NAME¶
perlxs - XS language reference manual
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 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¶
Many of the examples which follow will concentrate on creating an interface
between Perl and the ONC+ RPC bind library functions. The
rpcb_gettime() function is used to demonstrate many features of the XS
language. This function has two parameters; the first is an input parameter
and the second is an output parameter. The function also returns a status
value.
bool_t rpcb_gettime(const char *host, time_t *timep);
From C this function will be called with the following statements.
#include <rpc/rpc.h>
bool_t status;
time_t timep;
status = rpcb_gettime( "localhost", &timep );
If an XSUB is created to offer a direct translation between this function and
Perl, then this XSUB will be used from Perl with the following code. The
$status and $timep variables will contain the output of the function.
use RPC;
$status = rpcb_gettime( "localhost", $timep );
The following XS file shows an XS subroutine, or XSUB, which demonstrates one
possible interface to the
rpcb_gettime() function. This XSUB represents
a direct translation between C and Perl and so preserves the interface even
from Perl. This XSUB will be invoked from Perl with the 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¶
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¶
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¶
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 to
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¶
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¶
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¶
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¶
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 %s\n", 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¶
The NO_INIT keyword is used to indicate that a function parameter is being used
only as an output value. The
xsubpp compiler will normally generate
code to read the values of all function parameters from the argument stack and
assign them to C variables upon entry to the function. NO_INIT will tell the
compiler that some parameters will be used for output rather than for input
and that they will be handled before the function terminates.
The following example shows a variation of the
rpcb_gettime() function.
This function uses the timep variable only as an output variable and does not
care about its initial contents.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep = NO_INIT
OUTPUT:
timep
Initializing Function Parameters¶
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'ed 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 $type can be
used as in typemaps.
bool_t
rpcb_gettime(host,timep)
char *host = (char *)SvPV_nolen($arg);
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; /* \$v{timep}=@{[$v{timep}=$arg]} */
char *host + SvOK($v{timep}) ? SvPV_nolen($arg) : NULL;
OUTPUT:
timep
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 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 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, $month);
The "length(NAME)" Keyword¶
If one of the input arguments to the C function is the length of a string
argument "NAME", one can substitute the name of the length-argument
by "length(NAME)" in the XSUB declaration. This argument must be
omitted when the generated Perl function is called. E.g.,
void
dump_chars(char *s, short l)
{
short n = 0;
while (n < l) {
printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
n++;
}
}
MODULE = x PACKAGE = x
void dump_chars(char *s, short length(s))
should be called as "dump_chars($string)".
This directive is supported with ANSI-type function 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";
CODE:
if( items > 1 )
host = (char *)SvPV_nolen(ST(1));
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¶
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:
if( rpcb_gettime( host, &timep ) ){
ST(0) = sv_newmortal();
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!\n");
The VERSIONCHECK: Keyword¶
The VERSIONCHECK: keyword corresponds to
xsubpp's
"-versioncheck" and "-noversioncheck" options. This
keyword overrides the command line options. Version checking is enabled by
default. When version checking is enabled the XS module will attempt to verify
that its version matches the version of the PM module.
To enable version checking:
VERSIONCHECK: ENABLE
To disable version checking:
VERSIONCHECK: DISABLE
Note that if the version of the PM module is an NV (a floating point number), it
will be stringified with a possible loss of precision (currently chopping to
nine decimal places) so that it may not match the version of the XS module
anymore. Quoting the $VERSION declaration to make it a string is recommended
if long version numbers are used.
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";
CODE:
if( items > 1 )
host = (char *)SvPV_nolen(ST(1));
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.
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, separating 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 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_ANON(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 file
Rpcb1.xsh contains our "rpcb_gettime()" function:
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
The XS module can use INCLUDE: to pull that file into it.
INCLUDE: Rpcb1.xsh
If the parameters to the INCLUDE: keyword are followed by a pipe ("|")
then the compiler will interpret the parameters as a command. This feature is
mildly deprecated in favour of the "INCLUDE_COMMAND:" directive, as
documented below.
INCLUDE: cat Rpcb1.xsh |
Do not use this to run perl: "INCLUDE: perl |" will run the perl that
happens to be the first in your path and not necessarily the same perl that is
used to run "xsubpp". See "The INCLUDE_COMMAND: Keyword".
The INCLUDE_COMMAND: Keyword¶
Runs the supplied command and includes its output into the current XS document.
"INCLUDE_COMMAND" assigns special meaning to the $^X token in that
it runs the same perl interpreter that is running "xsubpp":
INCLUDE_COMMAND: cat Rpcb1.xsh
INCLUDE_COMMAND: $^X -e ...
The CASE: Keyword¶
The CASE: keyword allows an XSUB to have multiple distinct parts with each part
acting as a virtual XSUB. CASE: is greedy and if it is used then all other XS
keywords must be contained within a CASE:. This means nothing may precede the
first CASE: in the XSUB and anything following the last CASE: is included in
that case.
A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS:
variable (see "The ALIAS: Keyword"), or maybe via the
"items" variable (see "Variable-length Parameter Lists").
The last CASE: becomes the
default case if it is not associated with a
conditional. The following example shows CASE switched via "ix" with
a function "rpcb_gettime()" having an alias "x_gettime()".
When the function is called as "rpcb_gettime()" its parameters are
the usual "(char *host, time_t *timep)", but when the function is
called as "x_gettime()" its parameters are reversed, "(time_t
*timep, char *host)".
long
rpcb_gettime(a,b)
CASE: ix == 1
ALIAS:
x_gettime = 1
INPUT:
# 'a' is timep, 'b' is host
char *b
time_t a = NO_INIT
CODE:
RETVAL = rpcb_gettime( b, &a );
OUTPUT:
a
RETVAL
CASE:
# 'a' is host, 'b' is timep
char *a
time_t &b = NO_INIT
OUTPUT:
b
RETVAL
That function can be called with either of the following statements. Note the
different argument lists.
$status = rpcb_gettime( $host, $timep );
$status = x_gettime( $timep, $host );
The & Unary Operator¶
The "&" unary operator in the INPUT: section is used to tell
xsubpp that it should convert a Perl value to/from C using the C type
to the left of "&", but provide a pointer to this value when the
C function is called.
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_gettime(host, &timep)".
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
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 sections. 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 avoid making the comment
look like a C preprocessor directive, lest it be interpreted as such. The
simplest way to prevent this is to put whitespace in front of the
"#".
If you use preprocessor directives to choose one of two versions of a function,
use
#if ... version1
#else /* ... version2 */
#endif
and not
#if ... version1
#endif
#if ... version2
#endif
because otherwise
xsubpp will believe that you made a duplicate
definition of the function. Also, put a blank line before the #else/#endif so
it will not be seen as part of the function body.
Using XS With C++¶
If 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 parameter. In the
generated C++ code the object is called "THIS", and the method call
will be performed on this object. So in the C++ code the
blue() and
set_blue() methods will be called as this:
RETVAL = THIS->blue();
THIS->set_blue( val );
You could also write a single get/set method using an optional argument:
int
color::blue( val = NO_INIT )
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 variable 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\" );
XSRETURN_UNDEF;
}
Interface Strategy¶
When designing an interface between Perl and a C library a straight translation
from C to XS (such as created by "h2xs -x") is often sufficient.
However, sometimes the interface will look very C-like and occasionally
nonintuitive, especially when the C function modifies one of its parameters,
or returns failure inband (as in "negative return values mean
failure"). In cases where the programmer wishes to create a more
Perl-like interface the following strategy may help to identify the more
critical parts of the interface.
Identify the C functions 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 semantic 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 new types mapped to this C type, and
create separate
typemap entries for these new types. Use these types in
declarations of return type and parameters to XSUBs.
Perl Objects And C Structures¶
When dealing with C structures one should select either
T_PTROBJ or
T_PTRREF for the XS type. Both types are designed to handle pointers to
complex objects. The T_PTRREF type will allow the Perl object to be unblessed
while the T_PTROBJ type requires that the object be blessed. By using T_PTROBJ
one can achieve a form of type-checking because the XSUB will attempt to
verify that the Perl object is of the expected type.
The following XS code shows the
getnetconfigent() function which is used
with ONC+ TIRPC. The
getnetconfigent() function will return a pointer
to a C structure and has the C prototype shown below. The example will
demonstrate how the C pointer will become a Perl reference. Perl will consider
this reference to be a pointer to a blessed object and will attempt to call a
destructor for the object. A destructor will be provided in the XS source to
free the memory used by
getnetconfigent(). Destructors in XS can be
created by specifying an XSUB function whose name ends with the word
DESTROY. XS destructors can be used to free memory which may have been
malloc'd by another XSUB.
struct netconfig *getnetconfigent(const char *netid);
A "typedef" will be created for "struct netconfig". The Perl
object will be blessed in a class matching the name of the C type, with the
tag "Ptr" appended, and the name should not have embedded spaces if
it will be a Perl package name. The destructor will be placed in a class
corresponding to the class of the object and the PREFIX keyword will be used
to trim the name to the word DESTROY as Perl will expect.
typedef struct netconfig Netconfig;
MODULE = RPC PACKAGE = RPC
Netconfig *
getnetconfigent(netid)
char *netid
MODULE = RPC PACKAGE = NetconfigPtr PREFIX = rpcb_
void
rpcb_DESTROY(netconf)
Netconfig *netconf
CODE:
printf("Now in NetconfigPtr::DESTROY\n");
free( netconf );
This example requires the following typemap entry. Consult the typemap section
for more information about adding new typemaps for an extension.
TYPEMAP
Netconfig * T_PTROBJ
This example will be used with the following Perl statements.
use RPC;
$netconf = getnetconfigent("udp");
When Perl destroys the object referenced by $netconf it will send the object to
the supplied XSUB DESTROY function. Perl cannot determine, and does not care,
that this object is a C struct and not a Perl object. In this sense, there is
no difference between the object created by the
getnetconfigent() XSUB
and an object created by a normal Perl subroutine.
The Typemap¶
The typemap is a collection of code fragments which are used by the
xsubpp compiler to map C function parameters and values to Perl values.
The typemap file may consist of three sections labelled "TYPEMAP",
"INPUT", and "OUTPUT". 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 earlier demonstrates what may be the
typical use of extension typemaps. That typemap is used to equate a C
structure with the T_PTROBJ typemap. The typemap used by
getnetconfigent() is shown here. Note that the C type is separated from
the XS type with a tab and that the C unary operator "*" is
considered to be a part of the C type name.
TYPEMAP
Netconfig *<tab>T_PTROBJ
Here's a more complicated example: suppose that you wanted "struct
netconfig" to be blessed into the class "Net::Config". One way
to do this is to use underscores (_) to separate package names, as follows:
typedef struct netconfig * Net_Config;
And then provide a typemap entry "T_PTROBJ_SPECIAL" that maps
underscores to double-colons (::), and declare "Net_Config" to be of
that type:
TYPEMAP
Net_Config T_PTROBJ_SPECIAL
INPUT
T_PTROBJ_SPECIAL
if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")) {
IV tmp = SvIV((SV*)SvRV($arg));
$var = INT2PTR($type, tmp);
}
else
croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")
OUTPUT
T_PTROBJ_SPECIAL
sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",
(void*)$var);
The INPUT and OUTPUT sections substitute underscores for double-colons on the
fly, giving the desired effect. This example demonstrates some of the power
and versatility of the typemap facility.
The INT2PTR macro (defined in perl.h) casts an integer to a pointer, of a given
type, taking care of the possible different size of integers and pointers.
There are also PTR2IV, PTR2UV, PTR2NV macros, to map the other way, which may
be useful in OUTPUT sections.
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;
strcpy(MY_CXT.name[0], "None");
strcpy(MY_CXT.name[1], "None");
strcpy(MY_CXT.name[2], "None");
}
int
newMouse(char * name)
char * name;
PREINIT:
dMY_CXT;
CODE:
if (MY_CXT.count >= 3) {
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]);
void
CLONE(...)
CODE:
MY_CXT_CLONE;
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 naming 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 structure 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 BOOT: section. If you
are maintaining multiple interpreters, it should be called once in each
interpreter instance, except for interpreters cloned from existing ones.
(But see "MY_CXT_CLONE" below.)
- dMY_CXT
- Use the dMY_CXT macro (a declaration) in all the functions
that access MY_CXT.
- MY_CXT
- Use the MY_CXT macro to access members of the
"my_cxt_t" struct. For example, if "my_cxt_t" is
typedef struct {
int index;
} my_cxt_t;
then use this to access the "index" member
dMY_CXT;
MY_CXT.index = 2;
- aMY_CXT/pMY_CXT
- "dMY_CXT" may be quite expensive to calculate,
and to avoid the overhead of invoking it in each function it is possible
to pass the declaration onto other functions using the
"aMY_CXT"/"pMY_CXT" macros, eg
void sub1() {
dMY_CXT;
MY_CXT.index = 1;
sub2(aMY_CXT);
}
void sub2(pMY_CXT) {
MY_CXT.index = 2;
}
Analogously to "pTHX", there are equivalent forms for when the
macro is the first or last in multiple arguments, where an underscore
represents a comma, i.e. "_aMY_CXT", "aMY_CXT_",
"_pMY_CXT" and "pMY_CXT_".
- MY_CXT_CLONE
- By default, when a new interpreter is created as a copy of
an existing one (eg via "threads->create()"), both
interpreters share the same physical my_cxt_t structure. Calling
"MY_CXT_CLONE" (typically via the package's "CLONE()"
function), causes a byte-for-byte copy of the structure to be taken, and
any future dMY_CXT will cause the copy to be accessed instead.
- MY_CXT_INIT_INTERP(my_perl)
- dMY_CXT_INTERP(my_perl)
- These are versions of the macros which take an explicit
interpreter as an argument.
Note that these macros will only work together within the
same source
file; that is, a dMY_CTX in one source file will access a different structure
than a dMY_CTX in another source file.
Thread-aware system interfaces¶
Starting from Perl 5.8, in C/C++ level Perl knows how to wrap system/library
interfaces that have thread-aware versions (e.g.
getpwent_r()) into
frontend macros (e.g.
getpwent()) that correctly handle the
multithreaded interaction with the Perl interpreter. This will happen
transparently, the only thing you need to do is to instantiate a Perl
interpreter.
This wrapping happens always when compiling Perl core source (PERL_CORE is
defined) or the Perl core extensions (PERL_EXT is defined). When compiling XS
code outside of Perl core the wrapping does not take place. Note, however,
that intermixing the _r-forms (as Perl compiled for multithreaded operation
will do) and the _r-less forms is neither well-defined (inconsistent results,
data corruption, or even crashes become more likely), nor is it very portable.
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;
require DynaLoader;
@ISA = qw(Exporter DynaLoader);
@EXPORT = qw(rpcb_gettime getnetconfigent);
bootstrap RPC;
1;
File "rpctest.pl": Perl test program for the RPC extension.
use RPC;
$netconf = getnetconfigent();
$a = rpcb_gettime();
print "time = $a\n";
print "netconf = $netconf\n";
$netconf = getnetconfigent("tcp");
$a = rpcb_gettime("poplar");
print "time = $a\n";
print "netconf = $netconf\n";
XS VERSION¶
This document covers features supported by "xsubpp" 1.935.
AUTHOR¶
Originally written by Dean Roehrich <
roehrich@cray.com>.
Maintained since 1996 by The Perl Porters <
perlbug@perl.org>.
