From owner-freebsd-questions  Tue Mar 12 10:18:30 1996
Return-Path: owner-questions
Received: (from root@localhost)
          by freefall.freebsd.org (8.7.3/8.7.3) id KAA03070
          for questions-outgoing; Tue, 12 Mar 1996 10:18:30 -0800 (PST)
Received: from alpha.xerox.com (alpha.Xerox.COM [13.1.64.93])
          by freefall.freebsd.org (8.7.3/8.7.3) with SMTP id KAA03064
          for <questions@freebsd.org>; Tue, 12 Mar 1996 10:18:27 -0800 (PST)
Received: from gemini.sdsp.mc.xerox.com ([13.231.132.20]) by alpha.xerox.com with SMTP id <14745(11)>; Tue, 12 Mar 1996 10:17:47 PST
Received: from gnu.mc.xerox.com (gnu.sdsp.mc.xerox.com) by gemini.sdsp.mc.xerox.com (4.1/SMI-4.1)
	id AA03253; Tue, 12 Mar 96 13:17:48 EST
Received: by gnu.mc.xerox.com (4.1/SMI-4.1)
	id AA08720; Tue, 12 Mar 96 13:17:42 EST
Message-Id: <9603121817.AA08720@gnu.mc.xerox.com>
To: Luigi Rizzo <luigi@labinfo.iet.unipi.it>
Cc: msmith@atrad.adelaide.edu.au, questions@freebsd.org
Subject: Re: non-blocking read ? 
In-Reply-To: Your message of "Tue, 12 Mar 1996 10:06:55 PST."
             <199603121806.TAA20924@labinfo.iet.unipi.it> 
Date: Tue, 12 Mar 1996 10:17:39 PST
From: "Marty Leisner" <leisner@sdsp.mc.xerox.com>
Sender: owner-questions@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk


In message <199603121806.TAA20924@labinfo.iet.unipi.it>,   you write:
>	result=read(handle, buf, size);
>
>The lseek might be non blocking -- no problem with that.
>The read is non-blocking because of the previous call to fcntl().
>
>But how much data will be available when I get the SIGIO (or select
>will return that I/O is possible) ? The amount I requested (assuming it
>is available), or the system idea of a block, or what ?
>
>	Thanks
>	Luigi


Reads on files have to block...the only think you can do
is set a timer to go off in about 10 Msec, and if the read
isn't done yet, the alarm will knock you out of the read with
an -EINTR.

I don't know if the kernel will continue on the read...

GNU libc says:
- Macro: int O_NONBLOCK
     This prevents `open' from blocking for a "long time" to open the
     file.  This is only meaningful for some kinds of files, usually
     devices such as serial ports; when it is not meaningful, it is
     harmless and ignored.  Often opening a port to a modem blocks
     until the modem reports carrier detection; if `O_NONBLOCK' is
     specified, `open' will return immediately without a carrier.


files but design "don't block"

marty