H2XS(1perl)

📦

version 5.44.0 · 2 versions

5.44.0 · arch rolling (latest)
5.42.3 · arch rolling

💖 h2xs — the manual, but make it cute 💖

okay bestie, gather round 🫶 this is h2xs from the Perl Programmers Reference Guide (that's the H2XS(1perl) page, iykyk), and literally the whole point of it is turning C header files into Perl extensions. no cap.

✨ NAME ✨

h2xs — convert .h C header files to Perl extensions 🦋

✨ SYNOPSIS ✨

h2xs [OPTIONS ...] [headerfile ... [extra_libraries]]

h2xs -h|-?|--help

✨ DESCRIPTION ✨

so, h2xs builds a Perl extension out of C header files. the extension it spits out will include functions you can use to grab the value of any #define statement that was chillin' in those C header files. 💅

the module_name is what gets used as the name of the extension. and if you don't supply a module_name? it just takes the name of the first header file and capitalizes the first character. main character energy fr.

if your extension might need extra libraries, drop them here. the extension's Makefile.PL will handle checking whether those libraries actually exist and how they should be loaded — it's giving responsible. 🌸 the extra libraries go in the form -lm -lposix, etc, exactly like on the cc command line. by default, Makefile.PL searches through the library path that Configure determined. wanna add to that path? include arguments of the form -L/another/library/path in the extra-libraries argument. 💜

and here's the thing — in spite of its name, h2xs can also be used to create a skeleton pure Perl module. see the -X option. plot twist 🎀

✨ OPTIONS ✨

-A, --omit-autoload — omit all autoload facilities. this is the same as -c but it also removes the use AutoLoader statement from the .pm file.
-B, --beta-version — use an alpha/beta style version number. makes the version number "0.00_01" unless -v is specified. 🧚
-C, --omit-changes — omits creation of the Changes file, and adds a HISTORY section to the POD template.
-F, --cpp-flags=addflags — additional flags to specify to the C preprocessor when scanning the header for function declarations. it writes these options in the generated Makefile.PL too. 💅
-M, --func-mask=regular expression — selects functions/macros to process.
-O, --overwrite-ok — allows a pre-existing extension directory to be overwritten. (yes, this one will absolutely clobber stuff, be careful bestie 😭)
-P, --omit-pod — omit the autogenerated stub POD section.
-X, --omit-XS — omit the XS portion. used to generate a skeleton pure Perl module. -c and -f are implicitly enabled. ✨
-a, --gen-accessors — generate an accessor method for each element of structs and unions. the generated methods are named after the element name; they will return the current value of the element if called without additional arguments; and they will set the element to the supplied value (and return the new value) if called with an additional argument. embedded structures and unions are returned as a pointer rather than the complete structure, to facilitate chained calls.

these methods all apply to the Ptr type for the structure; additionally two methods are constructed for the structure type itself: _to_ptr, which returns a Ptr type pointing to the same structure, and a new method to construct and return a new structure, initialised to zeroes. 💖

-b, --compat-version=version — generates a .pm file which is backwards compatible with the specified perl version.

for versions < 5.6.0, the changes are:

no use of our (uses use vars instead)
no use warnings

specifying a compatibility version higher than the version of perl you're using to run h2xs will have no effect. if unspecified, h2xs will default to compatibility with the version of perl you're using to run h2xs.

-c, --omit-constant — omit constant() from the .xs file and the corresponding specialised AUTOLOAD from the .pm file.
-d, --debugging — turn on debugging messages. 🐛
-e, --omit-enums=[regular expression] — if regular expression isn't given, skip all constants that are defined in a C enumeration. otherwise skip only those constants that are defined in an enum whose name matches regular expression.

since regular expression is optional, make sure this switch is followed by at least one other switch if you omit regular expression and have some pending arguments such as header-file names. this is ok:

h2xs -e -n Module::Foo foo.h

this is not ok:

h2xs -n Module::Foo -e foo.h

in the latter, foo.h is taken as the regular expression. 💀

-f, --force — allows an extension to be created for a header even if that header is not found in standard include directories.
-g, --global — include code for safely storing static data in the .xs file. extensions that do no make use of static data can ignore this option.
-h, -?, --help — print the usage, help and version for this h2xs and exit. 🫶
-k, --omit-const-func — for function arguments declared as const, omit the const attribute in the generated XS code.
-m, --gen-tied-var — experimental: for each variable declared in the header file(s), declare a perl variable of the same name magically tied to the C variable. ✨
-n, --name=module_name — specifies a name to be used for the extension, e.g., -n RPC::DCE.
-o, --opaque-re=regular expression — use "opaque" data type for the C types matched by the regular expression, even if these types are typedef-equivalent to types from typemaps. should not be used without -x.

this may be useful since, say, types which are typedef-equivalent to integers may represent OS-related handles, and one may want to work with these handles in OO-way, as in $handle->do_something(). use -o . if you want to handle all the typedefed types as opaque types.

the type-to-match is whitewashed (except for commas, which have no whitespace before them, and multiple * which have no whitespace between them).

-p, --remove-prefix=prefix — specify a prefix which should be removed from the Perl function names, e.g., -p sec_rgy_. this sets up the XS PREFIX keyword and removes the prefix from functions that are autoloaded via the constant() mechanism.
-s, --const-subs=sub1,sub2 — create a perl subroutine for the specified macros rather than autoload with the constant() subroutine. these macros are assumed to have a return type of char *, e.g., -s sec_rgy_wildcard_name,sec_rgy_wildcard_sid. 🌈
-t, --default-type=type — specify the internal type that the constant() mechanism uses for macros. the default is IV (signed integer). currently all macros found during the header scanning process will be assumed to have this type. future versions of h2xs may gain the ability to make educated guesses. 🔮
--use-new-tests — when --compat-version (-b) is present, the generated tests will use Test::More rather than Test, which is the default for versions before 5.6.2. Test::More will be added to PREREQ_PM in the generated Makefile.PL.
--use-old-tests — will force the generation of test code that uses the older Test module.
--skip-exporter — do not use Exporter and/or export any symbol.
--skip-ppport — do not use Devel::PPPort: no portability to older version.
--skip-autoloader — do not use the module AutoLoader; but keep the constant() function and sub AUTOLOAD for constants.
--skip-strict — do not use the pragma strict.
--skip-warnings — do not use the pragma warnings.
-v, --version=version — specify a version number for this extension. this version number is added to the templates. the default is 0.01, or 0.00_01 if -B is specified. the version specified should be numeric. 🔢
-x, --autogen-xsubs — automatically generate XSUBs basing on function declarations in the header file. the package C::Scan should be installed. if this option is specified, the name of the header file may look like NAME1,NAME2. in this case NAME1 is used instead of the specified string, but XSUBs are emitted only for the declarations included from file NAME2. 👑

note that some types of arguments/return-values for functions may result in XSUB-declarations/typemap-entries which need hand-editing. such may be objects which cannot be converted from/to a pointer (like long long), pointers to functions, or arrays. see also the section on LIMITATIONS of -x. ⭐

✨ EXAMPLES ✨

ok bestie, theory time is OVER, here's the tea in actual commands 💅 every single one is here, comments and all, because the comments are lowkey the best part:

# Default behavior, extension is Rusers
h2xs rpcsvc/rusers

# Same, but extension is RUSERS
h2xs -n RUSERS rpcsvc/rusers

# Extension is rpcsvc::rusers. Still finds <rpcsvc/rusers.h>
h2xs rpcsvc::rusers

# Extension is ONC::RPC.  Still finds <rpcsvc/rusers.h>
h2xs -n ONC::RPC rpcsvc/rusers

# Without constant() or AUTOLOAD
h2xs -c rpcsvc/rusers

# Creates templates for an extension named RPC
h2xs -cfn RPC

# Extension is ONC::RPC.
h2xs -cfn ONC::RPC

# Extension is a pure Perl module with no XS code.
h2xs -X My::Module

# Extension is Lib::Foo which works at least with Perl5.005_03.
# Constants are created for all #defines and enums h2xs can find
# in foo.h.
h2xs -b 5.5.3 -n Lib::Foo foo.h

# Extension is Lib::Foo which works at least with Perl5.005_03.
# Constants are created for all #defines but only for enums
# whose names do not start with 'bar_'.
h2xs -b 5.5.3 -e '^bar_' -n Lib::Foo foo.h

# Makefile.PL will look for library -lrpc in
# additional directory /opt/net/lib
h2xs rpcsvc/rusers -L/opt/net/lib -lrpc

# Extension is DCE::rgynbase
# prefix "sec_rgy_" is dropped from perl function names
h2xs -n DCE::rgynbase -p sec_rgy_ dce/rgynbase

# Extension is DCE::rgynbase
# prefix "sec_rgy_" is dropped from perl function names
# subroutines are created for sec_rgy_wildcard_name and
# sec_rgy_wildcard_sid
h2xs -n DCE::rgynbase -p sec_rgy_ \
-s sec_rgy_wildcard_name,sec_rgy_wildcard_sid dce/rgynbase

# Make XS without defines in perl.h, but with function declarations
# visible from perl.h. Name of the extension is perl1.
# When scanning perl.h, define -DEXT=extern -DdEXT= -DINIT(x)=
# Extra backslashes below because the string is passed to shell.
# Note that a directory with perl header files would
#  be added automatically to include path.
h2xs -xAn perl1 -F "-DEXT=extern -DdEXT= -DINIT\(x\)=" perl.h

# Same with function declaration in proto.h as visible from perl.h.
h2xs -xAn perl2 perl.h,proto.h

# Same but select only functions which match /^av_/
h2xs -M '^av_' -xAn perl2 perl.h,proto.h

# Same but treat SV* etc as "opaque" types
h2xs -o '^[S]V \*$' -M '^av_' -xAn perl2 perl.h,proto.h

slay. notice how that one -F string is doing the absolute most with the backslashes — that's because the string gets handed off to the shell, so $x$ has to survive the journey 🙈 and yeah, a directory full of perl header files just gets added to the include path for free, no asking. iykyk.

🦋 Extension based on .h and .c files 🦋

say you've got a pile of C files doing some actual functionality, plus the matching header files, and you want that whole thing reachable from Perl. classic. here's the whole glow-up, step by step 🌸

this walkthrough pretends your headers are called interface_simple.h and interface_hairy.h, and that you want the perl module named "Ext::Ension". if you need preprocessor directives and/or linking against outside libraries, go check the flags "-F", "-L" and "-l" over in "OPTIONS", bestie, they have you covered.

🔍 Find the directory name

start with a lil dummy run of h2xs:

h2xs -Afn Ext::Ension

the ONLY point of this step is to make the directories exist and tell you what they're called. from the output you'll see the directory for this extension is Ext/Ension. that's it, that's the whole job 💖

📦 Copy C files

copy your header files and your C files into that directory, Ext/Ension.

🛠️ Create the extension

run h2xs, happily stomping over the older autogenerated files:

h2xs -Oxan Ext::Ension interface_simple.h interface_hairy.h

h2xs goes looking for header files after it changes into the extension directory, so it will find your header files just fine, no cap.

🎀 Archive and test

as per usual, run:

cd Ext/Ension
perl Makefile.PL
make dist
make
make test

💡 Hints

it is important to run make dist as early as you possibly can. that way, if you decide to edit your .h files and rerun h2xs, you can merge(1) your changes into the autogenerated files super easily. obsessed with this workflow honestly.

do not forget to edit the documentation in the generated .pm file. seriously. nobody else is gonna do it for you 😭

treat the autogenerated files as skeletons only — you can absolutely invent better interfaces than what h2xs was able to guess. you're the main character here.

and treat this whole section as a guideline only, because some other options of h2xs might suit your needs better. vibe check your own project, fr fr.

🌈 ENVIRONMENT 🌈

No environment variables are used.

that's it. nothing. clean slate, no hidden surprises, we love a low-drama situation 💜

👑 AUTHOR 👑

Larry Wall and others

🔥 SEE ALSO 🔥

perl, perlxstut, ExtUtils::MakeMaker, and AutoLoader.

💀 DIAGNOSTICS 💀

The usual warnings if it cannot read or write the files involved.

(translation: if it can't read or write the files in question, you get the standard warnings. nothing exotic, but still, read them.)

🙈 LIMITATIONS of -x 🙈

ok so here's the honest part, and you NEED to hear it.

h2xs would not distinguish whether an argument to a C function which is of the form, say, "int *", is an input, an output, or an input/output parameter. it simply cannot tell. so an argument declaration shaped like this:

int
foo(n)
    int *n

should be better rewritten as:

int
foo(n)
    int &n

if "n" is an input parameter. (yes, & vs *, that single character is carrying the entire meaning of your function's soul.)

additionally, h2xs has no facilities to intuit that a function like this:

int
foo(addr,l)
     char *addr
     int   l

takes a pair of address and length of data at that address. it can't read your mind, bestie. so it's better to rewrite this function as:

int
foo(sv)
        SV *addr
    PREINIT:
        STRLEN len;
        char *s;
    CODE:
        s = SvPV(sv,len);
        RETVAL = foo(s, len);
    OUTPUT:
        RETVAL

or alternately:

static int
my_foo(SV *sv)
{
    STRLEN len;
    char *s = SvPV(sv,len);

    return foo(s, len);
}

MODULE = foo        PACKAGE = foo   PREFIX = my_

int
foo(sv)
    SV *sv

See perlxs and perlxstut for additional details.

and that's the whole page, you made it to the end, proud of you 🫶 this one's from perl v5.44.0, dated 2026-10-17, and it lives in the manual as H2XS(1perl) — go forth and generate some extensions, mwah 💖✨🎀

📦 where this page comes from

arch rolling: perl 5.44.0-1 · Artistic-1.0-Perl AND GPL-1.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms