PERLXS(1) Perl Programmers Reference Guide 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 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 INPUT: Keyword
The XSUB's parameters are usually evaluated immediately after entering
the XSUB. The INPUT: keyword can be used to force those parameters to
be evaluated a little later. The INPUT: keyword can be used multiple
times within an XSUB and can be used to list one or more input
variables. This keyword is used with the PREINIT: keyword.
The following example shows how the input parameter "timep" can be
evaluated late, after a PREINIT.
bool_t
rpcb_gettime(host,timep)
char *host
PREINIT:
time_t tt;
INPUT:
time_t timep
CODE:
RETVAL = rpcb_gettime( host, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
The next example shows each input parameter evaluated late.
bool_t
rpcb_gettime(host,timep)
PREINIT:
time_t tt;
INPUT:
char *host
PREINIT:
char *h;
INPUT:
time_t timep
CODE:
h = host;
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
Since INPUT sections allow declaration of C variables which do not
appear in the parameter list of a subroutine, this may be shortened to:
bool_t
rpcb_gettime(host,timep)
time_t tt;
char *host;
char *h = host;
time_t timep;
CODE:
RETVAL = rpcb_gettime( h, &tt );
timep = tt;
OUTPUT:
timep
RETVAL
(We used our knowledge that input conversion for "char *" is a "simple"
one, thus "host" is initialized on the declaration line, and our
assignment "h = host" is not performed too early. Otherwise one would
need to have the assignment "h = host" in a CODE: or INIT: section.)
The IN/OUTLIST/IN_OUTLIST/OUT/IN_OUT Keywords
In the list of parameters for an XSUB, one can precede parameter names
by the "IN"/"OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords. "IN"
keyword is the default, the other keywords indicate how the Perl
interface should differ from the C interface.
Parameters preceded by "OUTLIST"/"IN_OUTLIST"/"OUT"/"IN_OUT" keywords
are considered to be used by the C subroutine via pointers.
"OUTLIST"/"OUT" keywords indicate that the C subroutine does not
inspect the memory pointed by this parameter, but will write through
this pointer to provide additional return values.
Parameters preceded by "OUTLIST" keyword do not appear in the usage
signature of the generated Perl function.
Parameters preceded by "IN_OUTLIST"/"IN_OUT"/"OUT" do appear as
parameters to the Perl function. With the exception of
"OUT"-parameters, these parameters are converted to the corresponding C
type, then pointers to these data are given as arguments to the C
function. It is expected that the C function will write through these
pointers.
The return list of the generated Perl function consists of the C return
value from the function (unless the XSUB is of "void" return type or
"The NO_OUTPUT Keyword" was used) followed by all the "OUTLIST" and
"IN_OUTLIST" parameters (in the order of appearance). On the return
from the XSUB the "IN_OUT"/"OUT" Perl parameter will be modified to
have the values written by the C function.
For example, an XSUB
void
day_month(OUTLIST day, IN unix_time, OUTLIST month)
int day
int unix_time
int month
should be used from Perl as
my ($day, $month) = day_month(time);
The C signature of the corresponding function should be
void day_month(int *day, int unix_time, int *month);
The "IN"/"OUTLIST"/"IN_OUTLIST"/"IN_OUT"/"OUT" keywords can be mixed
with ANSI-style declarations, as in
void
day_month(OUTLIST int day, int unix_time, OUTLIST int month)
(here the optional "IN" keyword is omitted).
The "IN_OUT" parameters are identical with parameters introduced with
"The & Unary Operator" and put into the "OUTPUT:" section (see "The
OUTPUT: Keyword"). The "IN_OUTLIST" parameters are very similar, the
only difference being that the value C function writes through the
pointer would not modify the Perl parameter, but is put in the output
list.
The "OUTLIST"/"OUT" parameter differ from "IN_OUTLIST"/"IN_OUT"
parameters only by the initial value of the Perl parameter not being
read (and not being given to the C function - which gets some garbage
instead). For example, the same C function as above can be interfaced
with as
void day_month(OUT int day, int unix_time, OUT int month);
or
void
day_month(day, unix_time, month)
int &day = NO_INIT
int unix_time
int &month = NO_INIT
OUTPUT:
day
month
However, the generated Perl function is called in very C-ish style:
my ($day, $month);
day_month($day, time, $month);
The "length(NAME)" Keyword
If one of the input arguments to the C function is the length of a
string argument "NAME", one can substitute the name of the length-
argument by "length(NAME)" in the XSUB declaration. This argument must
be omitted when the generated Perl function is called. E.g.,
void
dump_chars(char *s, short l)
{
short n = 0;
while (n < l) {
printf("s[%d] = \"\\%#03o\"\n", n, (int)s[n]);
n++;
}
}
MODULE = x PACKAGE = x
void dump_chars(char *s, short length(s))
should be called as "dump_chars($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.
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 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
Inserting POD, Comments and C Preprocessor Directives
C preprocessor directives are allowed within BOOT:, PREINIT: INIT:,
CODE:, PPCODE:, POSTCALL:, and CLEANUP: blocks, as well as outside the
functions. Comments are allowed anywhere after the MODULE keyword.
The compiler will pass the preprocessor directives through untouched
and will remove the commented lines. POD documentation is allowed at
any point, both in the C and XS language 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>.
perl v5.12.2 2010-09-06 PERLXS(1)
