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NAME

perlxstypemap - Perl XS C/Perl type mapping

DESCRIPTION

The more you think about interfacing between two languages, the more you'll realize that
the majority of programmer effort has to go into converting between the data structures
that are native to either of the languages involved. This trumps other matter such as
differing calling conventions because the problem space is so much greater. There are
simply more ways to shove data into memory than there are ways to implement a function
call.

Perl XS' attempt at a solution to this is the concept of typemaps. At an abstract level,
a Perl XS typemap is nothing but a recipe for converting from a certain Perl data
structure to a certain C data structure and vice versa. Since there can be C types that
are sufficiently similar to one another to warrant converting with the same logic, XS
typemaps are represented by a unique identifier, henceforth called an XS type in this
document. You can then tell the XS compiler that multiple C types are to be mapped with
the same XS typemap.

In your XS code, when you define an argument with a C type or when you are using a "CODE:"
and an "OUTPUT:" section together with a C return type of your XSUB, it'll be the
typemapping mechanism that makes this easy.

Anatomy of a typemap
In more practical terms, the typemap is a collection of code fragments which are used by
the xsubpp compiler to map C function parameters and values to Perl values. The typemap
file may consist of three sections labelled "TYPEMAP", "INPUT", and "OUTPUT". An
unlabelled initial section is assumed to be a "TYPEMAP" section. The INPUT section tells
the compiler how to translate Perl values into variables of certain C types. The OUTPUT
section tells the compiler how to translate the values from certain C types into values
Perl can understand. The TYPEMAP section tells the compiler which of the INPUT and OUTPUT
code fragments should be used to map a given C type to a Perl value. The section labels
"TYPEMAP", "INPUT", or "OUTPUT" must begin in the first column on a line by themselves,
and must be in uppercase.

Each type of section can appear an arbitrary number of times and does not have to appear
at all. For example, a typemap may commonly lack "INPUT" and "OUTPUT" sections if all it
needs to do is associate additional C types with core XS types like T_PTROBJ. Lines that
start with a hash "#" are considered comments and ignored in the "TYPEMAP" section, but
are considered significant in "INPUT" and "OUTPUT". Blank lines are generally ignored.

Traditionally, typemaps needed to be written to a separate file, conventionally called
"typemap" in a CPAN distribution. With ExtUtils::ParseXS (the XS compiler) version 3.12
or better which comes with perl 5.16, typemaps can also be embedded directly into XS code
using a HERE-doc like syntax:

TYPEMAP: <<HERE
...
HERE

where "HERE" can be replaced by other identifiers like with normal Perl HERE-docs. All
details below about the typemap textual format remain valid.

The "TYPEMAP" section should contain one pair of C type and XS type per line as follows.
An example from the core typemap file:

TYPEMAP
# all variants of char* is handled by the T_PV typemap
char * T_PV
const char * T_PV
unsigned char * T_PV
...

The "INPUT" and "OUTPUT" sections have identical formats, that is, each unindented line
starts a new in- or output map respectively. A new in- or output map must start with the
name of the XS type to map on a line by itself, followed by the code that implements it
indented on the following lines. Example:

INPUT
T_PV
$var = ($type)SvPV_nolen($arg)
T_PTR
$var = INT2PTR($type,SvIV($arg))

We'll get to the meaning of those Perlish-looking variables in a little bit.

Finally, here's an example of the full typemap file for mapping C strings of the "char *"
type to Perl scalars/strings:

TYPEMAP
char * T_PV

INPUT
T_PV
$var = ($type)SvPV_nolen($arg)

OUTPUT
T_PV
sv_setpv((SV*)$arg, $var);

Here's a more complicated example: suppose that you wanted "struct netconfig" to be
blessed into the class "Net::Config". One way to do this is to use underscores (_) to
separate package names, as follows:

typedef struct netconfig * Net_Config;

And then provide a typemap entry "T_PTROBJ_SPECIAL" that maps underscores to double-colons
(::), and declare "Net_Config" to be of that type:

TYPEMAP
Net_Config T_PTROBJ_SPECIAL

INPUT
T_PTROBJ_SPECIAL
if (sv_derived_from($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")){
IV tmp = SvIV((SV*)SvRV($arg));
$var = INT2PTR($type, tmp);
}
else
croak(\"$var is not of type ${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\")

OUTPUT
T_PTROBJ_SPECIAL
sv_setref_pv($arg, \"${(my $ntt=$ntype)=~s/_/::/g;\$ntt}\",
(void*)$var);

The INPUT and OUTPUT sections substitute underscores for double-colons on the fly, giving
the desired effect. This example demonstrates some of the power and versatility of the
typemap facility.

The "INT2PTR" macro (defined in perl.h) casts an integer to a pointer of a given type,
taking care of the possible different size of integers and pointers. There are also
"PTR2IV", "PTR2UV", "PTR2NV" macros, to map the other way, which may be useful in OUTPUT
sections.

The Role of the typemap File in Your Distribution
The default typemap in the lib/ExtUtils directory of the Perl source contains many useful
types which can be used by Perl extensions. Some extensions define additional typemaps
which they keep in their own directory. These additional typemaps may reference INPUT and
OUTPUT maps in the main typemap. The xsubpp compiler will allow the extension's own
typemap to override any mappings which are in the default typemap. Instead of using an
additional typemap file, typemaps may be embedded verbatim in XS with a heredoc-like
syntax. See the documentation on the "TYPEMAP:" XS keyword.

For CPAN distributions, you can assume that the XS types defined by the perl core are
already available. Additionally, the core typemap has default XS types for a large number
of C types. For example, if you simply return a "char *" from your XSUB, the core typemap
will have this C type associated with the T_PV XS type. That means your C string will be
copied into the PV (pointer value) slot of a new scalar that will be returned from your
XSUB to Perl.

If you're developing a CPAN distribution using XS, you may add your own file called
typemap to the distribution. That file may contain typemaps that either map types that
are specific to your code or that override the core typemap file's mappings for common C
types.

Sharing typemaps Between CPAN Distributions
Starting with ExtUtils::ParseXS version 3.13_01 (comes with perl 5.16 and better), it is
rather easy to share typemap code between multiple CPAN distributions. The general idea is
to share it as a module that offers a certain API and have the dependent modules declare
that as a built-time requirement and import the typemap into the XS. An example of such a
typemap-sharing module on CPAN is "ExtUtils::Typemaps::Basic". Two steps to getting that
module's typemaps available in your code:

· Declare "ExtUtils::Typemaps::Basic" as a build-time dependency in "Makefile.PL" (use
"BUILD_REQUIRES"), or in your "Build.PL" (use "build_requires").

· Include the following line in the XS section of your XS file: (don't break the line)

INCLUDE_COMMAND: $^X -MExtUtils::Typemaps::Cmd
-e "print embeddable_typemap(q{Basic})"

Writing typemap Entries
Each INPUT or OUTPUT typemap entry is a double-quoted Perl string that will be evaluated
in the presence of certain variables to get the final C code for mapping a certain C type.

This means that you can embed Perl code in your typemap (C) code using constructs such as
"${ perl code that evaluates to scalar reference here }". A common use case is to generate
error messages that refer to the true function name even when using the ALIAS XS feature:

${ $ALIAS ? \q[GvNAME(CvGV(cv))] : \qq[\"$pname\"] }

For many typemap examples, refer to the core typemap file that can be found in the perl
source tree at lib/ExtUtils/typemap.

The Perl variables that are available for interpolation into typemaps are the following:

· $var - the name of the input or output variable, eg. RETVAL for return values.

· $type - the raw C type of the parameter, any ":" replaced with "_". e.g. for a type
of "Foo::Bar", $type is "Foo__Bar"

· $ntype - the supplied type with "*" replaced with "Ptr". e.g. for a type of "Foo*",
$ntype is "FooPtr"

· $arg - the stack entry, that the parameter is input from or output to, e.g. ST(0)

· $argoff - the argument stack offset of the argument. ie. 0 for the first argument,
etc.

· $pname - the full name of the XSUB, with including the "PACKAGE" name, with any
"PREFIX" stripped. This is the non-ALIAS name.

· $Package - the package specified by the most recent "PACKAGE" keyword.

· $ALIAS - non-zero if the current XSUB has any aliases declared with "ALIAS".

Full Listing of Core Typemaps
Each C type is represented by an entry in the typemap file that is responsible for
converting perl variables (SV, AV, HV, CV, etc.) to and from that type. The following
sections list all XS types that come with perl by default.

T_SV
This simply passes the C representation of the Perl variable (an SV*) in and out of
the XS layer. This can be used if the C code wants to deal directly with the Perl
variable.

T_SVREF
Used to pass in and return a reference to an SV.

Note that this typemap does not decrement the reference count when returning the
reference to an SV*. See also: T_SVREF_REFCOUNT_FIXED

T_SVREF_FIXED
Used to pass in and return a reference to an SV. This is a fixed variant of T_SVREF
that decrements the refcount appropriately when returning a reference to an SV*.
Introduced in perl 5.15.4.

T_AVREF
From the perl level this is a reference to a perl array. From the C level this is a
pointer to an AV.

Note that this typemap does not decrement the reference count when returning an AV*.
See also: T_AVREF_REFCOUNT_FIXED

T_AVREF_REFCOUNT_FIXED
From the perl level this is a reference to a perl array. From the C level this is a
pointer to an AV. This is a fixed variant of T_AVREF that decrements the refcount
appropriately when returning an AV*. Introduced in perl 5.15.4.

T_HVREF
From the perl level this is a reference to a perl hash. From the C level this is a
pointer to an HV.

Note that this typemap does not decrement the reference count when returning an HV*.
See also: T_HVREF_REFCOUNT_FIXED

T_HVREF_REFCOUNT_FIXED
From the perl level this is a reference to a perl hash. From the C level this is a
pointer to an HV. This is a fixed variant of T_HVREF that decrements the refcount
appropriately when returning an HV*. Introduced in perl 5.15.4.

T_CVREF
From the perl level this is a reference to a perl subroutine (e.g. $sub = sub { 1 };).
From the C level this is a pointer to a CV.

Note that this typemap does not decrement the reference count when returning an HV*.
See also: T_HVREF_REFCOUNT_FIXED

T_CVREF_REFCOUNT_FIXED
From the perl level this is a reference to a perl subroutine (e.g. $sub = sub { 1 };).
From the C level this is a pointer to a CV.

This is a fixed variant of T_HVREF that decrements the refcount appropriately when
returning an HV*. Introduced in perl 5.15.4.

T_SYSRET
The T_SYSRET typemap is used to process return values from system calls. It is only
meaningful when passing values from C to perl (there is no concept of passing a system
return value from Perl to C).

System calls return -1 on error (setting ERRNO with the reason) and (usually) 0 on
success. If the return value is -1 this typemap returns "undef". If the return value
is not -1, this typemap translates a 0 (perl false) to "0 but true" (which is perl
true) or returns the value itself, to indicate that the command succeeded.

The POSIX module makes extensive use of this type.

T_UV
An unsigned integer.

T_IV
A signed integer. This is cast to the required integer type when passed to C and
converted to an IV when passed back to Perl.

T_INT
A signed integer. This typemap converts the Perl value to a native integer type (the
"int" type on the current platform). When returning the value to perl it is processed
in the same way as for T_IV.

Its behaviour is identical to using an "int" type in XS with T_IV.

T_ENUM
An enum value. Used to transfer an enum component from C. There is no reason to pass
an enum value to C since it is stored as an IV inside perl.

T_BOOL
A boolean type. This can be used to pass true and false values to and from C.

T_U_INT
This is for unsigned integers. It is equivalent to using T_UV but explicitly casts the
variable to type "unsigned int". The default type for "unsigned int" is T_UV.

T_SHORT
Short integers. This is equivalent to T_IV but explicitly casts the return to type
"short". The default typemap for "short" is T_IV.

T_U_SHORT
Unsigned short integers. This is equivalent to T_UV but explicitly casts the return to
type "unsigned short". The default typemap for "unsigned short" is T_UV.

T_U_SHORT is used for type "U16" in the standard typemap.

T_LONG
Long integers. This is equivalent to T_IV but explicitly casts the return to type
"long". The default typemap for "long" is T_IV.

T_U_LONG
Unsigned long integers. This is equivalent to T_UV but explicitly casts the return to
type "unsigned long". The default typemap for "unsigned long" is T_UV.

T_U_LONG is used for type "U32" in the standard typemap.

T_CHAR
Single 8-bit characters.

T_U_CHAR
An unsigned byte.

T_FLOAT
A floating point number. This typemap guarantees to return a variable cast to a
"float".

T_NV
A Perl floating point number. Similar to T_IV and T_UV in that the return type is cast
to the requested numeric type rather than to a specific type.

T_DOUBLE
A double precision floating point number. This typemap guarantees to return a variable
cast to a "double".

T_PV
A string (char *).

T_PTR
A memory address (pointer). Typically associated with a "void *" type.

T_PTRREF
Similar to T_PTR except that the pointer is stored in a scalar and the reference to
that scalar is returned to the caller. This can be used to hide the actual pointer
value from the programmer since it is usually not required directly from within perl.

The typemap checks that a scalar reference is passed from perl to XS.

T_PTROBJ
Similar to T_PTRREF except that the reference is blessed into a class. This allows
the pointer to be used as an object. Most commonly used to deal with C structs. The
typemap checks that the perl object passed into the XS routine is of the correct class
(or part of a subclass).

The pointer is blessed into a class that is derived from the name of type of the
pointer but with all '*' in the name replaced with 'Ptr'.

For "DESTROY" XSUBs only, a T_PTROBJ is optimized to a T_PTRREF. This means the class
check is skipped.

T_REF_IV_REF
NOT YET

T_REF_IV_PTR
Similar to T_PTROBJ in that the pointer is blessed into a scalar object. The
difference is that when the object is passed back into XS it must be of the correct
type (inheritance is not supported) while T_PTROBJ supports inheritance.

The pointer is blessed into a class that is derived from the name of type of the
pointer but with all '*' in the name replaced with 'Ptr'.

For "DESTROY" XSUBs only, a T_REF_IV_PTR is optimized to a T_PTRREF. This means the
class check is skipped.

T_PTRDESC
NOT YET

T_REFREF
Similar to T_PTRREF, except the pointer stored in the referenced scalar is
dereferenced and copied to the output variable. This means that T_REFREF is to
T_PTRREF as T_OPAQUE is to T_OPAQUEPTR. All clear?

Only the INPUT part of this is implemented (Perl to XSUB) and there are no known users
in core or on CPAN.

T_REFOBJ
Like T_REFREF, except it does strict type checking (inheritance is not supported).

For "DESTROY" XSUBs only, a T_REFOBJ is optimized to a T_REFREF. This means the class
check is skipped.

T_OPAQUEPTR
This can be used to store bytes in the string component of the SV. Here the
representation of the data is irrelevant to perl and the bytes themselves are just
stored in the SV. It is assumed that the C variable is a pointer (the bytes are copied
from that memory location). If the pointer is pointing to something that is
represented by 8 bytes then those 8 bytes are stored in the SV (and length() will
report a value of 8). This entry is similar to T_OPAQUE.

In principle the unpack() command can be used to convert the bytes back to a number
(if the underlying type is known to be a number).

This entry can be used to store a C structure (the number of bytes to be copied is
calculated using the C "sizeof" function) and can be used as an alternative to
T_PTRREF without having to worry about a memory leak (since Perl will clean up the
SV).

T_OPAQUE
This can be used to store data from non-pointer types in the string part of an SV. It
is similar to T_OPAQUEPTR except that the typemap retrieves the pointer directly
rather than assuming it is being supplied. For example, if an integer is imported into
Perl using T_OPAQUE rather than T_IV the underlying bytes representing the integer
will be stored in the SV but the actual integer value will not be available. i.e. The
data is opaque to perl.

The data may be retrieved using the "unpack" function if the underlying type of the
byte stream is known.

T_OPAQUE supports input and output of simple types. T_OPAQUEPTR can be used to pass
these bytes back into C if a pointer is acceptable.

Implicit array
xsubpp supports a special syntax for returning packed C arrays to perl. If the XS
return type is given as

array(type, nelem)

xsubpp will copy the contents of "nelem * sizeof(type)" bytes from RETVAL to an SV and
push it onto the stack. This is only really useful if the number of items to be
returned is known at compile time and you don't mind having a string of bytes in your
SV. Use T_ARRAY to push a variable number of arguments onto the return stack (they
won't be packed as a single string though).

This is similar to using T_OPAQUEPTR but can be used to process more than one element.

T_PACKED
Calls user-supplied functions for conversion. For "OUTPUT" (XSUB to Perl), a function
named "XS_pack_$ntype" is called with the output Perl scalar and the C variable to
convert from. $ntype is the normalized C type that is to be mapped to Perl.
Normalized means that all "*" are replaced by the string "Ptr". The return value of
the function is ignored.

Conversely for "INPUT" (Perl to XSUB) mapping, the function named "XS_unpack_$ntype"
is called with the input Perl scalar as argument and the return value is cast to the
mapped C type and assigned to the output C variable.

An example conversion function for a typemapped struct "foo_t *" might be:

static void
XS_pack_foo_tPtr(SV *out, foo_t *in)
{
dTHX; /* alas, signature does not include pTHX_ */
HV* hash = newHV();
hv_stores(hash, "int_member", newSViv(in->int_member));
hv_stores(hash, "float_member", newSVnv(in->float_member));
/* ... */

/* mortalize as thy stack is not refcounted */
sv_setsv(out, sv_2mortal(newRV_noinc((SV*)hash)));
}

The conversion from Perl to C is left as an exercise to the reader, but the prototype
would be:

static foo_t *
XS_unpack_foo_tPtr(SV *in);

Instead of an actual C function that has to fetch the thread context using "dTHX", you
can define macros of the same name and avoid the overhead. Also, keep in mind to
possibly free the memory allocated by "XS_unpack_foo_tPtr".

T_PACKEDARRAY
T_PACKEDARRAY is similar to T_PACKED. In fact, the "INPUT" (Perl to XSUB) typemap is
identical, but the "OUTPUT" typemap passes an additional argument to the
"XS_pack_$ntype" function. This third parameter indicates the number of elements in
the output so that the function can handle C arrays sanely. The variable needs to be
declared by the user and must have the name "count_$ntype" where $ntype is the
normalized C type name as explained above. The signature of the function would be for
the example above and "foo_t **":

static void
XS_pack_foo_tPtrPtr(SV *out, foo_t *in, UV count_foo_tPtrPtr);

The type of the third parameter is arbitrary as far as the typemap is concerned. It
just has to be in line with the declared variable.

Of course, unless you know the number of elements in the "sometype **" C array, within
your XSUB, the return value from "foo_t ** XS_unpack_foo_tPtrPtr(...)" will be hard to
decipher. Since the details are all up to the XS author (the typemap user), there are
several solutions, none of which particularly elegant. The most commonly seen
solution has been to allocate memory for N+1 pointers and assign "NULL" to the (N+1)th
to facilitate iteration.

Alternatively, using a customized typemap for your purposes in the first place is
probably preferable.

T_DATAUNIT
NOT YET

T_CALLBACK
NOT YET

T_ARRAY
This is used to convert the perl argument list to a C array and for pushing the
contents of a C array onto the perl argument stack.

The usual calling signature is

@out = array_func( @in );

Any number of arguments can occur in the list before the array but the input and
output arrays must be the last elements in the list.

When used to pass a perl list to C the XS writer must provide a function (named after
the array type but with 'Ptr' substituted for '*') to allocate the memory required to
hold the list. A pointer should be returned. It is up to the XS writer to free the
memory on exit from the function. The variable "ix_$var" is set to the number of
elements in the new array.

When returning a C array to Perl the XS writer must provide an integer variable called
"size_$var" containing the number of elements in the array. This is used to determine
how many elements should be pushed onto the return argument stack. This is not
required on input since Perl knows how many arguments are on the stack when the
routine is called. Ordinarily this variable would be called "size_RETVAL".

Additionally, the type of each element is determined from the type of the array. If
the array uses type "intArray *" xsubpp will automatically work out that it contains
variables of type "int" and use that typemap entry to perform the copy of each
element. All pointer '*' and 'Array' tags are removed from the name to determine the
subtype.

T_STDIO
This is used for passing perl filehandles to and from C using "FILE *" structures.

T_INOUT
This is used for passing perl filehandles to and from C using "PerlIO *" structures.
The file handle can used for reading and writing. This corresponds to the "+<" mode,
see also T_IN and T_OUT.

See perliol for more information on the Perl IO abstraction layer. Perl must have been
built with "-Duseperlio".

There is no check to assert that the filehandle passed from Perl to C was created with
the right "open()" mode.

Hint: The perlxstut tutorial covers the T_INOUT, T_IN, and T_OUT XS types nicely.

T_IN
Same as T_INOUT, but the filehandle that is returned from C to Perl can only be used
for reading (mode "<").

T_OUT
Same as T_INOUT, but the filehandle that is returned from C to Perl is set to use the
open mode "+>".

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