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Section: Perl Programmers Reference Guide (1)
Updated: 2019-01-14


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 extensionwill 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. Ifmodule_name is not supplied then the name of the first header filewill be used, with the first character capitalized.

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

In spite of its name, h2xs may also be used to create a skeleton purePerl module. See the -X option. 


-A, --omit-autoload
Omit all autoload facilities. This is the same as -c but alsoremoves the "use AutoLoader" statement from the .pm file.
-B, --beta-version
Use an alpha/beta style version number. Causes version number tobe ``0.00_01'' unless -v is specified.
-C, --omit-changes
Omits creation of the Changes file, and adds a HISTORY section tothe POD template.
-F, --cpp-flags=addflags
Additional flags to specify to C preprocessor when scanning header forfunction declarations. Writes these options in the generated Makefile.PLtoo.
-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 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. Thegenerated methods are named after the element name; will return the currentvalue of the element if called without additional arguments; and will setthe element to the supplied value (and return the new value) if called withan additional argument. Embedded structures and unions are returned as apointer rather than the complete structure, to facilitate chained calls.

These methods all apply to the Ptr type for the structure; additionallytwo 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 specifiedperl 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 youare using to run h2xs will have no effect. If unspecified h2xs will defaultto 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 ina C enumeration. Otherwise skip only those constants that are defined in anenum whose name matches regular expression.

Since regular expression is optional, make sure that this switch is followedby at least one other switch if you omit regular expression and have somepending 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 regular expression.

-f, --force
Allows an extension to be created for a header even if that header isnot 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 thegenerated XS code.
-m, --gen-tied-var
Experimental: for each variable declared in the header file(s), declarea 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 regularexpression, even if these types are "typedef"-equivalent to typesfrom typemaps. Should not be used without -x.

This may be useful since, say, types which are "typedef"-equivalentto integers may represent OS-related handles, and one may want to workwith these handles in OO-way, as in "$handle->do_something()".Use "-o ." if you want to handle all the "typedef"ed types as opaquetypes.

The type-to-match is whitewashed (except for commas, which have nowhitespace before them, and multiple "*" which have no whitespacebetween 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 removesthe 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 autoloadwith the constant() subroutine. These macros are assumed to have areturn 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 theheader scanning process will be assumed to have this type. Future versionsof "h2xs" may gain the ability to make educated guesses.
When --compat-version (-b) is present the generated tests will use"Test::More" rather than "Test" which is the default for versions before5.6.2. "Test::More" will be added to PREREQ_PM in the generated"Makefile.PL".
Will force the generation of test code that uses the older "Test" module.
Do not use "Exporter" and/or export any symbol.
Do not use "Devel::PPPort": no portability to older version.
Do not use the module "AutoLoader"; but keep the constant() functionand "sub AUTOLOAD" for constants.
Do not use the pragma "strict".
Do not use the pragma "warnings".
-v, --version=version
Specify a version number for this extension. This version number is addedto 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 theheader file. The package "C::Scan" should be installed. If thisoption is specified, the name of the header file may look like"NAME1,NAME2". In this case NAME1 is used instead of the specifiedstring, but XSUBs are emitted only for the declarations included fromfile NAME2.

Note that some types of arguments/return-values for functions mayresult in XSUB-declarations/typemap-entries which needhand-editing. Such may be objects which cannot be converted from/to apointer (like "long long"), pointers to functions, or arrays. Seealso the section on "LIMITATIONS of -x".



    # 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

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 whichmakes this functionality accessible in Perl? The example belowassumes that the header files are interface_simple.h andinterface_hairy.h, and you want the perl module be named as"Ext::Ension". If you need some preprocessor directives and/orlinking with external libraries, see the flags "-F", "-L" and "-l"in ``OPTIONS''.
Find the directory name
Start with a dummy run of h2xs:

  h2xs -Afn Ext::Ension

The only purpose of this step is to create the needed directories, andlet you know the names of these directories. From the output you cansee 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:

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

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

Archive and test
As usual, run

  cd Ext/Ension  perl Makefile.PL  make dist  make  make test
It is important to do "make dist" as early as possible. This way youcan easily merge(1) your changes to autogenerated files if you decideto 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 inventbetter interfaces than what h2xs could guess.

Consider this section as a guideline only, some other options of h2xsmay 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 functionwhich is of the form, say, "int *", is an input, output, orinput/output parameter. In particular, argument declarations of theform

    int    foo(n)        int *n

should be better rewritten as

    int    foo(n)        int &n

if "n" is an input parameter.

Additionally, h2xs has no facilities to intuit that a function

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

takes a pair of address and length of data at this address, so it is betterto 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.



Extension based on .h and .c files

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