On Wed, Oct 27, 2004 at 12:56:12AM +0000, John Goerzen wrote:
If you follow this a little bit, you'll find that forkProcess is *NOT* throwing the exception that is being reported here. The message is being printed by the RTS of the child process. No exception is thrown in the parent. (Believe me, I've tried to catch it!)
I don't have any idea what's happening here, except that it semes an obvious bug. I found that a work-around is to use exitImmediately in the child. This is often advisable after a fork anyway, because you don't want finalizers or atexit hooks to run in the child. Presumably, it is some such code causing the breakage, though I can't imagine how. On Wed, Oct 27, 2004 at 01:02:52AM +0000, John Goerzen wrote:
Yes. Its POSIX interface is, uhm, weird. I can't quite put my finger on it, but things like setting up a pipe to a child process's stdin just seem brittle and fragile with all sorts of weird errors. I can do this in my sleep in C, Perl, or Python but in Haskell I can barely make it work when I'm fully conscious :-)
Actually, your bug aside, and despite minimal documentation, I've found the Posix modules straightforward. I wrote a version of system that returns the output of the command as a string, and it worked on the first try! I'm sure there are better versions around, but: system :: [String] -> IO String system (cmd:args) = do (read, write) <- createPipe pid <- forkProcess (closeFd read >> child write) closeFd write ret <- fdToHandle read >>= hGetContents length ret `seq` getProcessStatus True False pid return ret where child write = do dupTo write stdOutput closeFd write executeFile cmd True -- path args Nothing -- env Andrew