On Friday 11 April 2008 05:39:54 am Duncan Coutts wrote:
On Thu, 2008-04-10 at 20:34 -0500, John Goerzen wrote:
I have created the named pipe from Haskell no problem.
But I can't use writeFile to write data to it. Worse, it returns:
*** Exception: /tmp/bakroller.zD0xHj/fifo: openFile: does not exist (No such device or address)
What's going on here? Am I going to have to resort to the System.Posix interface just to be able to write to a named pipe?
Named pipes have broken semantics for non-blocking IO, see google or the man pages on named pipes. GHC's standard Handle IO always sets file descriptor to non-blocking mode. That's the problem. That's why cat works, because it uses blocking IO.
OK, I have referred to fifo(7) regarding this point. It seems I may need a loop trying over and over to open the FIFO in write mode. It also appears that ReadWriteMode appearing to work is Linux-specific, and this behavior is left undefined in POSIX. Does openFd do a non-blocking open? (Brief testing suggests it does) I'm somewhat confused about its semantics, especially since it does not appear to correspond directly to open(2). O_CREAT, for instance, is missing.
You would indeed need to use System.Posix to get a FD in blocking mode. Then you have to worry a bit about blocking other Haskell thread when you block on writing to the pipe.
Fortunately, in this particular case, I'm using forking instead of threading so this won't be a problem. Thanks for the help. -- John