Problem with signal handling in sunview

[Marcus Gates]

I have a sunview program which sends/receives messages from/to other
programs (some sunview, some not).  The message passing is accomplished
using shared-memory and SIGUSR1.  The messages are placed into shared
memory (semaphores are used to lock shared memory so that only one program
can access it at a time) and then a SIGUSR1 signal is sent to the process
which must retrieve the message from its message queue.  For some reason
we occasionally lose a message.  The sunview code is something like:

main()
{
     notify_set_signal_func(sim_mod, get_input, SIGUSR1, NOTIFY_SYNC);
     window_main_loop()
}

get_input()
{
	mask = sigsetmask(0xffffffff);
	/*
	   done in while loop since other SIGUSR1 signals might come
	   in while we're here - so get messages until there are none.
	*/
	while (recv_msg(&msg) != ERROR)
	{
		printf("msg %d received\n", msg);
	}
	sigsetmask(mask);
}

recv_msg(msg)
{
	lock_semaphore();
	get_message from queue, adjust queue
	printf("message %d taken from queue\n", msg);
	unlock_semaphore()
	return()
}

The printfs were added to trace execution through these routines, what we
see when a message gets lost is:

  message 23 taken from queue
  message 12 taken from queue	/* 2 in a row from recv_msg */
  msg 12 received
  msg 12 received		/* 2 in a row from get_input */

Any ideas how code in recv_msg() could be executed twice (without
returning first?).  Any help would be greatly appreciated.

	marc gates
	UUCP: ...bpa!cp1!aplcen!aplvax!marc
	Internet: marc at aplpy.jhuapl.edu