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


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


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.


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.


signed integer, at least 32 bits.


unsigned integer, the same size as IV


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


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


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


A mortal SV .


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


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


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



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


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.


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 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 of the module. This must be specified


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


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


An array of constants’ names, either scalars containing names, or hashrefs as detailed in "C_constant".


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


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


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.


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


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


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


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 .


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


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.