FreeBSD + FIFO pipes

George Russell
Wed, 19 Feb 2003 13:23:48 +0100

Yes I know it's a FAQ, but despite the information in the GHC FAQ I can't get my
program to work.

I'm trying to get HTk to work on FreeBSD (actually FreeBSD running inside a VMware
virtual machine, but I don't think that should make any difference).  How HTk works
is it creates a couple of pipes

       (readIn,writeIn) <- Posix.createPipe
       (readOut,writeOut) <- Posix.createPipe

It then forks off a process which first dup2's readIn onto its stdin and writeOut onto
its stdout, and then runs the Tcl/Tk shell wish (which does graphics).  The idea is
that sending commands along writeIn will cause them to be executed by wish, and wish's
output can be read from readOut.

We do the reading/writing of readOut/writeIn using the Posix fdRead/fdWrite primitives.
In addition, before every call to fdRead we do a Posix.threadWaitRead on it, to make
sure there is actually input there.

All this works fine on Linux and Solaris by the way.  (On Windows we use a completely
different mechanism.)

The problem I am getting with FreeBSD is that fdRead sometimes reports EOF, even though
threadWaitRead has just reported the presence of data.  (And no, I don't think we have
a race condition here, the relevant bit of code is in an exclusive lock.)  If I tell it
to ignore the EOF and try to read again and again it just throws it into an endless loop,

The GHC FAQ says:

 > A workaround
 > for all systems is to open the FIFO for writing yourself,
 > before (or at the same time as) opening it for reading.
What does it mean "open the FIFO for writing yourself"?  I've tried putting in
the following lines immediately after creating the pipes

>       byteCountOut <- fdWrite writeOut "#"
>       (strIn,byteCountIn) <- fdRead readOut 1
>       case (byteCountOut,byteCountIn,strIn) of
>          (1,1,"#") -> done
>          x -> error ("ChildProcess.999 error"++show x)
>       byteCountOut <- fdWrite writeIn "#"
>       (strIn,byteCountIn) <- fdRead readIn 1
>       case (byteCountOut,byteCountIn,strIn) of
>          (1,1,"#") -> done
>          x -> error ("ChildProcess.9998 error"++show x)

which just write a single character down each pipe; unfortunately this doesn't seem
to help.

Any other suggestions?

By the way this is all GHC5.04.1, which seems to be the last FreeBSD version available,
at least with pkg_add.


George Russell