---
name: "ExtUtils::Constant"
section: "3perl"
platform: "linux"
package: "perl"
version: "5.44.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/ExtUtils::Constant/3perl"
---

# ExtUtils::Constant(3perl)

generate XS code to import C header constants

## NAME

ExtUtils::Constant - generate XS code to import C header constants

## SYNOPSIS

```
    use ExtUtils::Constant qw (WriteConstants);
    WriteConstants(
        NAME => 'Foo',
        NAMES => [qw(FOO BAR BAZ)],
    );
    # Generates wrapper code to make the values of the constants FOO BAR BAZ
    #  available to perl
```

## DESCRIPTION

ExtUtils::Constant facilitates generating C and XS wrapper code to allow perl modules to AUTOLOAD constants defined in C library header files. It is principally used by the `"h2xs"` utility, on which this code is based. It doesn't contain the routines to scan header files to extract these constants.

## USAGE

Generally one only needs to call the `"WriteConstants"` function, and then

```
    #include "const-c.inc"
```

in the C section of `"Foo.xs"`

```
    INCLUDE: const-xs.inc
```

in the XS section of `"Foo.xs"`.

For greater flexibility use `constant_types()`, `"C_constant"` and `"XS_constant"`, with which `"WriteConstants"` is implemented.

Currently this module understands the following types. h2xs may only know a subset. The sizes of the numeric types are chosen by the `"Configure"` script at compile time.

- IV

  signed integer, at least 32 bits.

- UV

  unsigned integer, the same size as *IV*

- NV

  floating point type, probably `"double"`, possibly `"long double"`

- PV

  NUL terminated string, length will be determined with `"strlen"`

- PVN

  A fixed length thing, given as a \[pointer, length\] pair. If you know the length of a string at compile time you may use this instead of *PV*

- SV

  A **mortal** SV.

- YES

  Truth.  (`"PL_sv_yes"`)  The value is not needed (and ignored).

- NO

  Defined Falsehood.  (`"PL_sv_no"`)  The value is not needed (and ignored).

- UNDEF

  `"undef"`.  The value of the macro is not needed.

## FUNCTIONS

- constant\_types

  A function returning a single scalar with `"#define"` definitions for the constants used internally between the generated C and XS functions.

- XS\_constant PACKAGE, TYPES, XS\_SUBNAME, C\_SUBNAME

  A function to generate the XS code to implement the perl subroutine *PACKAGE*::constant used by *PACKAGE*::AUTOLOAD to load constants. This XS code is a wrapper around a C subroutine usually generated by `"C_constant"`, and usually named `"constant"`.

  *TYPES* should be given either as a comma separated list of types that the C subroutine `"constant"` will generate or as a reference to a hash. It should be the same list of types as `"C_constant"` was given. \[Otherwise `"XS_constant"` and `"C_constant"` may have different ideas about the number of parameters passed to the C function `"constant"`\]

  You can call the perl visible subroutine something other than `"constant"` if you give the parameter *XS\_SUBNAME*. The C subroutine it calls defaults to the name of the perl visible subroutine, unless you give the parameter *C\_SUBNAME*.

- autoload PACKAGE, VERSION, AUTOLOADER

  A function to generate the AUTOLOAD subroutine for the module *PACKAGE* *VERSION* is the perl version the code should be backwards compatible with. It defaults to the version of perl running the subroutine.  If *AUTOLOADER* is true, the AUTOLOAD subroutine falls back on AutoLoader::AUTOLOAD for all names that the **constant()** routine doesn't recognise.

- WriteMakefileSnippet

  WriteMakefileSnippet ATTRIBUTE =\> VALUE \[, ...\]

  A function to generate perl code for Makefile.PL that will regenerate the constant subroutines.  Parameters are named as passed to `"WriteConstants"`, with the addition of `"INDENT"` to specify the number of leading spaces (default 2).

  Currently only `"INDENT"`, `"NAME"`, `"DEFAULT_TYPE"`, `"NAMES"`, `"C_FILE"` and `"XS_FILE"` are recognised.

- WriteConstants ATTRIBUTE =\> VALUE \[, ...\]

  Writes a file of C code and a file of XS code which you should `"#include"` and `"INCLUDE"` in the C and XS sections respectively of your module's XS code.  You probably want to do this in your `"Makefile.PL"`, so that you can easily edit the list of constants without touching the rest of your module. The attributes supported are

  - NAME

    Name of the module.  This must be specified

  - DEFAULT\_TYPE

    The default type for the constants.  If not specified `"IV"` is assumed.

  - BREAKOUT\_AT

    The names of the constants are grouped by length.  Generate child subroutines for each group with this number or more names in.

  - NAMES

    An array of constants' names, either scalars containing names, or hashrefs as detailed in "C\_constant".

  - PROXYSUBS

    If true, uses proxy subs. See ExtUtils::Constant::ProxySubs.

  - C\_FH

    A filehandle to write the C code to.  If not given, then *C\_FILE* is opened for writing.

  - C\_FILE

    The name of the file to write containing the C code.  The default is `"const-c.inc"`.  The `"-"` in the name ensures that the file can't be mistaken for anything related to a legitimate perl package name, and not naming the file `".c"` avoids having to override Makefile.PL's `".xs"` to `".c"` rules.

  - XS\_FH

    A filehandle to write the XS code to.  If not given, then *XS\_FILE* is opened for writing.

  - XS\_FILE

    The name of the file to write containing the XS code.  The default is `"const-xs.inc"`.

  - XS\_SUBNAME

    The perl visible name of the XS subroutine generated which will return the constants. The default is `"constant"`.

  - C\_SUBNAME

    The name of the C subroutine generated which will return the constants. The default is *XS\_SUBNAME*.  Child subroutines have `"_"` and the name length appended, so constants with 10 character names would be in `"constant_10"` with the default *XS\_SUBNAME*.

## AUTHOR

Nicholas Clark \<nick@ccl4.org\> based on the code in `"h2xs"` by Larry Wall and others