PERLXSTYPEMAP(1)

🎀 NAME 🎀

perlxstypemap — Perl XS C/Perl type mapping

straight from the Perl Programmers Reference Guide, bestie

✨ DESCRIPTION ✨

ngl, the more you think about making two languages talk to each other, the more you realize that most of the programmer effort goes into converting between the data structures that are native to each language. That matters more than other stuff like differing calling conventions, because the problem space is way bigger. There are just more ways to shove data into memory than there are ways to implement a function call.

Perl XS's solution to this whole situation 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 use a CODE: and an OUTPUT: section together with a C return type of your XSUB, it's the typemapping mechanism that makes this easy.

🧬 Anatomy of a typemap 🧬

In more practical terms, the typemap is a collection of code fragments 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 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, like this. Here's 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. iconic behavior fr fr ✨

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. cute, efficient, no notes 💅

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 build-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</p> 🦋

Perl-side: it's a reference to a perl array. C-side: it's a pointer to an AV. This is the fixed variant of T_AVREF — it actually decrements the refcount appropriately when returning an AV*. Introduced in perl 5.15.4. ✨

T_HVREF</p> 🎀

Perl-side: a reference to a perl hash. C-side: a pointer to an HV.

Heads up bestie: this typemap does not decrement the reference count when returning an HV*. See also: T_HVREF_REFCOUNT_FIXED. 💅

T_HVREF_REFCOUNT_FIXED</p> 🦋

Perl-side: a reference to a perl hash. C-side: a pointer to an HV. This is the fixed variant of T_HVREF, and it does decrement the refcount appropriately when returning an HV*. Introduced in perl 5.15.4. Slay. ✨

T_CVREF</p> 🌸

From the perl level this is a reference to a perl subroutine (like $sub = sub { 1 };). From the C level it's a pointer to a CV.

Okay so — the original text here literally says this typemap does not decrement the reference count when returning an HV*. Yes, HV*. In the CV entry. I'm just the messenger, no cap. 😭 See also: T_HVREF_REFCOUNT_FIXED.

T_CVREF_REFCOUNT_FIXED</p> 💜

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

T_SYSRET</p> ⚡

The T_SYSRET typemap is what processes return values from system calls. It only makes sense when passing values from C to perl — there's literally no concept of passing a system return value from Perl to C. 🫶

So: 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 it's not -1, this typemap translates a 0 (perl false) into "0 but true" (which is perl true), or just returns the value itself, to say the command succeeded. 🔥

The POSIX module makes extensive use of this type — iykyk.

T_UV</p> 🔢

An unsigned integer.

T_IV</p> 🔢

A signed integer. It's cast to the required integer type when passed to C, and converted to an IV when passed back to Perl.

T_INT</p> 🔢

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's processed exactly the same way as for T_IV.

Its behaviour is identical to using an int type in XS with T_IV. Slay, consistency. ✨

T_ENUM</p> 🎀

An enum value. Used to transfer an enum component from C. There's genuinely no reason to pass an enum value to C, since it's stored as an IV inside perl already. 💅

T_BOOL</p> 🦋

A boolean type. You can use this to pass true and false values to and from C. Periodt.

T_U_INT</p> 🔢

For unsigned integers. It's equivalent to using T_UV, but it explicitly casts the variable to type unsigned int. The default type for unsigned int is T_UV.

T_SHORT</p> 🔢

Short integers. Equivalent to T_IV, but it explicitly casts the return to type short. The default typemap for short is T_IV.

T_U_SHORT</p> 🔢

Unsigned short integers. Equivalent to T_UV, but it 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</p> 🔢

Long integers. Equivalent to T_IV, but it explicitly casts the return to type long. The default typemap for long is T_IV.

T_U_LONG</p> 🔢

Unsigned long integers. Equivalent to T_UV, but it 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</p> 🍓

Single 8-bit characters.

T_U_CHAR</p> 🍓

An unsigned byte.

T_FLOAT</p> 🌊

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

T_NV</p> 🌊

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</p> 🌊

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

T_PV</p> 📝

A string (char *).

T_PTR</p> 📍

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

T_PTRREF</p> 📍

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

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

T_PTROBJ</p> 👑

Similar to T_PTRREF, except the reference is blessed into a class. This lets the pointer be used as an object, and it's 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 derived from the name of the 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. Which means the class check is skipped. 💀

T_REF_IV_REF</p> 🙈

NOT YET

T_REF_IV_PTR</p> 🦋

Similar to T_PTROBJ in that the pointer is blessed into a scalar object. The difference? When the object is passed back into XS, it must be of the correct type — inheritance is not supported — whereas T_PTROBJ does support inheritance. 😤

The pointer is blessed into a class derived from the name of the 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. Which means the class check is skipped. 💀

T_PTRDESC</p> 🙈

NOT YET

T_REFREF</p> 🌀

Similar to T_PTRREF, except the pointer stored in the referenced scalar is dereferenced and copied to the output variable. So 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</p> 🌀

Like T_REFREF, except it does strict type checking — and nope, inheritance is not supported.

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

T_OPAQUEPTR</p> 🎁

This one stores bytes in the string component of the SV. The representation of the data is irrelevant to perl here — the bytes themselves are just stored in the SV. It's assumed the C variable is a pointer (the bytes get copied from that memory location). If the pointer points at something represented by 8 bytes, then those 8 bytes land 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 store a C structure (the number of bytes to be copied is calculated using the C sizeof function) and works as an alternative to T_PTRREF without having to worry about a memory leak, since Perl will clean up the SV. ✨

T_OPAQUE</p> 🎁

This stores data from non-pointer types in the string part of an SV. It's similar to T_OPAQUEPTR, except the typemap retrieves the pointer directly rather than assuming it's being supplied. For example, if an integer is imported into Perl using T_OPAQUE rather than T_IV, the underlying bytes representing the integer get 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</p> 🧚

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 it can process more than one element.

T_PACKED</p> 👑

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 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 its 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 dodge the overhead. Also — keep in mind to possibly free the memory allocated by XS_unpack_foo_tPtr. 🫶

🎀 T_PACKEDARRAY

Okay so T_PACKEDARRAY is basically T_PACKED's loud cousin 💅 — the INPUT (Perl → XSUB) typemap is literally identical, no notes. But the OUTPUT typemap? It passes one extra argument to the XS_pack_$ntype function. That third parameter tells the function how many elements are in the output, so it can actually handle C arrays sanely instead of just vibing and hoping 😭.

The variable has to be declared by you, bestie, and it must be named count_$ntype — where $ntype is the normalized C type name (the whole explanation of that is above, iykyk). So for the example above plus foo_t **, the function signature would look like:

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

The type of that third parameter is arbitrary as far as the typemap is concerned — it just has to be in line with the declared variable. No cap 🧢

Now the not-so-cute part 🙈 — 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(...) is gonna be hard to decipher. Like, genuinely. Since all the details are up to the XS author (that's you, the typemap user), there are several solutions, and ngl none of them are particularly elegant 💀.

The most commonly seen hack: allocate memory for N+1 pointers and assign NULL to the (N+1)th one so iteration is easy. Works! Iconic in a cursed way.

Alternatively, just using a customized typemap for your purposes in the first place is probably preferable. Highkey the move tbh ✨

---

🌸 T_DATAUNIT

NOT YET

🦋 T_CALLBACK

NOT YET

---

👑 T_ARRAY

This one converts the perl argument list to a C array, and also pushes the contents of a C array back onto the perl argument stack. Dual-purpose queen 💖

The usual calling signature is:

@out = array_func( @in );

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

When you're using it to pass a perl list to C, the XS writer has to provide a function to allocate the memory needed to hold the list — named after the array type, but with 'Ptr' substituted for '*'. It should return a pointer 🫶. Freeing that memory on exit from the function is on the XS writer (again: you). The variable ix_$var gets set to the number of elements in the new array.

When returning a C array to Perl, the XS writer has to provide an integer variable called size_$var that holds the number of elements in the array. That's what determines how many elements get pushed onto the return argument stack. This is not required on input, because Perl already knows how many arguments are on the stack when the routine is called — she's observant like that 😌. Ordinarily this variable would be called size_RETVAL.

Also! The type of each element is determined from the type of the array. So if your array uses type intArray *, xsubpp will automatically work out that it contains variables of type int and use that typemap entry to copy each element. All the pointer '*' and 'Array' tags get removed from the name to determine the subtype. Slay, honestly 🔥

---

💅 T_STDIO

This is the one for passing perl filehandles to and from C using FILE * structures 🎀

---

⭐ T_INOUT

This is for passing perl filehandles to and from C using PerlIO * structures. The filehandle can be used for reading and writing — it corresponds to the +< mode. See also T_IN and T_OUT 🦋

See perliol for more info 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. So like... be careful 💀

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's returned from C to Perl can only be used for reading (mode "<"). Read-only era 🙈

---

🍓 T_OUT

Same as T_INOUT, but the filehandle that's returned from C to Perl is set to use the open mode "+>". Write-only era ✨

---

And that's the whole vibe check, bestie — this page is from perl v5.42.3, dated 2026-11-03. Go forth and typemap responsibly 🫶💖🎀