Perl 5 version 8.8 documentation



h2xs - convert .h C header files to Perl extensions


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

h2xs -h|-?|--help


h2xs builds a Perl extension from C header files. The extension will include functions which can be used to retrieve the value of any #define statement which was in the C header files.

The module_name will be used for the name of the extension. If module_name is not supplied then the name of the first header file will be used, with the first character capitalized.

If the extension might need extra libraries, they should be included here. The extension Makefile.PL will take care of checking whether the libraries actually exist and how they should be loaded. The extra libraries should be specified in the form -lm -lposix, etc, just as on the cc command line. By default, the Makefile.PL will search through the library path determined by Configure. That path can be augmented by including arguments of the form -L/another/library/path in the extra-libraries argument.


  • -A, --omit-autoload

    Omit all autoload facilities. This is the same as -c but also removes the use AutoLoader statement from the .pm file.

  • -B, --beta-version

    Use an alpha/beta style version number. Causes version number to be "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 C preprocessor when scanning header for function declarations. 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.

  • -P, --omit-pod

    Omit the autogenerated stub POD section.

  • -X, --omit-XS

    Omit the XS portion. Used to generate templates for a module which is not XS-based. -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; will return the current value of the element if called without additional arguments; and 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 are using to run h2xs will have no effect. If unspecified h2xs will default to compatibility with the version of perl you are using to run h2xs.

  • -c, --omit-constant

    Omit constant() from the .xs file and corresponding specialised AUTOLOAD from the .pm file.

  • -d, --debugging

    Turn on debugging messages.

  • -e, --omit-enums=[regular expression]

    If regular expression is not 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 that 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:

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

    This is not ok:

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

    In the latter, foo.h is taken as 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 typedef ed 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 5.7.2 . 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 "b>" in LIMITATIONS of -x<.


  1. # Default behavior, extension is Rusers
  2. h2xs rpcsvc/rusers
  3. # Same, but extension is RUSERS
  4. h2xs -n RUSERS rpcsvc/rusers
  5. # Extension is rpcsvc::rusers. Still finds <rpcsvc/rusers.h>
  6. h2xs rpcsvc::rusers
  7. # Extension is ONC::RPC. Still finds <rpcsvc/rusers.h>
  8. h2xs -n ONC::RPC rpcsvc/rusers
  9. # Without constant() or AUTOLOAD
  10. h2xs -c rpcsvc/rusers
  11. # Creates templates for an extension named RPC
  12. h2xs -cfn RPC
  13. # Extension is ONC::RPC.
  14. h2xs -cfn ONC::RPC
  15. # Extension is Lib::Foo which works at least with Perl5.005_03.
  16. # Constants are created for all #defines and enums h2xs can find
  17. # in foo.h.
  18. h2xs -b 5.5.3 -n Lib::Foo foo.h
  19. # Extension is Lib::Foo which works at least with Perl5.005_03.
  20. # Constants are created for all #defines but only for enums
  21. # whose names do not start with 'bar_'.
  22. h2xs -b 5.5.3 -e '^bar_' -n Lib::Foo foo.h
  23. # Makefile.PL will look for library -lrpc in
  24. # additional directory /opt/net/lib
  25. h2xs rpcsvc/rusers -L/opt/net/lib -lrpc
  26. # Extension is DCE::rgynbase
  27. # prefix "sec_rgy_" is dropped from perl function names
  28. h2xs -n DCE::rgynbase -p sec_rgy_ dce/rgynbase
  29. # Extension is DCE::rgynbase
  30. # prefix "sec_rgy_" is dropped from perl function names
  31. # subroutines are created for sec_rgy_wildcard_name and
  32. # sec_rgy_wildcard_sid
  33. h2xs -n DCE::rgynbase -p sec_rgy_ \
  34. -s sec_rgy_wildcard_name,sec_rgy_wildcard_sid dce/rgynbase
  35. # Make XS without defines in perl.h, but with function declarations
  36. # visible from perl.h. Name of the extension is perl1.
  37. # When scanning perl.h, define -DEXT=extern -DdEXT= -DINIT(x)=
  38. # Extra backslashes below because the string is passed to shell.
  39. # Note that a directory with perl header files would
  40. # be added automatically to include path.
  41. h2xs -xAn perl1 -F "-DEXT=extern -DdEXT= -DINIT\(x\)=" perl.h
  42. # Same with function declaration in proto.h as visible from perl.h.
  43. h2xs -xAn perl2 perl.h,proto.h
  44. # Same but select only functions which match /^av_/
  45. h2xs -M '^av_' -xAn perl2 perl.h,proto.h
  46. # Same but treat SV* etc as "opaque" types
  47. h2xs -o '^[S]V \*$' -M '^av_' -xAn perl2 perl.h,proto.h

Extension based on .h and .c files

Suppose that you have some C files implementing some functionality, and the corresponding header files. How to create an extension which makes this functionality accessible in Perl? The example below assumes that the header files are interface_simple.h and interface_hairy.h, and you want the perl module be named as Ext::Ension . If you need some preprocessor directives and/or linking with external libraries, see the flags -F , -L and -l in OPTIONS.

  • Find the directory name

    Start with a dummy run of h2xs:

    1. h2xs -Afn Ext::Ension

    The only purpose of this step is to create the needed directories, and let you know the names of these directories. From the output you can see that the directory for the extension is Ext/Ension.

  • Copy C files

    Copy your header files and C files to this directory Ext/Ension.

  • Create the extension

    Run h2xs, overwriting older autogenerated files:

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

    h2xs looks for header files after changing to the extension directory, so it will find your header files OK.

  • Archive and test

    As usual, run

    1. cd Ext/Ension
    2. perl Makefile.PL
    3. make dist
    4. make
    5. make test
  • Hints

    It is important to do make dist as early as possible. This way you can easily merge(1) your changes to autogenerated files if you decide to edit your .h files and rerun h2xs.

    Do not forget to edit the documentation in the generated .pm file.

    Consider the autogenerated files as skeletons only, you may invent better interfaces than what h2xs could guess.

    Consider this section as a guideline only, some other options of h2xs may better suit your needs.


No environment variables are used.


Larry Wall and others


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


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


h2xs would not distinguish whether an argument to a C function which is of the form, say, int * , is an input, output, or input/output parameter. In particular, argument declarations of the form

  1. int
  2. foo(n)
  3. int *n

should be better rewritten as

  1. int
  2. foo(n)
  3. int &n

if n is an input parameter.

Additionally, h2xs has no facilities to intuit that a function

  1. int
  2. foo(addr,l)
  3. char *addr
  4. int l

takes a pair of address and length of data at this address, so it is better to rewrite this function as

  1. int
  2. foo(sv)
  3. SV *addr
  5. STRLEN len;
  6. char *s;
  7. CODE:
  8. s = SvPV(sv,len);
  9. RETVAL = foo(s, len);
  10. OUTPUT:
  11. RETVAL

or alternately

  1. static int
  2. my_foo(SV *sv)
  3. {
  4. STRLEN len;
  5. char *s = SvPV(sv,len);
  6. return foo(s, len);
  7. }
  8. MODULE = foo PACKAGE = foo PREFIX = my_
  9. int
  10. foo(sv)
  11. SV *sv

See perlxs and perlxstut for additional details.


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