[C2hs] distributing packages that use C2HS

Duncan Coutts duncan.coutts at worc.ox.ac.uk
Mon Jun 23 18:09:40 EDT 2008

On Mon, 2008-06-23 at 12:51 -0500, John Lato wrote:

> Okay, thanks.  I'll just need to live with the dependency on c2hs for
> end-users.  I suppose if that's a problem, I should have chosen
> something other than c2hs as a build tool.

In general though it does need to be done on the end users machine so
it's not just a c2hs limitation.

You may be lucky and know for certain that for the particular C API that
you are binding that it is the same on every platform and contains no
sizes or offsets of types (since sizes and offsets change between
platforms, especially between 32 and 64bit ones). In that happy
circumstance then it would be safe to bundle pre-generated .hs files. 

Cabal will not help you there however since it takes the conservative
position that the result of c2hs (and hsc2hs and other FFI
pre-processors) is platform dependent.

One way to check if its portable is to look at the .hs files generated
by c2hs and see if there are any numbers generated. If so then it is
platform dependent. Another way of saying it is that call and fun hooks
are ok but sizeof, get and set hooks are not.

However even if there are no numbers in the generated .hs that doesn't
guarantee you're safe. If you're binding to a .h file that might be
different on different systems then you've got problems. It is quite
common for lots of system headers to use slightly different types on
different systems. If you're binding to a C library that has the same
implementation on each system or is otherwise guaranteed to have the
same C interface then you'd be ok.

If you were *absolutely* sure then you could manually include the
generated .hs files in dist/build/ in the same path in the tarball and
it would work without c2hs installed (assuming you also removed the c2hs
build-tool dep). For example see the tarball for happy and where the
pre-generated .hs file lives in the tarball (of course for happy, cabal
does this automatically because it knows the output of happy is platform


More information about the C2hs mailing list