On Fri, Apr 11, 2008 at 12:39:54PM +0200, 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.
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.
You could also work around this, at least on Linux, by opening the file in ReadMode before opening it in WriteMode. Hold the reading file handle open until you close the writing handle. Never read from the reading handle. In my testing, this worked well, but bear in mind that when the other readers close the pipe, further writes will "block" rather than produce a SIGPIPE. Matt