FFI imports packaging utility

Abstract

The Haskell Cabal [1] is a framework which defines a common interface for authors to more easily build their applications in a portable way. The Haskell Foreign Functions Import Generator (hsffig) [3] is a tool to convert a C header file (.h) into Haskell code containing FFI [2] import statements for all entities whose declarations are found in the header file. The FFI Packaging Utility (ffipkg) is a tool that integrates the functionality of hsffig with the Cabal framework allowing for building and installation of packages entirely consisting of foreign functions imports.

It is recommended that readers of this document be familiar with the documents referred to as [3] and [4].

Benefits of Packaging FFI Imports

To build a Haskell application linked to a foreign library, it is necessary to specify the locations of certain files (C headers and static or shared library files) for the Haskell compiler, and this information must be remembered for every application using the library. Building a FFI package means that all such information is contained within the package descriptor, and all that needs to be remembered is just name of the package.

Purpose

The ffipkg utility accepts locations of C header and foreign library files as command line arguments and produces Haskell source files with FFI declarations, a Makefile, a Cabal package descriptor file, and a Setup.hs file suitable for running the Cabal package setup program. The utility acts as a "driver" running the C preprocessor, the equivalent of the hsffig program, and the source splitter. The Makefile created allows for compilation of Haskell source files into split object files: a feature provided by GHC. This technique is discussed in [4].

Command Line Options

Synopsis

Usage: ffipkg [OPTION...] include-file...
  -v      --verbose              provide verbose output
  -n      --new-hooks            use newer userHooks interface
  -i      --header               stop after writing package include file
  -?, -h  --help                 print this help message
  -I                             include files location (may be multiple)
  -L                             library files location (may be multiple)
  -l                             library file to link (may be multiple)
  -V      --version              show program version number
  -w 0.0  --package-version=0.0  specify version of the package
  -p      --package-name=        name the package (will be uppercased)
          --with-make=make       path to make
          --with-awk=awk         path to awk
          --with-ar=ar           path to ar
          --with-ghc=ghc         path to ghc
          --with-gcc=gcc         path to gcc
          --with-hsc2hs=hsc2hs   path to hsc2hs

Package Naming and Versioning

Per the Cabal specification, the two fields are mandatory for a package descriptor file: Name and Version. The -p option sets the name of the package into its argument uppercased. If omitted, name of the first include file found on the command line will be used for package name, uppercased, with directory part and file name suffix stripped. The -w option sets the version field of the package descriptor file to its argument. The version supplied is checked for correctness using the same parser Cabal itself uses. If the syntax of the version is incorrect, or if the option is omitted, the default version string "0.0" will be used.

For FFI packages, versioning does not carry as much sense as it does for native library packages. In some cases, as shown in the Berkeley DB Binding example, it may be set to the version of the library used, but this is totally up to the FFI package creator. It is generally safe to omit this option unless there are separate packages created for different versions of the same library.

Location of Libraries and Include (Header) Files

Similarly to GCC, the -I option is used to specify location(s) where header files will be searched for, and the -L option is used to specify location(s) where library files will be searched for. The -l option is to specify name(s) of library files to link the resulting executable against, and all non-option command line arguments regardless of their position will be treated as include file names (although it is advisable to place all the option arguments on the command line first, and then all non-option arguments).

The ffipkg utility itself does not check for validity or existence of directories and files supplied this way; it only places this information in appropriate fields of the Cabal package descriptor file created by the utility.

Include file names may or may not include the directory part. If included, that may be either relative or absolute paths. The utility creates a small include file which in turn contains #include directives for all include files found on the command line.

For example, if the command line contains:

db.h sys/stat.h

then the include file will look like:

/* File is generated automatically: do not edit */
#include "db.h"
#include "sys/stat.h"

Library file names may be anything a particular linker would accept. It is recommended though to keep with the standard practice not to include directory part into library file names, but use the -L option instead.

References

[1] The Haskell Cabal The Common Architecture for Building Applications and Libraries

[2] Foreign Function Interface

[3] The HSFFIG Tutorial

[4] HSFFIG Linkage Optimization

[5] Berkeley DB

[6] X Window System Protocol X Consortium Standard X Version 11, Release 6.7 DRAFT