Path: utzoo!attcan!uunet!yale!cmcl2!polyof!john
From: john@polyof.UUCP ( John Buck )
Newsgroups: comp.sys.att
Subject: Re: termio(7) and read(2) problem!
Summary: Non-blocking reads do work
Message-ID: <427@polyof.UUCP>
Date: 9 Jan 89 23:58:12 GMT
References: <137@zebra.UUCP>
Distribution: na
Organization: Polytechnic University, Farmingdale NY
Lines: 42

In article <137@zebra.UUCP>, vern@zebra.UUCP (Vernon C. Hoxie) writes:
> It is connected to /dev/tty000.  The program opens this file with:
> 	fd = open("/dev/tty000", O_RDWR | O_NDELAY);
> Then after some other housekeeping:
> 	char buf[512];
> 	.
> 	.
> 	k = read(fd, buf, 211);
> Vernon C. Hoxie		       {ncar,nbires,boulder,isis}!scicom!zebra!vern

Yeah, this is a common problem.  the open(...O_NDELAY) is the culprit,
for lack of a better word.  After the open(), do a

	if(fcntl(fd, F_SETFL, 0) == -1){
		perror("F_SETFL");
		....
	}

The O_NDELAY, while causing the open() to return without waiting for
CARRIER, also causes read()'s to return without waiting for data.
Had you bothered to read the manual:
READ(2)...
	When attempting to read a file associated with a tty that has no
	data currently available:

		If O_NDELAY is set, the read will return -1 (or 0 on
			some systems... JB)
		If O_NDELAY is clear, the read will block until data
			becomes available.

The latter is the normal case.

FCNTL(2) explains how to turn certain file flags on and off...

Hope this helps.

John Buck
john@polyof.poly.edu
john@polygraf.bitnet
trixie!polyof!john  (UUCP)
Polytechnic University
Farmingdale, NY