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H2XS(1) Perl Programmers Reference Guide H2XS(1)


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.

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


-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 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;
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: 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 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.

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”.

Will force the generation of test code that uses the older “Test”

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()
function and “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 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”.

# Default behavior, extension is Rusers
h2xs rpcsvc/rusers

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

# Extension is rpcsvc::rusers. Still finds
h2xs rpcsvc::rusers

# Extension is ONC::RPC. Still finds
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 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:

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:

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

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

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

int *n

should be better rewritten as

int &n

if “n” is an input parameter.

Additionally, h2xs has no facilities to intuit that a function

char *addr
int l

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

SV *addr
char *s;
s = SvPV(sv,len);
RETVAL = foo(s, len);

or alternately

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

return foo(s, len);

MODULE = foo PACKAGE = foo PREFIX = my_

SV *sv

See perlxs and perlxstut for additional details.

perl v5.22.1 2016-03-13 H2XS(1)