In an interesting c.l.t posting, Colin Macleod describes what it takes to make a Unix FIFO or named piped event-based:
I happen to be working on a project just now where I want to have an event-driven Tcl process read from a fifo. (This is on Solaris 2.6, in case it makes a difference.) My first attempts suggested that this could only be done by polling, but after digging around more I found that I could get fileevent to work as follows:
proc readpipe pipe {
set data [read $pipe]
... process $data ...
}
### Open the fifo, set up event-driven input from it.
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
}
The tricky bits are:
- 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.
- 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.
- 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).
(Explain how general file append operations can be detected as events only through polling.)