Re: building and analysing C structs with Storable

On Sun, Oct 14, 2007 at 09:19:24PM +0200, Henning Thielemann wrote:

On Sat, 13 Oct 2007, Adam Langley wrote:

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On 10/13/07, Henning Thielemann wrote:

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Are there analogons to ByteString's Put and Get Monads to construct and disect binary data in a C struct for exchange with a C program?

The memory layout of structs depends on the ABI of system. You can take a guess and write code in the Get and Put monads. Your guess will probably work pretty well within a single architecture (e.g. x86 or x86-64).

However, the portable way to do this is to get the information from the C compiler, which is the approach that c2hs[1] and friends use. I would recommend this unless there's some good reason to think otherwise. (see [2] for the exact hook)

[1] http://www.cse.unsw.edu.au/~chak/haskell/c2hs/ [2] http://www.cse.unsw.edu.au/~chak/haskell/c2hs/docu/c2hs-2.html#ss2.8

I do not quite see the need for an extra tool. I thought it must be possible to ship a Haskell compiler with modules that depend on the system's C compiler, just like the modules that are implemented differently for Windows and Unix. Such modules could provide the functionality to create and inspect C structs for exchange with system libraries.

You say "the system's C compiler" as if there was only one. It's quite common for UNIXoid systems to have several C compilers installed simultaneously; and if you use the module corresponding to the wrong compiler, you get silent data loss. I wouldn't risk it. Stefan