alarm / setitimer question and serial record communication

[Peter Dufault]

I have some serial port code that I want to run on both RTEMS and a 
POSIX system without change, and preferably, because I want to, without 
any ifdefs.

The POSIX termios VMIN / VTIME interface is bizarre in that I can't 
(though I've looked at it many times) put together a bullet proof 
interface that handles the remote end disappearing and the local end 
then recovering while I'm only the termios timeout interface.

If you set an initial timeout, then that timeout corresponds only to 
the first received byte, and the read returns after that one byte.  If 
you set an interbyte timeout, then the first byte timeout has to be 
infinite since the interbyte timeout won't come into effect until the 
first byte is received.

To work around this I usually set a timeout on the first received byte 
to know that the remote end is alive, I then check the received byte 
for consistency, and then I set the interbyte timeout for the balance 
of the record for the number of bytes I expect.  Then, in case the 
remote end dies after starting to talk and before finishing talking, I 
wrap it the read call with an alarm that will interrupt the read in the 
rare case that the remote end dies.

This code doesn't work on RTEMS.  It seems to work unless the remote 
device disappears after the first byte is received, but that means it 
doesn't work. "setitimer" doesn't exist on RTEMS, and "alarm" doesn't 
interrupt the thread doing a read even if I brute-force enable SIGALRM.

Am I missing a way to keep this part of my library portable (without 
ifdefs)?  There is one way I don't like, which is to revert to doing 
single byte I/O using the POSIX initial timeout (the timeout that 
returns as soon as the timeout elapses or a single byte is received).  
That approach is OK on an embedded system but isn't OK for a portable 
POSIX library that doesn't like ifdefs.

Is there is a more modern (than alarm for example) POSIX approach that 
will work on both RTEMS and modern POSIX systems?

Peter


Peter Dufault
HD Associates, Inc.