Olivier Andrieu wrote:

 >> and daemon modes with no apparent problems. On the second box, normal
 >> mode works fine; the app will also run in daemon mode. But when I
 >> attempt to send a command to the app in daemon mode, I get the following
 >> error message:
 >>
 >>   ** (bantam.bin:13244): CRITICAL **: GIOChannel watch: callback 
raised \
 >>      an exception
 >
 > That's because your callback raised an exception (!). LablGTK traps
 > uncaught exceptions from callbacks instead of re-raising them and exiting
 > the main  loop. For signals, there is a user-overridable handler that
 > simply prints the exception name to stderr ; for other types of callbacks
 > (IO watches, timeouts, etc.) you simply get this error message.
 >
 > In your code you should wrap your "watcher" function in a try...with 
block
 > and print the exception using Printexc.to_string to see what's going
 > wrong.

Thanks for the tip ... I suppose I should have thought of it myself.

Anyway, I seem to have solved the problem ... I had:

   msg_fd <-
     Unix.openfile msg_pipe [Unix.O_RDONLY; Unix.O_NONBLOCK] 0o600;

Apparently the file descriptor should *not* be opened in non-blocking 
mode (is that really so obvious that it doesn't need to be documented 
anywhere??). But I'm still puzzled, because the behavior was very 
inconsistent: it worked on one box but not another--though perhaps the 
difference in GTK versions would account for that. What was really 
strange, though, was that if I attempted to display the GUI in the 
normal way, i.e. with the command

   $ bantam -s

'bantam' being a shell script which, given the '-s' option, does the 
following:

   echo '%show' > ${TMPDIR}/bantam-${USER}/message.pipe

(which in my case resolves to

   echo '%show' > /tmp/bantam-matt/message.pipe

) --this caused the I/O error. Yet, if invoked the same 'echo' command 
directly in an interactive shell (in this case, bash running in an 
rxvt), it never caused the error. Why on earth would that be, I wonder?

-- 
Matt Gushee
The Reluctant Geek: http://matt.gushee.net/rg/