On Wed, Oct 17, 2007 at 08:46:41AM -0700, Donn Cave wrote:
On Wed, 17 Oct 2007, Simon Marlow wrote: ...
Note that forkProcess doesn't currently work with +RTS -N2 (or any value larger than 1), and it isn't likely to in the future. I suspect forkProcess should be deprecated.
The POSIX spec is pretty strict about what system calls you can make in the child process of a fork in a multithreaded program, and those same restrictions apply in Haskell, and they apply not only to the Haskell code but also to the runtime (e.g. what if the child process needs more memory and the runtime calls mmap(), that's not allowed). We get away with it most of the time because the OSs we run on are less strict than POSIX. However, in general I think forking should be restricted to C code that you invoke via the FFI.
Just to be precise about it, though, there's nothing about Haskell per se that causes trouble with fork, right? This is a GHC implementation issue.
Forking in the presense of multiple threads is a semantic nightmare. Anything any Haskell implementation does is going to be wrong in at least two ways, so it's probably best not to try. Write your fork-child code in C. Stefan