NAME

FFI - Perl Foreign Function Interface

SYNOPSIS

    use FFI;
    $addr = <address of a C function>
    $signature = <function signature>
    $ret = FFI::call($addr, $signature, ...);

    $cb = FFI::callback($signature, sub {...});
    $ret = FFI::call($addr, $signature, $cb->addr, ...);

DESCRIPTION

This module provides a low-level foreign function interface to Perl. It allows the calling of any function for which the user can supply an address and calling signature. Furthermore, it provides a method of encapsulating Perl subroutines as callback functions whose addresses can be passed to C code.

FUNCTION SIGNATURES

Function interfaces are defined by signatures. A function's signature is a string which specifies the function's return type, argument types and calling convention. The first character of the string is the function's calling convention. This is one of

    s   The standard calling convention for dynamically linked functions
    c   The calling convention used by C functions

Note that on many platforms, these two calling conventions may be identical. On the Windows platform, the s code corresponds to the stdcall calling convention, which is used for most dynamic link libraries. The c code corresponds to the cdecl calling convention, which is used for C functions, such as those in the C runtime library.

The remaining characters of the string are the return type of the function, followed by the argument types, in left-to-right order. Valid values are based on the codes used for the pack function, namely

    c   A signed char value.
    C   An unsigned char value.
    s   A signed short value.
    S   An unsigned short value.
    i   A signed integer value.
    I   An unsigned integer value.
    l   A signed long value.
    L   An unsigned long value.
    f   A single-precision float.
    d   A double-precision float.
    p   A pointer.
    v   No value (only valid as a return type).

Note that all of the above codes refer to ``native'' format values.

The p code as an argument type simply passes the address of the Perl value's memory to the foreign function. It is the caller's responsibility to be sure that the called function does not overwrite memory outside that allocated by Perl.

The p code as a return type treats the returned value as a null-terminated string, and passes it back to Perl as such. There is currently no support for functions which return pointers to structures, or to other blocks of memory which do not contain strings, nor for functions which return memory which the caller must free.

To pass pointers to strings, use the p code. Perl ensures that strings are null-terminated for you. To pass pointers to structures, use pack. To pass an arbitrary block of memory, use something like the following:

    $buf = ' ' x 100;
    # Use $buf via a 'p' parameter as a 100-byte memory block

At the present time, there is no direct support for passing pointers to 'native' types (like int). To work around this, use $buf = pack('i', 12); to put an integer into a block of memory, then use the p pointer type, and obtain any returned value using $n = unpack('i', $buf); In the future, better support may be added (but remember that this is intended as a low-level interface!)

EXAMPLES

It is somewhat difficult to provide examples of using this module in isolation, as it is necessary to (somehow) obtain the address of a function to call. In general, this task is delegated to higher-level wrapper modules.

However, the standard DynaLoader module returns symbol references via the DynaLoader::dl_find_symbol() function. While these references are not documented as being addresses, in practice, they seem to be. Code to obtain the address of various C library functions can be built around this

    $clib_file = ($^O eq "MSWin32") ? "MSVCRT40.DLL" : "-lc";
    $clib = DynaLoader::dl_findfile($clib_file);
    $strlen = DynaLoader::dl_find_symbol($clib, "strlen");
    $n = FFI::call($strlen, "cIp", $my_string);
    DynaLoader::dl_free_file($clib);

Clearly, code like this needs to be encapsulated in a module of some form...

NOTE: In fact, the DynaLoader interface has problems in ActiveState Perl, and probably in other binary distributions of Perl. (The issue is related to the way in which the DynaLoader module is built, and may be addressed in future versions of Perl). In the interim, the higher-level wrapper module FFI::Library does not use DynaLoader on Win32 - it uses the (deprecated, but still available) Win32::LoadLibrary and related calls.

TODO

LICENSE

Substantial portions of the code for this module (the underlying FFI code) are licensed under the Gnu General Public License. Under the terms of that license, my understanding is that this module has to be distrubuted under that same license.

My personal preference would be to distribute this module under the same terms as Perl. However, I understand that this is not possible, given the licensing of the FFI code.

AUTHOR

Paul Moore, gustav@morpheus.demon.co.uk

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