a line in the INPUT: section. The only exception happens
if this ";" terminates the line, then this ";" is
quietly ignored.
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
in the declaration for the input variable, replacing the
initialization supplied by the typemap. If the initialization
begins with ";" or "+", then it is performed after
all of the input variables have been declared. In the ";"
case the initialization normally supplied by the typemap
is not performed. For the "+" case, the declaration for
the variable will include the initialization from the
typemap. A global variable, %v, is available for the
truly rare case where information from one initialization
is needed in another initialization.
Here's a truly obscure example:
bool_t
rpcb_gettime(host,timep)
time_t &timep ; /* {timep}=@{[$v{timep}=$arg]}
*/
char *host + SvOK($v{timep}) ? SvPV($arg,PL_na)
: NULL;
OUTPUT:
timep
The construct "{timep}=@{[$v{timep}=$arg]}" used in the
above example has a two-fold purpose: first, when this
line is processed by xsubpp, the Perl snippet
"$v{timep}=$arg" is evaluated. Second, the text of the
evaluated snippet is output into the generated C file
(inside a C comment)! During the processing of "char
*host" line, $arg will evaluate to ST(0), and $v{timep}
will evaluate to ST(1).
Default Parameter Values [Toc] [Back]
Default values for XSUB arguments can be specified by
placing an assignment statement in the parameter list.
The default value may be a number, a string or the special
string "NO_INIT". Defaults should always be used on the
right-most parameters only.
To allow the XSUB for rpcb_gettime() to have a default
host value the parameters to the XSUB could be rearranged.
The XSUB will then call the real rpcb_gettime() function
with the parameters in the correct order. This XSUB can
be called from Perl with either of the following statements:
$status = rpcb_gettime( $timep, $host );
$status = rpcb_gettime( $timep );
The XSUB will look like the code which follows. A
CODE: block is used to call the real rpcb_gettime() function
with the parameters in the correct order for that
function.
bool_t
rpcb_gettime(timep,host="localhost")
char *host
time_t timep = NO_INIT
CODE:
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The PREINIT: Keyword
The PREINIT: keyword allows extra variables to be declared
immediately before or after the declarations of the parameters
from the INPUT: section are emitted.
If a variable is declared inside a CODE: section it will
follow any typemap code that is emitted for the input
parameters. This may result in the declaration ending up
after C code, which is C syntax error. Similar errors may
happen with an explicit ";"-type or "+"-type initialization
of parameters is used (see "Initializing Function
Parameters"). Declaring these variables in an INIT: section
will not help.
In such cases, to force an additional variable to be
declared together with declarations of other variables,
place the declaration into a PREINIT: section. The
PREINIT: keyword may be used one or more times within an
XSUB.
The following examples are equivalent, but if the code is
using complex typemaps then the first example is safer.
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
CODE:
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
For this particular case an INIT: keyword would generate
the same C code as the PREINIT: keyword. Another correct,
but error-prone example:
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
CODE:
char *host = "localhost";
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
Another way to declare "host" is to use a C block in the
CODE: section:
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
CODE:
{
char *host = "localhost";
RETVAL = rpcb_gettime( host, &timep );
}
OUTPUT:
timep
RETVAL
The ability to put additional declarations before the
typemap entries are processed is very handy in the cases
when typemap conversions manipulate some global state:
MyObject
mutate(o)
PREINIT:
MyState st = global_state;
INPUT:
MyObject o;
CLEANUP:
reset_to(global_state, st);
Here we suppose that conversion to "MyObject" in the
INPUT: section and from MyObject when processing RETVAL
will modify a global variable "global_state". After these
conversions are performed, we restore the old value of
"global_state" (to avoid memory leaks, for example).
There is another way to trade clarity for compactness:
INPUT sections allow declaration of C variables which do
not appear in the parameter list of a subroutine. Thus
the above code for mutate() can be rewritten as
MyObject
mutate(o)
MyState st = global_state;
MyObject o;
CLEANUP:
reset_to(global_state, st);
and the code for rpcb_gettime() can be rewritten as
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
char *host = "localhost";
C_ARGS:
host, &timep
OUTPUT:
timep
RETVAL
The SCOPE: Keyword
The SCOPE: keyword allows scoping to be enabled for a particular
XSUB. If enabled, the XSUB will invoke ENTER and
LEAVE automatically.
To support potentially complex type mappings, if a typemap
entry used by an XSUB contains a comment like "/*scope*/"
then scoping will be automatically 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.
bool_t
rpcb_gettime(host,timep)
char *host
PREINIT:
time_t tt;
INPUT:
time_t timep
CODE:
RETVAL = rpcb_gettime( host, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
The next example shows each input parameter evaluated
late.
bool_t
rpcb_gettime(host,timep)
PREINIT:
time_t tt;
INPUT:
char *host
PREINIT:
char *h;
INPUT:
time_t timep
CODE:
h = host;
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
Since INPUT sections allow declaration of C variables
which do not appear in the parameter list of a subroutine,
this may be shortened to:
bool_t
rpcb_gettime(host,timep)
time_t tt;
char *host;
char *h = host;
time_t timep;
CODE:
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
(We used our knowledge that input conversion for "char *"
is a "simple" one, thus "host" is initialized on the declaration
line, and our assignment "h = host" is not performed
too early. Otherwise one would need to have the
assignment "h = host" in a CODE: or INIT: section.)
The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
In the list of parameters for an XSUB, one can precede
parameter names by the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT"
keywords. "IN" keyword is the
default, the other keywords indicate how the Perl interface
should differ from the C interface.
Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT"
keywords are considered to be used by
the C subroutine via pointers. "OUTLIST"/"OUT" keywords
indicate that the C subroutine does not inspect the memory
pointed by this parameter, but will write through this
pointer to provide additional return values.
Parameters preceded by "OUTLIST" keyword do not appear in
the usage signature of the generated Perl function.
Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do
appear as parameters to the Perl function. With the
exception of "OUT"-parameters, these parameters are converted
to the corresponding C type, then pointers to these
data are given as arguments to the C function. It is
expected that the C function will write through these
pointers.
The return list of the generated Perl function consists of
the C return value from the function (unless the XSUB is
of "void" return type or "The NO_OUTPUT Keyword" was used)
followed by all the "OUTLIST" and "IN_OUTLIST" parameters
(in the order of appearance). On the return from the XSUB
the "IN_OUT"/"OUT" Perl parameter will be modified to have
the values written by the C function.
For example, an XSUB
void
day_month(OUTLIST day, IN unix_time, OUTLIST month)
int day
int unix_time
int month
should be used from Perl as
my ($day, $month) = day_month(time);
The C signature of the corresponding function should be
void day_month(int *day, int unix_time, int *month);
The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords
can be mixed with ANSI-style declarations, as in
void
day_month(OUTLIST int day, int unix_time, OUTLIST int
month)
(here the optional "IN" keyword is omitted).
The "IN_OUT" parameters are identical with parameters
introduced with "The & Unary Operator" and put into the
"OUTPUT:" section (see "The OUTPUT: Keyword"). The
"IN_OUTLIST" parameters are very similar, the only difference
being that the value C function writes through the
pointer would not modify the Perl parameter, but is put in
the output list.
The "OUTLIST"/"OUT" parameter differ from
"IN_OUTLIST"/"IN_OUT" parameters only by the initial value
of the Perl parameter not being read (and not being given
to the C function - which gets some garbage instead). For
example, the same C function as above can be interfaced
with as
void day_month(OUT int day, int unix_time, OUT int
month);
or
void
day_month(day, unix_time, month)
int &day = NO_INIT
int unix_time
int &month = NO_INIT
OUTPUT:
day
month
However, the generated Perl function is called in very
C-ish style:
my ($day, $month);
day_month($day, time, $month);
The "length(NAME)" Keyword
If one of the input arguments to the C function is the
length of a string argument "NAME", one can substitute the
name of the length-argument by "length(NAME)" in the XSUB
declaration. This argument must be omited when the generated
Perl function is called. E.g.,
void
dump_chars(char *s, short l)
{
short n = 0;
while (n < l) {
printf("s[%d] =
n++;
}
}
MODULE = x PACKAGE = x
void dump_chars(char *s, short length(s))
should be called as "dump_chars($string)".
This directive is supported with ANSI-type function declarations
only.
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 );
$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 PPCODE: section. The contents of
the C_ARGS: paragraph is put as the argument to the called
C function without any change.
For example, suppose that a C function is declared as
symbolic nth_derivative(int n, symbolic function, int
flags);
and that the default flags are kept in a global C variable
"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 single
value. In these cases one must use PPCODE: and then
explicitly push the list of values on the stack. The
PPCODE: and CODE: keywords should not be used together
within the same XSUB.
The actual difference between PPCODE: and CODE: sections
is in the initialization of "SP" macro (which stands for
the current Perl stack pointer), and in the handling of
data on the stack when returning from an XSUB. In CODE:
sections SP preserves the value which was on entry to the
XSUB: SP is on the function pointer (which follows the
last parameter). In PPCODE: sections SP is moved backward
to the beginning of the parameter list, which allows
"PUSH*()" macros to place output values in the place Perl
expects them to be when the XSUB returns back to Perl.
The generated trailer for a CODE: section ensures that the
number of return values Perl will see is either 0 or 1
(depending on the "void"ness of the return value of the C
function, and heuristics mentioned in "The RETVAL Variable").
The trailer generated for a PPCODE: section is
based on the number of return values and on the number of
times "SP" was updated by "[X]PUSH*()" macros.
Note that macros ST(i), "XST_m*()" and "XSRETURN*()" work
equally well in CODE: sections and PPCODE: sections.
The following XSUB will call the C rpcb_gettime() function
and will return its two output values, timep and status,
to Perl as a single list.
void
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
bool_t status;
PPCODE:
status = rpcb_gettime( host, &timep );
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(status)));
PUSHs(sv_2mortal(newSViv(timep)));
Notice that the programmer must supply the C code necessary
to have the real rpcb_gettime() function called and
to have the return values properly placed on the argument
stack.
The "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 perlguts for
details about 'set' magic.
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
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.
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 perlapi
for other "XSRETURN" macros.
Since "XSRETURN_*" macros can be used with CODE blocks as
well, one can rewrite this example as:
int
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
CODE:
RETVAL = rpcb_gettime( host, &timep );
if (RETVAL == 0)
XSRETURN_UNDEF;
OUTPUT:
RETVAL
In fact, one can put this check into a POSTCALL: section
as well. Together with PREINIT: simplifications, this
leads to:
int
rpcb_gettime(host)
char *host
time_t timep;
POSTCALL:
if (RETVAL == 0)
XSRETURN_UNDEF;
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 POSTCALL: Keyword
This keyword can be used when an XSUB requires special
procedures executed after the C subroutine call is performed.
When the POSTCALL: keyword is used it must precede
OUTPUT: and CLEANUP: blocks which are present in the
XSUB.
See examples in "The NO_OUTPUT Keyword" and "Returning
Undef And Empty Lists".
The POSTCALL: block does not make a lot of sense when the
C subroutine call is supplied by user by providing either
CODE: or PPCODE: section.
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!0);
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 "Prototypes" in perlsub for information
about Perl prototypes.
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
If the prototypes are enabled, you can disable it locally
for a given XSUB as in the following example:
void
rpcb_gettime_noproto()
PROTOTYPE: DISABLE
...
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 fullyqualified
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 = %d0, ix );
OUTPUT:
timep
The OVERLOAD: Keyword
Instead of writing an overloaded interface using pure
Perl, you can also use the OVERLOAD keyword to define
additional Perl names for your functions (like the ALIAS:
keyword above). However, the overloaded functions must be
defined with three parameters (except for the nomethod()
function which needs four parameters). If any function
has the OVERLOAD: keyword, several additional lines will
be defined in the c file generated by xsubpp in order to
register with the overload magic.
Since blessed objects are actually stored as RV's, it is
useful to use the typemap features to preprocess parameters
and extract the actual SV stored within the blessed
RV. See the sample for T_PTROBJ_SPECIAL below.
To use the OVERLOAD: keyword, create an XS function which
takes three input parameters ( or use the c style '...'
definition) like this:
SV *
cmp (lobj, robj, swap)
My_Module_obj lobj
My_Module_obj robj
IV swap
OVERLOAD: cmp <=>
{ /* function defined here */}
In this case, the function will overload both of the three
way comparison operators. For all overload operations
using non-alpha characters, you must type the parameter
without quoting, seperating multiple overloads with
whitespace. Note that "" (the stringify overload) should
be entered as
The FALLBACK: Keyword
In addition to the OVERLOAD keyword, if you need to control
how Perl autogenerates missing overloaded operators,
you can set the FALLBACK keyword in the module header section,
like this:
MODULE = RPC PACKAGE = RPC
FALLBACK: TRUE
...
where FALLBACK can take any of the three values TRUE,
FALSE, or UNDEF. If you do not set any FALLBACK value
when using OVERLOAD, it defaults to UNDEF. FALLBACK is
not used except when one or more functions using OVERLOAD
have been defined. Please see "Fallback" in overload for
more details.
The INTERFACE: Keyword
This keyword declares the current XSUB as a keeper of the
given calling signature. If some text follows this keyword,
it is considered as a list of functions which have
this signature, and should be attached to the current
XSUB.
For example, if you have 4 C functions multiply(),
divide(), add(), subtract() all having the signature:
symbolic f(symbolic, symbolic);
you can make them all to use the same XSUB using this:
symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE:
multiply divide
add subtract
(This is the complete XSUB code for 4 Perl functions!)
Four generated Perl function share names with corresponding
C functions.
The advantage of this approach comparing to ALIAS: keyword
is that there is no need to code a switch statement, each
Perl function (which shares the same XSUB) knows which C
function it should call. Additionally, one can attach an
extra function remainder() at runtime by using
CV *mycv = newXSproto("Symbolic::remainder",
XS_Symbolic_interface_s_ss,
__FILE__, "$$");
XSINTERFACE_FUNC_SET(mycv, remainder);
say, from another XSUB. (This example supposes that there
was no INTERFACE_MACRO: section, otherwise one needs to
use something else instead of "XSINTERFACE_FUNC_SET", see
the next section.)
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,
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 t
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