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From: mcooper@usc-oberon.UUCP (Michael A. Cooper)
Newsgroups: net.unix-wizards
Subject: alarms during a read()
Message-ID: <667@usc-oberon.UUCP>
Date: Mon, 4-Aug-86 19:38:53 EDT
Article-I.D.: usc-ober.667
Posted: Mon Aug  4 19:38:53 1986
Date-Received: Thu, 7-Aug-86 03:07:46 EDT
Distribution: net
Organization: USC Computing Services, Los Angeles
Lines: 44

-=-=

[VAX 4.2BSD]

I've been trying to figure out how to jump out of a read()
via a signal alarm.  My problem is that things wedge up
when (apparantly) read() is restarted when the alarm goes
off.  This occurs when there is no input for read() to read.
Hence, wakeup() is never called by the alarm.  Below is the 
basic code:

			signal(SIGALRM, wakeup);
			alarm(3);
			recvbuf[0] = getch();
			alarm(0);

And here's getch()...

			getch()
			{
				char c;

				(void) read(0, &c, 1);
				return(c & 0377);
			}

According to the signal(3c) manual:

    "If a caught signal occurs during certain system calls, caus-
     ing the call to terminate prematurely, the call is automati-
     cally restarted.  In particular this can occur during a read
     or write(2) on a slow device (such as a terminal; but not a
     file) and during a wait(2)."

I assume this is my problem.  Now, has anyone figured out a way
around this?  It seems that catching a signal during certain
system calls should not be a problem.  It's not too much to ask
for, is it?


-- 
Michael Cooper, U of Southern California Computing Services, (213) 743-3462
  UUCP: {sdcrdcf, uscvax}!usc-oberon!mcooper
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