Provided by: perl-doc_5.30.0-9ubuntu0.5_all 
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 <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. 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 <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 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: <<HERE
... your typemap code here ...
HERE
where the "TYPEMAP" keyword must appear in the first column of a new line.
Refer to perlxstypemap for details on writing typemaps.
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:, 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 in such a way as to accept the number of parameters supplied by perl's overload
system. For most overload methods, it will be three parameters; for the "nomethod" function it will be
four. However, the bitwise operators "&", "|", "^", and "~" may be called with three or five arguments
(see overload).
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).
Since, as mentioned above, bitwise operators may take extra arguments, you may want to use something like
"(lobj, robj, swap, ...)" (with literal "...") as your parameter list.
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 the Perl core, the wrapping does not
take place before Perl 5.28. Starting in that release you can
#define PERL_REENTRANT
in your code to enable the wrapping. It is advisable to do so if you are using such functions, as
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. Unfortunately, not all systems have all the "_r" forms, but using this "#define"
gives you whatever protection that Perl is aware is available on each system.
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 <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. (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 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 the others up. XS code should avoid changing
the locale, as it can adversely affect other, unrelated, code and may not be thread-safe. To
minimize problems, the macros "STORE_LC_NUMERIC_SET_TO_NEEDED" in perlapi,
"STORE_LC_NUMERIC_FORCE_TO_UNDERLYING" in perlapi, and "RESTORE_LC_NUMERIC" in perlapi should be used
to affect any needed change.
But, starting with Perl v5.28, locales are thread-safe on platforms that support this functionality.
Windows has this starting with Visual Studio 2005. Many other modern platforms support the thread-
safe POSIX 2008 functions. The C "#define" "USE_THREAD_SAFE_LOCALE" will be defined iff this build
is using these. From Perl-space, the read-only variable "${SAFE_LOCALES}" is 1 if either the build
is not threaded, or if "USE_THREAD_SAFE_LOCALE" is defined; otherwise it is 0.
The way this works under-the-hood is that every thread has a choice of using a locale specific to it
(this is the Windows and POSIX 2008 functionality), or the global locale that is accessible to all
threads (this is the functionality that has always been there). The implementations for Windows and
POSIX are completely different. On Windows, the runtime can be set up so that the standard
setlocale(3) function either only knows about the global locale or the locale for this thread. On
POSIX, "setlocale" always deals with the global locale, and other functions have been created to
handle per-thread locales. Perl makes this transparent to perl-space code. It continues to use
"POSIX::setlocale()", and the interpreter translates that into the per-thread functions.
All other locale-senstive functions automatically use the per-thread locale, if that is turned on,
and failing that, the global locale. Thus calls to "setlocale" are ineffective on POSIX systems for
the current thread if that thread is using a per-thread locale. If perl is compiled for single-
thread operation, it does not use the per-thread functions, so "setlocale" does work as expected.
If you have loaded the "POSIX" module you can use the methods given in perlcall to call
"POSIX::setlocale" to safely change or query the locale (on systems where it is safe to do so), or
you can use the new 5.28 function "Perl_setlocale" in perlapi instead, which is a drop-in replacement
for the system setlocale(3), and handles single-threaded and multi-threaded applications
transparently.
There are some locale-related library calls that still aren't thread-safe because they return data in
a buffer global to all threads. In the past, these didn't matter as locales weren't thread-safe at
all. But now you have to be aware of them in case your module is called in a multi-threaded
application. The known ones are
asctime()
ctime()
gcvt() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
getdate()
wcrtomb() if its final argument is NULL
wcsrtombs() if its final argument is NULL
wcstombs()
wctomb()
Some of these shouldn't really be called in a Perl application, and for others there are thread-safe
versions of these already implemented:
asctime_r()
ctime_r()
Perl_langinfo()
The "_r" forms are automatically used, starting in Perl 5.28, if you compile your code, with
#define PERL_REENTRANT
See also "Perl_langinfo" in perlapi. You can use the methods given in perlcall, to get the best
available locale-safe versions of these
POSIX::localeconv()
POSIX::wcstombs()
POSIX::wctomb()
And note, that some items returned by "Localeconv" are available through "Perl_langinfo" in perlapi.
The others shouldn't be used in a threaded application.
Some modules may call a non-perl library that is locale-aware. This is fine as long as it doesn't
try to query or change the locale using the system "setlocale". But if these do call the system
"setlocale", those calls may be ineffective. Instead, "Perl_setlocale" works in all circumstances.
Plain setlocale is ineffective on multi-threaded POSIX 2008 systems. It operates only on the global
locale, whereas each thread has its own locale, paying no attention to the global one. Since
converting these non-Perl libraries to "Perl_setlocale" is out of the question, there is a new
function in v5.28 "switch_to_global_locale" that will switch the thread it is called from so that any
system "setlocale" calls will have their desired effect. The function "sync_locale" must be called
before returning to perl.
This thread can change the locale all it wants and it won't affect any other thread, except any that
also have been switched to the global locale. This means that a multi-threaded application can have
a single thread using an alien library without a problem; but no more than a single thread can be so-
occupied. Bad results likely will happen.
In perls without multi-thread locale support, some alien libraries, such as "Gtk" change locales.
This can cause problems for the Perl core and other modules. For these, before control is returned
to perl, 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));
or use the methods given in perlcall.
XS VERSION
This document covers features supported by "ExtUtils::ParseXS" (also known as "xsubpp") 3.13_01.
AUTHOR
Originally written by Dean Roehrich <roehrich@cray.com>.
Maintained since 1996 by The Perl Porters <perlbug@perl.org>.