perlxs - Online in the Cloud

PROGRAM:

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 enabled by default. When
prototypes are enabled XSUBs will be given Perl prototypes. This keyword may be used
multiple times in an XS module to enable and disable prototypes for different parts of the
module.

To enable prototypes:

PROTOTYPES: ENABLE

To disable prototypes:

PROTOTYPES: DISABLE

The PROTOTYPE: Keyword
This keyword is similar to the PROTOTYPES: keyword above but can be used to force xsubpp
to use a specific prototype for the XSUB. This keyword overrides all other prototype
options and keywords but affects only the current XSUB. Consult "Prototypes" in perlsub
for information about Perl prototypes.

bool_t
rpcb_gettime(timep, ...)
time_t timep = NO_INIT
PROTOTYPE: $;$
PREINIT:
char *host = "localhost";
CODE:
if( items > 1 )
host = (char *)SvPV_nolen(ST(1));
RETVAL = rpcb_gettime( host, &timep );
OUTPUT:
timep
RETVAL

If the prototypes are enabled, you can disable it locally for a given XSUB as in the
following example:

void
rpcb_gettime_noproto()
PROTOTYPE: DISABLE
...

The ALIAS: Keyword
The ALIAS: keyword allows an XSUB to have two or more unique Perl names and to know which
of those names was used when it was invoked. The Perl names may be fully-qualified with
package names. Each alias is given an index. The compiler will setup a variable called
"ix" which contain the index of the alias which was used. When the XSUB is called with
its declared name "ix" will be 0.

The following example will create aliases "FOO::gettime()" and "BAR::getit()" for this
function.

bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
ALIAS:
FOO::gettime = 1
BAR::getit = 2
INIT:
printf("# ix = %d\n", ix );
OUTPUT:
timep

The OVERLOAD: Keyword
Instead of writing an overloaded interface using pure Perl, you can also use the OVERLOAD
keyword to define additional Perl names for your functions (like the ALIAS: keyword
above). However, the overloaded functions must be defined with three parameters (except
for the nomethod() function which needs four parameters). If any function has the
OVERLOAD: keyword, several additional lines will be defined in the c file generated by
xsubpp in order to register with the overload magic.

Since blessed objects are actually stored as RV's, it is useful to use the typemap
features to preprocess parameters and extract the actual SV stored within the blessed RV.
See the sample for T_PTROBJ_SPECIAL below.

To use the OVERLOAD: keyword, create an XS function which takes three input parameters (
or use the c style '...' definition) like this:

SV *
cmp (lobj, robj, swap)
My_Module_obj lobj
My_Module_obj robj
IV swap
OVERLOAD: cmp <=>
{ /* function defined here */}

In this case, the function will overload both of the three way comparison operators. For
all overload operations using non-alpha characters, you must type the parameter without
quoting, separating multiple overloads with whitespace. Note that "" (the stringify
overload) should be entered as \"\" (i.e. escaped).

The FALLBACK: Keyword
In addition to the OVERLOAD keyword, if you need to control how Perl autogenerates missing
overloaded operators, you can set the FALLBACK keyword in the module header section, like
this:

MODULE = RPC PACKAGE = RPC

FALLBACK: TRUE
...

where FALLBACK can take any of the three values TRUE, FALSE, or UNDEF. If you do not set
any FALLBACK value when using OVERLOAD, it defaults to UNDEF. FALLBACK is not used except
when one or more functions using OVERLOAD have been defined. Please see "fallback" in
overload for more details.

The INTERFACE: Keyword
This keyword declares the current XSUB as a keeper of the given calling signature. If
some text follows this keyword, it is considered as a list of functions which have this
signature, and should be attached to the current XSUB.

For example, if you have 4 C functions multiply(), divide(), add(), subtract() all having
the signature:

symbolic f(symbolic, symbolic);

you can make them all to use the same XSUB using this:

symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE:
multiply divide
add subtract

(This is the complete XSUB code for 4 Perl functions!) Four generated Perl function share
names with corresponding C functions.

The advantage of this approach comparing to ALIAS: keyword is that there is no need to
code a switch statement, each Perl function (which shares the same XSUB) knows which C
function it should call. Additionally, one can attach an extra function remainder() at
runtime by using

CV *mycv = newXSproto("Symbolic::remainder",
XS_Symbolic_interface_s_ss, __FILE__, "$$");
XSINTERFACE_FUNC_SET(mycv, remainder);

say, from another XSUB. (This example supposes that there was no INTERFACE_MACRO:
section, otherwise one needs to use something else instead of "XSINTERFACE_FUNC_SET", see
the next section.)

The INTERFACE_MACRO: Keyword
This keyword allows one to define an INTERFACE using a different way to extract a function
pointer from an XSUB. The text which follows this keyword should give the name of macros
which would extract/set a function pointer. The extractor macro is given return type,
"CV*", and "XSANY.any_dptr" for this "CV*". The setter macro is given cv, and the
function pointer.

The default value is "XSINTERFACE_FUNC" and "XSINTERFACE_FUNC_SET". An INTERFACE keyword
with an empty list of functions can be omitted if INTERFACE_MACRO keyword is used.

Suppose that in the previous example functions pointers for multiply(), divide(), add(),
subtract() are kept in a global C array "fp[]" with offsets being "multiply_off",
"divide_off", "add_off", "subtract_off". Then one can use

#define XSINTERFACE_FUNC_BYOFFSET(ret,cv,f) \
((XSINTERFACE_CVT_ANON(ret))fp[CvXSUBANY(cv).any_i32])
#define XSINTERFACE_FUNC_BYOFFSET_set(cv,f) \
CvXSUBANY(cv).any_i32 = CAT2( f, _off )

in C section,

symbolic
interface_s_ss(arg1, arg2)
symbolic arg1
symbolic arg2
INTERFACE_MACRO:
XSINTERFACE_FUNC_BYOFFSET
XSINTERFACE_FUNC_BYOFFSET_set
INTERFACE:
multiply divide
add subtract

in XSUB section.

The INCLUDE: Keyword
This keyword can be used to pull other files into the XS module. The other files may have
XS code. INCLUDE: can also be used to run a command to generate the XS code to be pulled
into the module.

The file Rpcb1.xsh contains our "rpcb_gettime()" function:

bool_t
rpcb_gettime(host,timep)
char *host
time_t &timep
OUTPUT:
timep

The XS module can use INCLUDE: to pull that file into it.

INCLUDE: Rpcb1.xsh

If the parameters to the INCLUDE: keyword are followed by a pipe ("|") then the compiler
will interpret the parameters as a command. This feature is mildly deprecated in favour of
the "INCLUDE_COMMAND:" directive, as documented below.

INCLUDE: cat Rpcb1.xsh |

Do not use this to run perl: "INCLUDE: perl |" will run the perl that happens to be the
first in your path and not necessarily the same perl that is used to run "xsubpp". See
"The INCLUDE_COMMAND: Keyword".

The INCLUDE_COMMAND: Keyword
Runs the supplied command and includes its output into the current XS document.
"INCLUDE_COMMAND" assigns special meaning to the $^X token in that it runs the same perl
interpreter that is running "xsubpp":

INCLUDE_COMMAND: cat Rpcb1.xsh

INCLUDE_COMMAND: $^X -e ...

The CASE: Keyword
The CASE: keyword allows an XSUB to have multiple distinct parts with each part acting as
a virtual XSUB. CASE: is greedy and if it is used then all other XS keywords must be
contained within a CASE:. This means nothing may precede the first CASE: in the XSUB and
anything following the last CASE: is included in that case.

A CASE: might switch via a parameter of the XSUB, via the "ix" ALIAS: variable (see "The
ALIAS: Keyword"), or maybe via the "items" variable (see "Variable-length Parameter
Lists"). The last CASE: becomes the default case if it is not associated with a
conditional. The following example shows CASE switched via "ix" with a function
"rpcb_gettime()" having an alias "x_gettime()". When the function is called as
"rpcb_gettime()" its parameters are the usual "(char *host, time_t *timep)", but when the
function is called as "x_gettime()" its parameters are reversed, "(time_t *timep, char
*host)".

long
rpcb_gettime(a,b)
CASE: ix == 1
ALIAS:
x_gettime = 1
INPUT:
# 'a' is timep, 'b' is host
char *b
time_t a = NO_INIT
CODE:
RETVAL = rpcb_gettime( b, &a );
OUTPUT:
a
RETVAL
CASE:
# 'a' is host, 'b' is timep
char *a
time_t &b = NO_INIT
OUTPUT:
b
RETVAL

That function can be called with either of the following statements. Note the different
argument lists.

$status = rpcb_gettime( $host, $timep );

$status = x_gettime( $timep, $host );

The 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 <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 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.

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