proc readpipe pipe {
    set data [read $pipe]
    ... process $data ...
}

proc open_pipe pipename {
    exec /bin/sh -c "sleep 10 > $pipename" &
    set pipe [open $pipename r]
    fconfigure $pipe -blocking 0
    fileevent $pipe readable [list readpipe $pipe]
    set dummy [open $pipename w]
    after 20000 {exec /bin/true}; # to get zombie sleep reaped
}

1. The process opening the fifo to read will block until another process opens it to write. I get past this by exec-ing a shell which opens the pipe, writes nothing, then exits after 10 seconds. Note - it is critical that the "> $pipename" redirection is done by the shell (after it is forked as a new process) and not by Tcl.
2. If the fifo is closed by the writing process, fileevent will fire continuously because it sees end-of-file on the fifo. We can avoid this by making sure that at least one process always has the fifo open for writing, so I also open the fifo for writing from the same Tcl process and never write to it or close it. The 10-second delay in step 1 is to give this time to happen.
3. Finally, I noticed that the exec-ed shell would hang around as a zombie after the sleep finished. The delayed and apparently pointless exec at the end is because running exec again has the side-effect of wait-ing for terminated background processes - documented under Tcl_DetachPids(3).

1. Opening a pipe for reading is no problem, provided that you use the {RDONLY NONBLOCK} access mode; open returns immediately, and you can configure the channel as you wish.
2. Opening a pipe for writing is hazardous, because the open will block until the pipe has also been opened for reading; the access mode {WRONLY NONBLOCK} is rejected by the OS (throws error). The work-around is to first open the pipe for reading as above, second open it for writing (no NONBLOCK), and third close the read channel to the pipe.

## ********************************************************
##
## Name: fifo
##
## Description:
## Provide an anonymous fifo using cat.
##
## Parameters:
##
## Usage:
## trivial handler example:
##
## proc handle { fifo } {
##      puts -nonewline [ gets $fifo ]
## }
##
## set fifo [ fifo handle ]
## puts $fifo peep!
##
## Comments:
## The input side of the fifo should never be flushed!
## The fifo can be closed like an ordinary file.

proc fifo { { handler "" } } {
    if { [ catch {
        set fifo [ open |cat a+ ]
        fconfigure $fifo -blocking off
        fconfigure $fifo -buffering none
        if { [ string length $handler ] } {
            fileevent $fifo readable "$handler $fifo"
        }
    } err ] } {
        return -code error "[ myName ]: $err"
    }
    return $fifo
}
## ********************************************************

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