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.

Memory footprint and run-time performance is not as good as specialized perfect hashes as with XSConfig or Win32::GUI::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.

C_constant

A function to generate the C code in const-c.inc to implement the perl subroutine PACKAGE::constant.

The $what paramater 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 XS_constant was given. Otherwise XS_constant and C_constant may have different ideas about the number of parameters passed to the C function constant.

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.

This is needed unless you use PROXYSUBS = {autoload=>1}>, but which generates code unusable earlier than 5.8.

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. PROXYSUBS create CONSTSUB's for each defined constant upfront, while without PROXYSUBS every constant is looked up at run-time. Thus it trades memory footprint for faster run-time performance.

Options: autoload, push, croak_on_error or croak_on_read with most of the options being exclusive, and croak_on_read usable since 5.24.

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.

PERFORMANCE

You can calculate simple performance numbers with perl -Mblib t/Constant.t --bench --memtest /dev/null 2>bench.lst> and grep ^# bench.lst for 19 constants.

    Option              Memory [b]   Time [s]
    <none>              1612758      0.023946
    PROXYSUBS           1593553      0.020061
    PROXYSUBS autoload  1588555      0.024906
    PROXYSUBS push      1608709      0.023361
    PROXYSUBS 
    croak_on_error      1590267      0.023052
    PROXYSUBS
    croak_on_read       1599747      0.021710
    PROXYSUBS
    autoload push       1589737      0.021917
    PROXYSUBS
    croak_on_error push 1590606      0.021669

PROXYSUBS without any option is the fastest, and on the lower side of memory.

PROXYSUBS autoload is the slowest, but easiest to use.

The old method without PROXYSUBS uses the most memory and is 2nd slowest.

With >500 constants:

    Option              Memory [b]   Time [s]
    <none>              2572650      0.025867
    PROXYSUBS           2145701      0.024928
    PROXYSUBS autoload  2133418      0.024154
    PROXYSUBS push      2173807      0.030029
    PROXYSUBS 
    croak_on_error      2142773      0.025776
    PROXYSUBS
    croak_on_read       2144780      0.027649
    PROXYSUBS
    autoload push       2161453      0.026092
    PROXYSUBS
    croak_on_error push 2170791      0.027433

There are no numbers compared to perfect hashes yet. The estimation is that EU-C is ~50% slower and bigger, as seen with Win32::GUI::Constants and XSConfig. See also Perfect::Hash.

AUTHOR

Reini Urban <rurban@cpan.org> fixed up ProxySubs and took over maintainance. Nicholas Clark <nick@ccl4.org> wrote it based on the code in h2xs by Larry Wall and others.