I have some code which forks off a new process which does some non-Haskelly stuff. Thus it does the standard things, something like
processId <- Posix.forkProcess case processId of Just _ -> -- parent process, continue normally ... Nothing -> -- child process do ... -- rearrange plumbing of stdin/stdout/stderr Posix.executeFile [blah]
The GHC runProcess function uses a similar mechanism.
Unfortunately this has developed an extremely irritating bug. The problem is that my program has a lot of worker threads which do things like communicating with servers, already running programs, and so on. These worker threads get replicated in the child process (like everything else) which means that you now have two clients communicating with the server, where
be one, result chaos. (Or in this case, the entire program coming to a halt.) I've had this code for several years (I inherited it), but the problem seems to have recently got much worse; I don't know whether this is because of a change in GHC's scheduling with ghc5.04, or because I am now compiling much more complicated programs.
Whatever, I simply couldn't find a way of fixing this in Haskell. It would be good if there were a way of telling GHC's RTS scheduler "Please don't run any other threads apart from this one until further notice". If so, you could do
[block other threads] [do fork] [if parent process then unblock other threads ... else -- child process rearrange plumbing and do exec ]
But there isn't.
So in the end I resorted to writing a C function which does both the forking and replumbing and execing. Since GHC doesn't run native threads while foreign C code is executing,
Have a look at GHC.Conc.forkProcess, which Volker Stolz
contributed a while ago. Does what you want, but I think you're
actually doing the Right Thing by implementing the fork() & exec()
outside of Haskell.
--sigbjorn
----- Original Message -----
From: "George Russell"
It would be nice if there were a better way . . . _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users