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.
Before writing XS, read the "CAVEATS" section below.
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.
For more information on typemaps, see perlxstypemap.
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 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
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. A #define for "PERL_NO_GET_CONTEXT" should be present
to fetch the interpreter context more efficiently, see perlguts for
details.
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include
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 perlxstypemap 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 *", "CV *", and "SVREF"
(which indicates a scalar reference, not a general "SV *"). In XS code
on perls starting with perl 5.16, you can override the typemaps for any
of these types with a version that has proper handling of refcounts. In
your "TYPEMAP" section, do
AV* T_AVREF_REFCOUNT_FIXED
to get the repaired variant. For backward compatibility with older
versions of perl, you can instead decrement the reference count manually
when you're returning one of the aforementioned types using
"sv_2mortal":
AV *
array()
CODE:
RETVAL = newAV();
sv_2mortal((SV*)RETVAL);
/* do something with RETVAL */
OUTPUT:
RETVAL
Remember that you don't have to do this for an "SV *". The reference
documentation for all core typemaps can be found in perlxstypemap.
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
The TYPEMAP: Keyword
Starting with Perl 5.16, you can embed typemaps into your XS code
instead of or in addition to typemaps in a separate file. Multiple such
embedded typemaps will be processed in order of appearance in the XS
code and like local typemap files take precedence over the default
typemap, the embedded typemaps may overwrite previous definitions of
TYPEMAP, INPUT, and OUTPUT stanzas. The syntax for embedded typemaps is
TYPEMAP: < 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:, 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 disabled 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. Note that xsubpp will nag you if you
don't explicitly enable or disable prototypes, with:
Please specify prototyping behavior for Foo.xs (see perlxs manual)
To enable prototypes:
PROTOTYPES: ENABLE
To disable prototypes:
PROTOTYPES: DISABLE
The PROTOTYPE: Keyword
This keyword is similar to the PROTOTYPES: keyword above but can be used
to force xsubpp to use a specific prototype for the XSUB. This keyword
overrides all other prototype options and keywords but affects only the
current XSUB. Consult "Prototypes" in perlsub for information about Perl
prototypes.
bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PROTOTYPE: $;$
PREINIT:
char *host = "localhost";
CODE:
if( items > 1 )
host = (char *)SvPV_nolen(ST(1));
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL
If the prototypes are enabled, you can disable it locally for a given
XSUB as in the following example:
void
rpcb_gettime_noproto()
PROTOTYPE: DISABLE
...
The ALIAS: Keyword
The ALIAS: keyword allows an XSUB to have two or more unique Perl names
and to know which of those names was used when it was invoked. The Perl
names may be fully-qualified with package names. Each alias is given an
index. The compiler will setup a variable called "ix" which contain the
index of the alias which was used. When the XSUB is called with its
declared name "ix" will be 0.
The following example will create aliases "FOO::gettime()" and
"BAR::getit()" for this function.
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
ALIAS:
FOO::gettime = 1
BAR::getit = 2
INIT:
printf("# ix = %d\n", ix );
OUTPUT:
timep
The OVERLOAD: Keyword
Instead of writing an overloaded interface using pure Perl, you can also
use the OVERLOAD keyword to define additional Perl names for your
functions (like the ALIAS: keyword above). However, the overloaded
functions must be defined with three parameters (except for the
nomethod() function which needs four parameters). If any function has
the OVERLOAD: keyword, several additional lines will be defined in the c
file generated by xsubpp in order to register with the overload magic.
Since blessed objects are actually stored as RV's, it is useful to use
the typemap features to preprocess parameters and extract the actual SV
stored within the blessed RV. See the sample for T_PTROBJ_SPECIAL below.
To use the OVERLOAD: keyword, create an XS function which takes three
input parameters ( or use the c style '...' definition) like this:
SV *
cmp (lobj, robj, swap)
My_Module_obj lobj
My_Module_obj robj
IV swap
OVERLOAD: cmp <=>
{ /* function defined here */}
In this case, the function will overload both of the three way
comparison operators. For all overload operations using non-alpha
characters, you must type the parameter without quoting, separating
multiple overloads with whitespace. Note that "" (the stringify
overload) should be entered as \"\" (i.e. escaped).
The FALLBACK: Keyword
In addition to the OVERLOAD keyword, if you need to control how Perl
autogenerates missing overloaded operators, you can set the FALLBACK
keyword in the module header section, like this:
MODULE = RPC PACKAGE = RPC
FALLBACK: TRUE
...
where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF.
If you do not set any FALLBACK value when using OVERLOAD, it defaults to
UNDEF. FALLBACK is not used except when one or more functions using
OVERLOAD have been defined. Please see "fallback" in overload for more
details.
The INTERFACE: Keyword
This keyword declares the current XSUB as a keeper of the given calling
signature. If some text follows this keyword, it is considered as a list
of functions which have this signature, and should be attached to the
current XSUB.
For example, if you have 4 C functions multiply(), divide(), add(),
subtract() all having the signature:
symbolic f(symbolic, symbolic);
you can make them all to use the same XSUB using this:
symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE:
multiply divide
add subtract
(This is the complete XSUB code for 4 Perl functions!) Four generated
Perl function share names with corresponding C functions.
The advantage of this approach comparing to ALIAS: keyword is that there
is no need to code a switch statement, each Perl function (which shares
the same XSUB) knows which C function it should call. Additionally, one
can attach an extra function remainder() at runtime by using
CV *mycv = newXSproto("Symbolic::remainder",
XS_Symbolic_interface_s_ss, __FILE__, "$$");
XSINTERFACE_FUNC_SET(mycv, remainder);
say, from another XSUB. (This example supposes that there was no
INTERFACE_MACRO: section, otherwise one needs to use something else
instead of "XSINTERFACE_FUNC_SET", see the next section.)
The INTERFACE_MACRO: Keyword
This keyword allows one to define an INTERFACE using a different way to
extract a function pointer from an XSUB. The text which follows this
keyword should give the name of macros which would extract/set a
function pointer. The extractor macro is given return type, "CV*", and
"XSANY.any_dptr" for this "CV*". The setter macro is given cv, and the
function pointer.
The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET". An
INTERFACE keyword with an empty list of functions can be omitted if
INTERFACE_MACRO keyword is used.
Suppose that in the previous example functions pointers for multiply(),
divide(), add(), subtract() are kept in a global C array "fp[]" with
offsets being "multiply_off", "divide_off", "add_off", "subtract_off".
Then one can use
#define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
((XSINTERFACE_CVT_ANON(ret))fp[CvXSUBANY(cv).any_i32])
#define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
CvXSUBANY(cv).any_i32 = CAT2( f, _off )
in C section,
symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE_MACRO:
XSINTERFACE_FUNC_BYOFFSET
XSINTERFACE_FUNC_BYOFFSET_set
INTERFACE:
multiply divide
add subtract
in XSUB section.
The INCLUDE: Keyword
This keyword can be used to pull other files into the XS module. The
other files may have XS code. INCLUDE: can also be used to run a command
to generate the XS code to be pulled into the module.
The file Rpcb1.xsh contains our "rpcb_gettime()" function:
bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep
The XS module can use INCLUDE: to pull that file into it.
INCLUDE: Rpcb1.xsh
If the parameters to the INCLUDE: keyword are followed by a pipe ("|")
then the compiler will interpret the parameters as a command. This
feature is mildly deprecated in favour of the "INCLUDE_COMMAND:"
directive, as documented below.
INCLUDE: cat Rpcb1.xsh |
Do not use this to run perl: "INCLUDE: perl |" will run the perl that
happens to be the first in your path and not necessarily the same perl
that is used to run "xsubpp". See "The INCLUDE_COMMAND: Keyword".
The INCLUDE_COMMAND: Keyword
Runs the supplied command and includes its output into the current XS
document. "INCLUDE_COMMAND" assigns special meaning to the $^X token in
that it runs the same perl interpreter that is running "xsubpp":
INCLUDE_COMMAND: cat Rpcb1.xsh
INCLUDE_COMMAND: $^X -e ...
The CASE: Keyword
The CASE: keyword allows an XSUB to have multiple distinct parts with
each part acting as a virtual XSUB. CASE: is greedy and if it is used
then all other XS keywords must be contained within a CASE:. This means
nothing may precede the first CASE: in the XSUB and anything following
the last CASE: is included in that case.
A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS:
variable (see "The ALIAS: Keyword"), or maybe via the "items" variable
(see "Variable-length Parameter Lists"). The last CASE: becomes the
default case if it is not associated with a conditional. The following
example shows CASE switched via "ix" with a function "rpcb_gettime()"
having an alias "x_gettime()". When the function is called as
"rpcb_gettime()" its parameters are the usual "(char *host, time_t
*timep)", but when the function is called as "x_gettime()" its
parameters are reversed, "(time_t *timep, char *host)".
long
rpcb_gettime(a,b)
CASE: ix == 1
ALIAS:
x_gettime = 1
INPUT:
# 'a' is timep, 'b' is host
char *b
time_t a = NO_INIT
CODE:
RETVAL = rpcb_gettime( b, &a );
OUTPUT:
a
RETVAL
CASE:
# 'a' is host, 'b' is timep
char *a
time_t &b = NO_INIT
OUTPUT:
b
RETVAL
That function can be called with either of the following statements.
Note the different argument lists.
$status = rpcb_gettime( $host, $timep );
$status = x_gettime( $timep, $host );
The EXPORT_XSUB_SYMBOLS: Keyword
The EXPORT_XSUB_SYMBOLS: keyword is likely something you will never
need. In perl versions earlier than 5.16.0, this keyword does nothing.
Starting with 5.16, XSUB symbols are no longer exported by default. That
is, they are "static" functions. If you include
EXPORT_XSUB_SYMBOLS: ENABLE
in your XS code, the XSUBs following this line will not be declared
"static". You can later disable this with
EXPORT_XSUB_SYMBOLS: DISABLE
which, again, is the default that you should probably never change. You
cannot use this keyword on versions of perl before 5.16 to make XSUBs
"static".
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 perlxstypemap
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.
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.
#define PERL_NO_GET_CONTEXT
#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)
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);
}
OUTPUT:
RETVAL
char *
get_mouse_name(index)
int index
PREINIT:
dMY_CXT;
CODE:
if (index > MY_CXT.count)
croak("There are only 3 blind mice.");
else
RETVAL = MY_CXT.name[index - 1];
OUTPUT:
RETVAL
void
CLONE(...)
CODE:
MY_CXT_CLONE;
MY_CXT 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 initializes 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.
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include
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. (cf. perlxstypemap)
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";
CAVEATS
XS code has full access to system calls including C library functions.
It thus has the capability of interfering with things that the Perl core
or other modules have set up, such as signal handlers or file handles.
It could mess with the memory, or any number of harmful things. Don't.
Some modules have an event loop, waiting for user-input. It is highly
unlikely that two such modules would work adequately together in a
single Perl application.
In general, the perl interpreter views itself as the center of the
universe as far as the Perl program goes. XS code is viewed as a
help-mate, to accomplish things that perl doesn't do, or doesn't do fast
enough, but always subservient to perl. The closer XS code adheres to
this model, the less likely conflicts will occur.
One area where there has been conflict is in regards to C locales. (See
perllocale.) perl, with one exception and unless told otherwise, sets up
the underlying locale the program is running in to the locale passed
into it from the environment. This is an important difference from a
generic C language program, where the underlying locale is the "C"
locale unless the program changes it. As of v5.20, this underlying
locale is completely hidden from pure perl code outside the lexical
scope of "use locale" except for a couple of function calls in the POSIX
module which of necessity use it. But the underlying locale, with that
one exception is exposed to XS code, affecting all C library routines
whose behavior is locale-dependent. Your XS code better not assume that
the underlying locale is "C". The exception is the "LC_NUMERIC" locale
category, and the reason it is an exception is that experience has shown
that it can be problematic for XS code, whereas we have not had reports
of problems with the other locale categories. And the reason for this
one category being problematic is that the character used as a decimal
point can vary. Many European languages use a comma, whereas English,
and hence Perl are expecting a dot (U+002E: FULL STOP). Many modules can
handle only the radix character being a dot, and so perl attempts to
make it so. Up through Perl v5.20, the attempt was merely to set
"LC_NUMERIC" upon startup to the "C" locale. Any setlocale() otherwise
would change it; this caused some failures. Therefore, starting in
v5.22, perl tries to keep "LC_NUMERIC" always set to "C" for XS code.
To summarize, here's what to expect and how to handle locales in XS
code:
Non-locale-aware XS code
Keep in mind that even if you think your code is not locale-aware,
it may call a C library function that is. Hopefully the man page for
such a function will indicate that dependency, but the documentation
is imperfect.
The current locale is exposed to XS code except possibly
"LC_NUMERIC" (explained in the next paragraph). There have not been
reports of problems with the other categories. Perl initializes
things on start-up so that the current locale is the one which is
indicated by the user's environment in effect at that time. See
"ENVIRONMENT" in perllocale.
However, up through v5.20, Perl initialized things on start-up so
that "LC_NUMERIC" was set to the "C" locale. But if any code
anywhere changed it, it would stay changed. This means that your
module can't count on "LC_NUMERIC" being something in particular,
and you can't expect floating point numbers (including version
strings) to have dots in them. If you don't allow for a non-dot,
your code could break if anyone anywhere changed the locale. For
this reason, v5.22 changed the behavior so that Perl tries to keep
"LC_NUMERIC" in the "C" locale except around the operations
internally where it should be something else. Misbehaving XS code
will always be able to change the locale anyway, but the most common
instance of this is checked for and handled.
Locale-aware XS code
If the locale from the user's environment is desired, there should
be no need for XS code to set the locale except for "LC_NUMERIC", as
perl has already set it up. XS code should avoid changing the
locale, as it can adversely affect other, unrelated, code and may
not be thread safe. However, some alien libraries that may be called
do set it, such as "Gtk". This can cause problems for the perl core
and other modules. Starting in v5.20.1, calling the function
sync_locale() from XS should be sufficient to avoid most of these
problems. Prior to this, you need a pure Perl statement that does
this:
POSIX::setlocale(LC_ALL, POSIX::setlocale(LC_ALL));
In the event that your XS code may need the underlying "LC_NUMERIC"
locale, there are macros available to access this; see
"Locale-related functions and macros" in perlapi.
XS VERSION
This document covers features supported by "ExtUtils::ParseXS" (also
known as "xsubpp") 3.13_01.
AUTHOR
Originally written by Dean Roehrich .
Maintained since 1996 by The Perl Porters .