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"ExtUtils::H2PM" − automatically generate perl modules to wrap C header files


This module assists in generating wrappers around system functionallity, such as "socket()" types or "ioctl()" calls, where the only interesting features required are the values of some constants or layouts of structures normally only known to the C header files. Rather than writing an entire XS module just to contain some constants and pack/unpack functions, this module allows the author to generate, at module build time, a pure perl module containing constant declarations and structure utility functions. The module then requires no XS module to be loaded at run time.

In comparison to h2ph, "C::Scan::Constants", and so on, this module works by generating a small C program containing "printf()" lines to output the values of the constants, compiling it, and running it. This allows it to operate without needing tricky syntax parsing or guessing of the contents of C header files.

It can also automatically build pack/unpack functions for simple structure layouts, whose members are all simple integer or character array fields. It is not intended as a full replacement of arbitrary code written in XS modules. If structures should contain pointers, or require special custom handling, then likely an XS module will need to be written.


module $name
Sets the name of the perl module to generate. This will apply a "package" header.

include $file
Adds a file to the list of headers which will be included by the C program, to obtain the constants or structures from

constant $name, %args
Adds a numerical constant.

The following additional named arguments are also recognised:

name => STRING

Use the given name for the generated constant function. If not specified, the C name for the constant will be used.

ifdef => STRING

If present, guard the constant with an "#ifdef STRING" preprocessor macro. If the given string is not defined, no constant will be generated.

structure $name, %args
Adds a structure definition. This requires a named argument, "members". This should be an ARRAY ref containing an even number of name-definition pairs. The first of each pair should be a member name. The second should be one of the following structure member definitions.

The following additional named arguments are also recognised:

pack_func => STRING

unpack_func => STRING

Use the given names for the generated pack or unpack functions.

with_tail => BOOL

If true, the structure is a header with more data behind it. The pack function takes an optional extra string value for the data tail, and the unpack function will return an extra string value containing it.

no_length_check => BOOL

If true, the generated unpack function will not first check the length of its argument before attempting to unpack it. If the buffer is not long enough to unpack all the required values, the remaining ones will not be returned. This may be useful, for example, in cases where various versions of a structure have been designed, later versions adding extra members, but where the exact version found may not be easy to determine beforehand.

arg_style => STRING

Defines the style in which the functions take arguments or return values. Defaults to "list", which take or return a list of values in the given order. The other allowed value is "hashref", where the pack function takes a HASH reference and the unpack function returns one. Each will consist of keys named after the structure members. If a data tail is included, it will use the hash key of "_tail".

ifdef => STRING

If present, guard the structure with an "#ifdef STRING" preprocessor macro. If the given string is not defined, no functions will be generated.

The following structure member definitions are allowed:


The field contains a single signed or unsigned number. Its size and signedness will be automatically detected.


The field contains a NULL-padded string of characters. Its size will be automatically detected.


The field contains a single number as for "member_numeric". Instead of consuming/returning a value in the arguments list, this member will be packed from an expression, or asserted that it contains the given value. The string $code will be inserted into the generated pack and unpack functions, so it can be used for constants generated by the "constant" directive.

The structure definition results in two new functions being created, "pack_$name" and "unpack_$name", where $name is the name of the structure (with the leading "struct" prefix stripped). These behave similarly to the familiar functions such as "pack_sockaddr_in"; the "pack_" function will take a list of fields and return a packed string, the "unpack_" function will take a string and return a list of fields.

no_export, use_export, use_export_ok
Controls the export behaviour of the generated symbols. "no_export" creates symbols that are not exported by their package, they must be used fully− qualified. "use_export" creates symbols that are exported by default. "use_export_ok" creates symbols that are exported if they are specifically requested at "use" time.

The mode can be changed at any time to affect only the symbols that follow it. It defaults to "use_export_ok".

$perl = gen_output
Returns the generated perl code. This is used internally for testing purposes but normally would not be necessary; see instead "write_output".

write_output $filename
Write the generated perl code into the named file. This would normally be used as the last function in the containing script, to generate the output file. In the case of "ExtUtils::MakeMaker" or "Module::Build" invoking the script, the path to the file to be generated should be given in $ARGV[0]. Normally, therefore, the script would end with

 write_output $ARGV[0];

Adds an include path to the list of paths used by the compiler

 include_path $path

Adds a symbol to be defined on the compiler’s commandline, by using the "−D" option. This is sometimes required to turn on particular optional parts of the included files. An optional value can also be specified.

 define $symbol
 define $symbol, $value;


Normally this module would be used by another module at build time, to construct the relevant constants and structure functions from system headers.

For example, suppose your operating system defines a new type of socket, which has its own packet and address families, and perhaps some new socket options which are valid on this socket. We can build a module to contain the relevant constants and structure functions by writing, for example:

 use ExtUtils::H2PM;
 module "Socket::Moonlaser";
 include "moon/laser.h";
 constant "AF_MOONLASER";
 constant "PF_MOONLASER";
 constant "SOL_MOONLASER";
 constant "MOONLASER_POWER",      name => "POWER";
 structure "struct laserwl",
    members => [
       lwl_nm_coarse => member_numeric,
       lwl_nm_fine   => member_numeric,
 write_output $ARGV[0];

If we save this script as, say, lib/Socket/, then when the distribution is built, the script will be used to generate the contents of the file lib/Socket/ Once installed, any other code can simply

 use Socket::Moonlaser qw( AF_MOONLASER );

to import a constant.

The method described above doesn’t allow us any room to actually include other code in the module. Perhaps, as well as these simple constants, we’d like to include functions, documentation, etc... To allow this, name the script instead something like lib/Socket/, so that this is the name used for the generated output. The code can then be included in the actual lib/Socket/ (which will just be a normal perl module) by

 package Socket::Moonlaser;
 use Socket::Moonlaser_const;
 sub get_power
    getsockopt( $_[0], SOL_MOONLASER, POWER );
 sub set_power
    setsockopt( $_[0], SOL_MOONLASER, POWER, $_[1] );
 sub get_wavelength
    my $wl = getsockopt( $_[0], SOL_MOONLASER, WAVELENGTH );
    defined $wl or return;
    unpack_laserwl( $wl );
 sub set_wavelength
    my $wl = pack_laserwl( $_[1], $_[2] );
    setsockopt( $_[0], SOL_MOONLASER, WAVELENGTH, $wl );

Sometimes, the actual C structure layout may not exactly match the semantics we wish to present to perl modules using this extension wrapper. Socket address structures typically contain their address family as the first member, whereas this detail isn’t exposed by, for example, the "sockaddr_in" and "sockaddr_un" functions. To cope with this case, the low-level structure packing and unpacking functions can be generated with a different name, and wrapped in higher-level functions in the main code. For example, in

 structure "struct sockaddr_ml",
    pack_func   => "_pack_sockaddr_ml",
    unpack_func => "_unpack_sockaddr_ml",
    members => [
       ml_family    => member_numeric,
       ml_lat_deg   => member_numeric,
       ml_long_deg  => member_numeric,
       ml_lat_fine  => member_numeric,
       ml_long_fine => member_numeric,

This will generate a pack/unpack function pair taking or returning five arguments; these functions will not be exported. In our main file we can wrap these to actually expose a different API:

 sub pack_sockaddr_ml
    @_ == 2 or croak "usage: pack_sockaddr_ml(lat, long)";
    my ( $lat, $long ) = @_;
    return _pack_sockaddr_ml( AF_MOONLASER, int $lat, int $long,
      ($lat − int $lat) * 1_000_000, ($long − int $long) * 1_000_000);
 sub unpack_sockaddr_ml
    my ( $family, $lat, $long, $lat_fine, $long_fine ) =
       _unpack_sockaddr_ml( $_[0] );
    $family == AF_MOONLASER or croak "expected family AF_MOONLASER";
    return ( $lat + $lat_fine/1_000_000, $long + $long_fine/1_000_000 );

Sometimes, a structure will contain members which are themselves structures. Suppose a different definition of the above address, which at the C layer is defined as

 struct angle
    short         deg;
    unsigned long fine;
 struct sockaddr_ml
    short        ml_family;
    struct angle ml_lat, ml_long;

We can instead "flatten" this structure tree to obtain the five fields by naming the sub-members of the outer structure:

 structure "struct sockaddr_ml",
    members => [
       "ml_family"    => member_numeric,
       "ml_lat.deg"   => member_numeric,
       "ml_lat.fine"  => member_numeric,
       "ml_long.deg"  => member_numeric,
       "ml_long.fine" => member_numeric,


Consider more structure members. With strings comes the requirement to have members that store a size. This requires cross-referential members. And while we’re at it it might be nice to have constant members; fill in constants without consuming arguments when packing, assert the right value on unpacking.


Paul Evans <leonerd AT leonerd DOT org DOT uk>