perlxs
DESCRIPTION
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 inter
face 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 val
ues 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 sup
plied from the Perl side.
The above is a somewhat simplified view of what really
happens. Since Perl allows more flexible calling conven
tions than C, XSUBs may do much more in practice, such as
checking input parameters for validity, throwing excep
tions (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 dec
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 sec
tions, which allows the XS file to contain embedded docu
mentation.
See perlxstut for a tutorial on the whole extension cre
ation process.
Note: For some extensions, Dave Beazley's SWIG system may
provide a significantly more convenient mechanism for cre
ating the extension glue code. See http://www.swig.org/
for more information.
On The Road
Many of the examples which follow will concentrate on cre
ating 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 func
tion 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 transla
tion between C and Perl and so preserves the interface
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 pro
gram 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 differ
ent 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 parame
ters. This abstraction will allow the programmer to cre
ate a more Perl-like interface to the C function.
The Anatomy of an XSUB
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 for
mats 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
Parameters with C pointer types can have different seman
tic: 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 key
word, such as INIT: or CLEANUP:. However, the first two
lines of an XSUB always contain the same data: descrip
tions 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 key
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 outgo
ing 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 program
mer must supply the code.
The RETVAL Variable
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" func
tions. 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 with
POD stripped, but otherwise untouched. Every XS module
will have a bootstrap function which is used to hook the
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 pro
vides 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 MOD
ULE keyword.
MODULE = RPC PREFIX = rpc_
MODULE = RPC PACKAGE = RPCB PREFIX = rpcb_
The OUTPUT: Keyword
The OUTPUT: keyword indicates that certain function param
eters 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 func
tions 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.
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 parame
ters 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 "SET
MAGIC: DISABLE" line to disable it for the remainder of
the parameters in the OUTPUT section. Likewise, "SET
MAGIC: ENABLE" can be used to reenable it for the remain
der of the OUTPUT section. See perlguts for more details
about 'set' magic.
The NO_OUTPUT Keyword
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 cre
ated, 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 use
ful to make a function interface more Perl-like, espe
cially when the C return value is just an error condition
indicator. For example,
NO_OUTPUT int
delete_file(char *name)
POSTCALL:
if (RETVAL != 0)
The following XSUB is for a C function which requires spe
cial handling of its parameters. The Perl usage is given
first.
$status = rpcb_gettime( "localhost", $timep );
The XSUB follows.
bool_t
rpcb_gettime(host,timep)
char *host
time_t timep
CODE:
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The INIT: Keyword
The INIT: keyword allows initialization to be inserted
into the XSUB before the compiler generates the call to
the C function. Unlike the CODE: keyword above, this key
word does not affect the way the compiler handles RETVAL.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
INIT:
printf("# Host is %s\n", host );
OUTPUT:
timep
Another use for the INIT: section is to check for precon
ditions before making a call to the C function:
long long
lldiv(a,b)
long long a
long long b
INIT:
if (a == 0 && b == 0)
XSRETURN_UNDEF;
if (b == 0)
croak("lldiv: cannot divide by 0");
The NO_INIT Keyword
The NO_INIT keyword is used to indicate that a function
parameter is being used only as an output value. The
char *host
time_t &timep = NO_INIT
OUTPUT:
timep
Initializing Function Parameters
C function parameters are normally initialized with their
values from the argument stack (which in turn contains the
parameters that were passed to the XSUB from Perl). The
typemaps contain the code segments which are used to
translate the Perl values to the C parameters. The pro
grammer, however, is allowed to override the typemaps and
supply alternate (or additional) initialization code.
Initialization code starts with the first "=", ";" or "+"
on a line in the INPUT: section. The only exception hap
pens if this ";" terminates the line, then this ";" is
quietly ignored.
The following code demonstrates how to supply initializa
tion code for function parameters. The initialization
code is eval'd within double quotes by the compiler before
it is added to the output so anything which should be
interpreted literally [mainly "$", "@", 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 initial
ization 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.
The construct "\$v{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
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 state
ments:
$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() func
tion 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 param
eters 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
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:
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 par
ticular 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.
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 dec
laration line, and our assignment "h = host" is not per
formed 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
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 con
verted 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, $month) = day_month(time);
The C signature of the corresponding function should be
void day_month(int *day, int unix_time, int *month);
The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords
can be mixed with ANSI-style declarations, as in
void
day_month(OUTLIST int day, int unix_time, OUTLIST int month)
(here the optional "IN" keyword is omitted).
The "IN_OUT" parameters are identical with parameters
introduced with "The & Unary Operator" and put into the
"OUTPUT:" section (see "The OUTPUT: Keyword"). The
"IN_OUTLIST" parameters are very similar, the only differ
ence being that the value C function writes through the
pointer would not modify the Perl parameter, but is put in
the output list.
The "OUTLIST"/"OUT" parameter differ from "IN_OUT
int &month = NO_INIT
OUTPUT:
day
month
However, the generated Perl function is called in very
C-ish style:
my ($day, $month);
day_month($day, time, $month);
The "length(NAME)" Keyword
If one of the input arguments to the C function is the
length of a string argument "NAME", one can substitute the
name of the length-argument by "length(NAME)" in the XSUB
declaration. This argument must be omited when the gener
ated Perl function is called. E.g.,
void
dump_chars(char *s, short l)
{
short n = 0;
while (n < l) {
printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
n++;
}
}
MODULE = x PACKAGE = x
void dump_chars(char *s, short length(s))
should be called as "dump_chars($string)".
This directive is supported with ANSI-type function decla
rations only.
Variable-length Parameter Lists
XSUBs can have variable-length parameter lists by specify
ing 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
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: key
word and is used to tell the xsubpp compiler that the pro
grammer 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 sin
gle 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:
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 neces
sary 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: direc
tive.
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
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
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 "XSRE
TURN_EMPTY" should be used, instead. This will ensure
that the XSUB stack is properly adjusted. Consult perlapi
for other "XSRETURN" macros.
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 ver
sion of the xsubpp compiler needed to compile the XS mod
ule. 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 per
formed. When the POSTCALL: keyword is used it must pre
cede 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
# bootstrap function executes.
printf("Hello from the bootstrap!\n");
The VERSIONCHECK: Keyword
The VERSIONCHECK: keyword corresponds to xsubpp's "-ver
sioncheck" 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 "-proto
types" and "-noprototypes" options. This keyword over
rides 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 dif
ferent 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 proto
type for the XSUB. This keyword overrides all other pro
totype options and keywords but affects only the current
XSUB. Consult "Prototypes" in perlsub for information
about Perl prototypes.
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 fully-
qualified with package names. Each alias is given an
index. The compiler will setup a variable called "ix"
which contain the index of the alias which was used. When
the XSUB is called with its declared name "ix" will be 0.
The following example will create aliases "FOO::gettime()"
and "BAR::getit()" for this function.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
ALIAS:
FOO::gettime = 1
BAR::getit = 2
INIT:
printf("# ix = %d\n", ix );
OUTPUT:
timep
The 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.
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 \"\" (i.e. escaped).
The FALLBACK: Keyword
In addition to the OVERLOAD keyword, if you need to con
trol how Perl autogenerates missing overloaded operators,
you can set the FALLBACK keyword in the module header sec
tion, 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 key
word, 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
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 "XSINTER
FACE_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 to
be pulled into the module.
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 parame
ters 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 pre
cede 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 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_get
time()" having an alias "x_gettime()". When the function
is called as "rpcb_gettime()" its parameters are the usual
"(char *host, time_t *timep)", but when the function is
called as "x_gettime()" its parameters are reversed,
"(time_t *timep, char *host)".
long
rpcb_gettime(a,b)
CASE: ix == 1
ALIAS:
x_gettime = 1
INPUT:
# 'a' is timep, 'b' is host
char *b
time_t a = NO_INIT
CODE:
RETVAL = rpcb_gettime( b, &a );
OUTPUT:
a
RETVAL
CASE:
# 'a' is host, 'b' is timep
char *a
time_t &b = NO_INIT
OUTPUT:
b
RETVAL
This is useful to avoid a CODE: block for a C function
which takes a parameter by reference. Typically, the
parameter should be not a pointer type (an "int" or "long"
but not an "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 "&"
through, so the function call looks like "rpcb_get
time(host, &timep)".
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
Inserting POD, Comments and C Preprocessor Directives
C preprocessor directives are allowed within BOOT:,
PREINIT: INIT:, CODE:, PPCODE:, POSTCALL:, 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. POD documentation is
allowed at any point, both in the C and XS language sec
tions. POD must be terminated with a "=cut" command;
"xsubpp" will exit with an error if it does not. It is
very unlikely that human generated C code will be mistaken
for POD, as most indenting styles result in whitespace in
front of any line starting with "=". Machine generated XS
files may fall into this trap unless care is taken to
ensure that a space breaks the sequence "\n=".
Comments can be added to XSUBs by placing a "#" as the
first non-whitespace of a line. Care should be taken to
#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 an XSUB name contains "::", it is considered to be a
C++ method. The generated Perl function will assume that
its first argument is an object pointer. The object
pointer will be stored in a variable called THIS. The
object should have been created by C++ with the new()
function and should be blessed by Perl with the
sv_setref_pv() macro. The blessing of the object by Perl
can be handled by a typemap. An example typemap is shown
at the end of this section.
If the return type of the XSUB includes "static", the
method is considered to be a static method. 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 Perl functions will expect an object as the first
int val
PROTOTYPE $;$
CODE:
if (items > 1)
THIS->set_blue( val );
RETVAL = THIS->blue();
OUTPUT:
RETVAL
If the function's name is DESTROY then the C++ "delete"
function will be called and "THIS" will be given as its
parameter. The generated C++ code for
void
color::DESTROY()
will look like this:
color *THIS = ...; // Initialized as in typemap
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 vari
able called "CLASS", to be given as the first argument.
color *
color::new()
The generated 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\" );
parts of the interface.
Identify the C functions with input/output or output
parameters. The XSUBs for these functions may be able to
return lists to Perl.
Identify the C functions which use some inband info as an
indication of failure. They may be candidates to return
undef or an empty list in case of failure. If the failure
may be detected without a call to the C function, you may
want to use an INIT: section to report the failure. For
failures detectable after the C function returns one may
want to use a POSTCALL: section to process the failure.
In more complicated cases use CODE: or PPCODE: sections.
If many functions use the same failure indication based on
the return value, you may want to create a special typedef
to handle this situation. Put
typedef int negative_is_failure;
near the beginning of XS file, and create an OUTPUT
typemap entry for "negative_is_failure" which converts
negative values to "undef", or maybe croak()s. After this
the return value of type "negative_is_failure" will create
more Perl-like interface.
Identify which values are used by only the C and XSUB
functions themselves, say, when a parameter to a function
should be a contents of a global variable. 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 may be used to implement
input/output or output parameters, they can be handled in
XS with the "&" unary operator, and, possibly, using the
NO_INIT keyword. Some others will require handling of
types like "int *", and one needs to decide what a useful
Perl translation will do in such a case. When the seman
tic is clear, it is advisable to put the translation into
a typemap file.
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. (This is handled automatically by "h2xs
-x".)
If the same C type is used in several different contexts
which require different translations, "typedef" several
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 pack
age name. The destructor will be placed in a class corre
sponding 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. Con
sult the typemap section for more information about adding
new typemaps for an extension.
TYPEMAP
normal Perl subroutine.
The Typemap
The typemap is a collection of code fragments which are
used by the xsubpp compiler to map C function parameters
and values to Perl values. The typemap file may consist
of three sections labelled "TYPEMAP", "INPUT", and "OUT
PUT". An unlabelled initial section is assumed to be a
"TYPEMAP" section. 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. The section
labels "TYPEMAP", "INPUT", or "OUTPUT" must begin in the
first column on a line by themselves, and must be in
uppercase.
The default typemap in the "lib/ExtUtils" 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 ear
lier 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 getnetcon
figent() 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::Con
fig". One way to do this is to use underscores (_) to
separate package names, as follows:
typedef struct netconfig * Net_Config;
And then provide a typemap entry "T_PTROBJ_SPECIAL" that
maps underscores to double-colons (::), and declare
"Net_Config" to be of that type:
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.
Safely Storing Static Data in XS
Starting with Perl 5.8, a macro framework has been defined
to allow static data to be safely stored in XS modules
that will be accessed from a multi-threaded Perl.
Although primarily designed for use with multi-threaded
Perl, the macros have been designed so that they will work
with non-threaded Perl as well.
It is therefore strongly recommended that these macros be
used by all XS modules that make use of static data.
The easiest way to get a template set of macros to use is
by specifying the "-g" ("--global") option with h2xs (see
h2xs).
Below is an example module that makes use of the macros.
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
/* Global Data */
#define MY_CXT_KEY "BlindMice::_guts" XS_VERSION
typedef struct {
int count;
char name[3][100];
} my_cxt_t;
START_MY_CXT
MODULE = BlindMice PACKAGE = BlindMice
BOOT:
{
MY_CXT_INIT;
MY_CXT.count = 0;
warn("Already have 3 blind mice") ;
RETVAL = 0;
}
else {
RETVAL = ++ MY_CXT.count;
strcpy(MY_CXT.name[MY_CXT.count - 1], name);
}
char *
get_mouse_name(index)
int index
CODE:
dMY_CXT;
RETVAL = MY_CXT.lives ++;
if (index > MY_CXT.count)
croak("There are only 3 blind mice.");
else
RETVAL = newSVpv(MY_CXT.name[index - 1]);
REFERENCE
MY_CXT_KEY
This macro is used to define a unique key to refer to
the static data for an XS module. The suggested nam
ing scheme, as used by h2xs, is to use a string that
consists of the module name, the string "::_guts" and
the module version number.
#define MY_CXT_KEY "MyModule::_guts" XS_VERSION
typedef my_cxt_t
This struct typedef must always be called "my_cxt_t"
-- the other "CXT*" macros assume the existence of
the "my_cxt_t" typedef name.
Declare a typedef named "my_cxt_t" that is a struc
ture that contains all the data that needs to be
interpreter-local.
typedef struct {
int some_value;
} my_cxt_t;
START_MY_CXT
Always place the START_MY_CXT macro directly after
the declaration of "my_cxt_t".
MY_CXT_INIT
The MY_CXT_INIT macro initialises storage for the
"my_cxt_t" struct.
It must be called exactly once -- typically in a
then use this to access the "index" member
dMY_CXT;
MY_CXT.index = 2;
EXAMPLES
File "RPC.xs": Interface to some ONC+ RPC bind library
functions.
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <rpc/rpc.h>
typedef struct netconfig Netconfig;
MODULE = RPC PACKAGE = RPC
SV *
rpcb_gettime(host="localhost")
char *host
PREINIT:
time_t timep;
CODE:
ST(0) = sv_newmortal();
if( rpcb_gettime( host, &timep ) )
sv_setnv( ST(0), (double)timep );
Netconfig *
getnetconfigent(netid="udp")
char *netid
MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
void
rpcb_DESTROY(netconf)
Netconfig *netconf
CODE:
printf("NetconfigPtr::DESTROY\n");
free( netconf );
File "typemap": Custom typemap for RPC.xs.
TYPEMAP
Netconfig * T_PTROBJ
File "RPC.pm": Perl module for the RPC extension.
package RPC;
require Exporter;
print "time = $a\n";
print "netconf = $netconf\n";
$netconf = getnetconfigent("tcp");
$a = rpcb_gettime("poplar");
print "time = $a\n";
print "netconf = $netconf\n";
XS VERSION
This document covers features supported by "xsubpp" 1.935.
AUTHOR
Originally written by Dean Roehrich <roehrich@cray.com>.
Maintained since 1996 by The Perl Porters <perl
bug@perl.org>.
perl v5.8.1 2003-09-02 PERLXS(1)
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