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PERLXS(1)
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 the
perlpod manpage) in both the C and XS language sections, which allows the
XS file to contain embedded documentation.
See the perlxstut manpage 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 section on "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 section on "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 The
INPUT: Keyword entry elsewhere in this document.)
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.)
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 ]
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 the perlguts manpage 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 The RETVAL Variable entry elsewhere in this
document 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)
POST_CALL:
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'd within double
quotes by the compiler before it is added to the output so anything which
should be interpreted literally [mainly "$", "@", or "\\"] must be
protected with backslashes. The variables $var, $arg, and $type can be
used as in typemaps.
bool_t
rpcb_gettime(host,timep)
char *host = (char *)SvPV($arg,PL_na);
time_t &timep = 0;
OUTPUT:
timep
This should not be used to supply default values for parameters. One would
normally use this when a function parameter must be processed by another
library function before it can be used. Default parameters are covered in
the next section.
If the initialization begins with "=", then it is output in the declaration
for the input variable, replacing the initialization supplied by the
typemap. If the initialization begins with ";" or "+", then it is
performed after all of the input variables have been declared. In the ";"
case the initialization normally supplied by the typemap is not performed.
For the "+" case, the declaration for the variable will include the
initialization from the typemap. A global variable, "%v", is available for
the truly rare case where information from one initialization is needed in
another initialization.
Here's a truly obscure example:
bool_t
rpcb_gettime(host,timep)
time_t &timep ; /* \$v{timep}=@{[$v{timep}=$arg]} */
char *host + SvOK($v{timep}) ? SvPV($arg,PL_na) : 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 the section on "Initializing Function Parameters").
Declaring these variables in an INIT: section will not help.
In such cases, to force an additional variable to be declared together with
declarations of other variables, place the declaration into a PREINIT:
section. The PREINIT: keyword may be used one or more times within an
XSUB.
The following examples are equivalent, but if the code is using complex
typemaps then the first example is safer.
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
CODE:
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
For this particular case an INIT: keyword would generate the same C code as
the PREINIT: keyword. Another correct, but error-prone example:
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
CODE:
char *host = "localhost";
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
Another way to declare "host" is to use a C block in the CODE: section:
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
CODE:
{
char *host = "localhost";
RETVAL = rpcb_gettime( host, &timep );
}
OUTPUT:
timep
RETVAL
The ability to put additional declarations before the typemap entries are
processed is very handy in the cases when typemap conversions manipulate
some global state:
MyObject
mutate(o)
PREINIT:
MyState st = global_state;
INPUT:
MyObject o;
CLEANUP:
reset_to(global_state, st);
Here we suppose that conversion to "MyObject" in the INPUT: section and
from MyObject when processing RETVAL will modify a global variable
"global_state". After these conversions are performed, we restore the old
value of "global_state" (to avoid memory leaks, for example).
There is another way to trade clarity for compactness: INPUT sections allow
declaration of C variables which do not appear in the parameter list of a
subroutine. Thus the above code for mutate() can be rewritten as
MyObject
mutate(o)
MyState st = global_state;
MyObject o;
CLEANUP:
reset_to(global_state, st);
and the code for rpcb_gettime() can be rewritten as
bool_t
rpcb_gettime(timep)
time_t timep = NO_INIT
char *host = "localhost";
C_ARGS:
host, &timep
OUTPUT:
timep
RETVAL
The SCOPE: Keyword
The SCOPE: keyword allows scoping to be enabled for a particular XSUB. If
enabled, the XSUB will invoke ENTER and LEAVE automatically.
To support potentially complex type mappings, if a typemap entry used by an
XSUB contains a comment like "/*scope*/" then scoping will be automatically
enabled for that XSUB.
To enable scoping:
SCOPE: ENABLE
To disable scoping:
SCOPE: DISABLE
The INPUT: Keyword
The XSUB's parameters are usually evaluated immediately after entering the
XSUB. The INPUT: keyword can be used to force those parameters to be
evaluated a little later. The INPUT: keyword can be used multiple times
within an XSUB and can be used to list one or more input variables. This
keyword is used with the PREINIT: keyword.
The following example shows how the input parameter "timep" can be
evaluated late, after a PREINIT.
bool_t
rpcb_gettime(host,timep)
char *host
PREINIT:
time_t tt;
INPUT:
time_t timep
CODE:
RETVAL = rpcb_gettime( host, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
The next example shows each input parameter evaluated late.
bool_t
rpcb_gettime(host,timep)
PREINIT:
time_t tt;
INPUT:
char *host
PREINIT:
char *h;
INPUT:
time_t timep
CODE:
h = host;
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
Since INPUT sections allow declaration of C variables which do not appear
in the parameter list of a subroutine, this may be shortened to:
bool_t
rpcb_gettime(host,timep)
time_t tt;
char *host;
char *h = host;
time_t timep;
CODE:
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
(We used our knowledge that input conversion for "char *" is a "simple"
one, thus "host" is initialized on the declaration line, and our assignment
"h = host" is not performed too early. Otherwise one would need to have
the assignment "h = host" in a CODE: or INIT: section.)
The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
In the list of parameters for an XSUB, one can precede parameter names by
the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords. "IN" keyword is
the default, the other keywords indicate how the Perl interface should
differ from the C interface.
Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords are
considered to be used by the C subroutine via pointers. "OUTLIST"/"OUT"
keywords indicate that the C subroutine does not inspect the memory pointed
by this parameter, but will write through this pointer to provide
additional return values.
Parameters preceded by "OUTLIST" keyword do not appear in the usage
signature of the generated Perl function.
Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do appear as parameters
to the Perl function. With the exception of "OUT"-parameters, these
parameters are converted to the corresponding C type, then pointers to
these data are given as arguments to the C function. It is expected that
the C function will write through these pointers.
The return list of the generated Perl function consists of the C return
value from the function (unless the XSUB is of "void" return type or "The
NO_OUTPUT Keyword" was used) followed by all the "OUTLIST" and "IN_OUTLIST"
parameters (in the order of appearance). On the return from the XSUB the
"IN_OUT"/"OUT" Perl parameter will be modified to have the values written
by the C function.
For example, an XSUB
void
day_month(OUTLIST day, IN unix_time, OUTLIST month)
int day
int unix_time
int month
should be used from Perl as
my ($day, $month) = day_month(time);
The C signature of the corresponding function should be
void day_month(int *day, int unix_time, int *month);
The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords can be mixed with
ANSI-style declarations, as in
void
day_month(OUTLIST int day, int unix_time, OUTLIST int month)
(here the optional "IN" keyword is omitted).
The "IN_OUT" parameters are identical with parameters introduced with the
The & Unary Operator entry elsewhere in this document and put into the
"OUTPUT:" section (see the The OUTPUT: Keyword entry elsewhere in this
document). The "IN_OUTLIST" parameters are very similar, the only
difference being that the value C function writes through the pointer would
not modify the Perl parameter, but is put in the output list.
The "OUTLIST"/"OUT" parameter differ from "IN_OUTLIST"/"IN_OUT" parameters
only by the the initial value of the Perl parameter not being read (and not
being given to the C function - which gets some garbage instead). For
example, the same C function as above can be interfaced with as
void day_month(OUT int day, int unix_time, OUT int month);
or
void
day_month(day, unix_time, month)
int &day = NO_INIT
int unix_time
int &month = NO_INIT
OUTPUT:
day
month
However, the generated Perl function is called in very C-ish style:
my ($day, $month);
day_month($day, time, $month);
Variable-length Parameter Lists
XSUBs can have variable-length parameter lists by specifying an ellipsis
"(...)" in the parameter list. This use of the ellipsis is similar to that
found in ANSI C. The programmer is able to determine the number of
arguments passed to the XSUB by examining the "items" variable which the
xsubpp compiler supplies for all XSUBs. By using this mechanism one can
create an XSUB which accepts a list of parameters of unknown length.
The host parameter for the rpcb_gettime() XSUB can be optional so the
ellipsis can be used to indicate that the XSUB will take a variable number
of parameters. Perl should be able to call this XSUB with either of the
following statements.
$status = rpcb_gettime( $timep, $host );
$status = rpcb_gettime( $timep );
The XS code, with ellipsis, follows.
bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PREINIT:
char *host = "localhost";
STRLEN n_a;
CODE:
if( items > 1 )
host = (char *)SvPV(ST(1), n_a);
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
The C_ARGS: Keyword
The C_ARGS: keyword allows creating of XSUBS which have different calling
sequence from Perl than from C, without a need to write CODE: or PPCODE:
section. The contents of the C_ARGS: paragraph is put as the argument to
the called C function without any change.
For example, suppose that a C function is declared as
symbolic nth_derivative(int n, symbolic function, int flags);
and that the default flags are kept in a global C variable "default_flags".
Suppose that you want to create an interface which is called as
$second_deriv = $function->nth_derivative(2);
To do this, declare the XSUB as
symbolic
nth_derivative(function, n)
symbolic function
int n
C_ARGS:
n, function, default_flags
The PPCODE: Keyword
The PPCODE: keyword is an alternate form of the CODE: keyword and is used
to tell the xsubpp compiler that the programmer is supplying the code to
control the argument stack for the XSUBs return values. Occasionally one
will want an XSUB to return a list of values rather than a single value.
In these cases one must use PPCODE: and then explicitly push the list of
values on the stack. The PPCODE: and CODE: keywords should not be used
together within the same XSUB.
The actual difference between PPCODE: and CODE: sections is in the
initialization of "SP" macro (which stands for the current Perl stack
pointer), and in the handling of data on the stack when returning from an
XSUB. In CODE: sections SP preserves the value which was on entry to the
XSUB: SP is on the function pointer (which follows the last parameter). In
PPCODE: sections SP is moved backward to the beginning of the parameter
list, which allows "PUSH*()" macros to place output values in the place
Perl expects them to be when the XSUB returns back to Perl.
The generated trailer for a CODE: section ensures that the number of return
values Perl will see is either 0 or 1 (depending on the "void"ness of the
return value of the C function, and heuristics mentioned in the section on
"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 the perlguts manpage 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:
ST(0) = sv_newmortal();
if( rpcb_gettime( host, &timep ) ){
sv_setnv( ST(0), (double)timep);
}
else{
ST(0) = &PL_sv_undef;
}
To return an empty list one must use a PPCODE: block and then not push
return values on the stack.
void
rpcb_gettime(host)
char *host
PREINIT:
time_t timep;
PPCODE:
if( rpcb_gettime( host, &timep ) )
PUSHs(sv_2mortal(newSViv(timep)));
else{
/* Nothing pushed on stack, so an empty
* list is implicitly returned. */
}
Some people may be inclined to include an explicit "return" in the above
XSUB, rather than letting control fall through to the end. In those
situations "XSRETURN_EMPTY" should be used, instead. This will ensure that
the XSUB stack is properly adjusted. Consult the API LISTING entry in the
perlguts manpage 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 POST_CALL: section as well.
Together with PREINIT: simplifications, this leads to:
int
rpcb_gettime(host)
char *host
time_t timep;
POST_CALL:
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 POST_CALL: Keyword
This keyword can be used when an XSUB requires special procedures executed
after the C subroutine call is performed. When the POST_CALL: keyword is
used it must precede OUTPUT: and CLEANUP: blocks which are present in the
XSUB.
The POST_CALL: 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
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 the Prototypes entry in the perlsub manpage for
information about Perl prototypes.
bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PROTOTYPE: $;$
PREINIT:
char *host = "localhost";
STRLEN n_a;
CODE:
if( items > 1 )
host = (char *)SvPV(ST(1), n_a);
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
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 INTERFACE: Keyword
This keyword declares the current XSUB as a keeper of the given calling
signature. If some text follows this keyword, it is considered as a list
of functions which have this signature, and should be attached to the
current XSUB.
For example, if you have 4 C functions multiply(), divide(), add(),
subtract() all having the signature:
symbolic f(symbolic, symbolic);
you can make them all to use the same XSUB using this:
symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE:
multiply divide
add subtract
(This is the complete XSUB code for 4 Perl functions!) Four generated Perl
function share names with corresponding C functions.
The advantage of this approach comparing to ALIAS: keyword is that there is
no need to code a switch statement, each Perl function (which shares the
same XSUB) knows which C function it should call. Additionally, one can
attach an extra function remainder() at runtime by using
CV *mycv = newXSproto("Symbolic::remainder",
XS_Symbolic_interface_s_ss, __FILE__, "$$");
XSINTERFACE_FUNC_SET(mycv, remainder);
say, from another XSUB. (This example supposes that there was no
INTERFACE_MACRO: section, otherwise one needs to use something else instead
of "XSINTERFACE_FUNC_SET", see the next section.)
The INTERFACE_MACRO: Keyword
This keyword allows one to define an INTERFACE using a different way to
extract a function pointer from an XSUB. The text which follows this
keyword should give the name of macros which would extract/set a function
pointer. The extractor macro is given return type, "CV*", and
"XSANY.any_dptr" for this "CV*". The setter macro is given cv, and the
function pointer.
The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET". An
INTERFACE keyword with an empty list of functions can be omitted if
INTERFACE_MACRO keyword is used.
Suppose that in the previous example functions pointers for multiply(),
divide(), add(), subtract() are kept in a global C array "fp[]" with
offsets being "multiply_off", "divide_off", "add_off", "subtract_off".
Then one can use
#define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
((XSINTERFACE_CVT(ret,))fp[CvXSUBANY(cv).any_i32])
#define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
CvXSUBANY(cv).any_i32 = CAT2( f, _off )
in C section,
symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE_MACRO:
XSINTERFACE_FUNC_BYOFFSET
XSINTERFACE_FUNC_BYOFFSET_set
INTERFACE:
multiply divide
add subtract
in XSUB section.
The INCLUDE: Keyword
This keyword can be used to pull other files into the XS module. The other
files may have XS code. INCLUDE: can also be used to run a command to
generate the XS code to be pulled into the module.
The file Rpcb1.xsh contains our "rpcb_gettime()" function:
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
The XS module can use INCLUDE: to pull that file into it.
INCLUDE: Rpcb1.xsh
If the parameters to the INCLUDE: keyword are followed by a pipe ("|") then
the compiler will interpret the parameters as a command.
INCLUDE: cat Rpcb1.xsh |
The CASE: Keyword
The CASE: keyword allows an XSUB to have multiple distinct parts with each
part acting as a virtual XSUB. CASE: is greedy and if it is used then all
other XS keywords must be contained within a CASE:. This means nothing may
precede the first CASE: in the XSUB and anything following the last CASE:
is included in that case.
A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS:
variable (see the section on "The ALIAS: Keyword"), or maybe via the
"items" variable (see the section on "Variable-length Parameter Lists").
The last CASE: becomes the default case if it is not associated with a
conditional. The following example shows CASE switched via "ix" with a
function "rpcb_gettime()" having an alias "x_gettime()". When the function
is called as "rpcb_gettime()" its parameters are the usual "(char *host,
time_t *timep)", but when the function is called as "x_gettime()" its
parameters are reversed, "(time_t *timep, char *host)".
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 a "int*" or "long*").
The following XSUB will generate incorrect C code. The xsubpp compiler
will turn this into code which calls "rpcb_gettime()" with parameters
"(char *host, time_t timep)", but the real "rpcb_gettime()" wants the
"timep" parameter to be of type "time_t*" rather than "time_t".
bool_t
rpcb_gettime(host,timep)
char *host
time_t timep
OUTPUT:
timep
That problem is corrected by using the "&" operator. The xsubpp compiler
will now turn this into code which calls "rpcb_gettime()" correctly with
parameters "(char *host, time_t *timep)". It does this by carrying the "&"
through, so the function call looks like "rpcb_gettime(host, &timep)".
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
Inserting POD, Comments and C Preprocessor Directives
C preprocessor directives are allowed within BOOT:, PREINIT: INIT:, CODE:,
PPCODE:, POST_CALL:, 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
POST_CALL: 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));
$var = ($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.
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>.
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