[Haskell-cafe] Re: Bug in runInteractiveProcess?

Stefan O'Rear stefanor at cox.net
Wed Oct 17 21:11:12 EDT 2007

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.

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