Usage of C➔Haskell

Let's have a brief look at how to call the tool and how to use the generated interfaces.

Usage of c2hs

C➔Haskell is implemented by the executable c2hs. The simplest form of usage is

c2hs Lib.chs

where Lib.chs is the Haskell binding module defining the Haskell interface to a C library together with the required marshalling code. If c2hs is invoked in this manner, the binding module must contain a cpp #include directive to determine the C-side interface of the library. Alternatively, a C header file can be specified on the command line, as in

c2hs lib.h Lib.chs

However, the latter option is only preserved for backwards compatibility and not recommended. If no errors occur, c2hs generates three files:

  1. a pure Haskell module Lib.hs, which implements the Haskell API of the library

  2. a C header file Lib.h which some Haskell systems need to compile the generated Haskell code.

  3. a c2hs interface file Lib.chi that is used by other binding modules that import Lib.hs using an import hook (see the section called “Import Hooks”the section on import hooks for details).

The executable c2hs has a couple more options:

Usage: c2hs [ option... ] [header-file] binding-file

  -C CPPOPTS   --cppopts=CPPOPTS    pass CPPOPTS to the C preprocessor
  -c CPP       --cpp=CPP            use executable CPP to invoke C preprocessor
  -d TYPE      --dump=TYPE          dump internal information (for debugging)
  -h, -?       --help               brief help (the present message)
  -i INCLUDE   --include=INCLUDE    include paths for .chi files
  -k           --keep               keep pre-processed C header
  -l           --copy-library       copy `C2HS' library module in
  -o FILE      --output=FILE        output result to FILE (should end in .hs)
  -p PLATFORM  --platform=PLATFORM  platform to use for cross compilation
  -t PATH      --output-dir=PATH    place generated files in PATH
  -v           --version            show version information
               --numeric-version    show version number

The header file must be a C header file matching the given binding file.
The dump TYPE can be
  trace   -- trace compiler phases
  genbind -- trace binding generation
  ctrav   -- trace C declaration traversal
  chs     -- dump the binding file (adds `.dump' to the name)
PLATFORM can be x86_64-linux, i686-linux, m68k-palmos

The most useful of these is probably --cppopts= (or -C). If the C header file needs any special options (like -D or -I) to go through the C pre-processor, here is the place to pass them. A call may look like this:

c2hs --cppopts='-I/some/obscure/dir' --cppopts=-DEXTRA' Lib.chs

If you have more than one option that you want to pass to the pre-processor it is best to use multiple --cppopts= flags. That way there is no need to worry about quoting.

Often, lib.h will not be in the current directory, but in one of the header file directories. c2hs leaves locating the header file to the standard C preprocessor, which usually looks in two places for the header: first, in the standard include directory of the used system, this is usually /usr/include and /usr/local/include; and second, it will look in every directory that is mentioned in a -IDIR option passed to the pre-processor via --cppopts.

If the compiled binding module contains import hooks, C➔Haskell needs to find the .chi (C➔Haskell interface files) produced while compiling the corresponding binding modules. By default, they will be searched for in the current working directory. If they are located elsewhere, the --include=INCLUDE option has to be used to indicate the location, where INCLUDE is a colon-separated list of directories. Multiple such options are admissible. Paths specified later are searched first.

Compilation of a Generated Haskell API

C➔Haskell comes with a marshalling library, called C2HS, which needs to be explicitly imported into Haskell binding modules. The library contains functions that users might use explicitly, but also functions that C➔Haskell will generate for some classes of bindings. The library takes the form of a single Haskell module, which c2hs places in the same directory as the generated binding whenever it is given the --copy-library (or -l) option.