From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 1318 invoked by alias); 17 Aug 2014 22:19:52 -0000 Mailing-List: contact cygwin-help@cygwin.com; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner@cygwin.com Mail-Followup-To: cygwin@cygwin.com Received: (qmail 1294 invoked by uid 89); 17 Aug 2014 22:19:49 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-0.7 required=5.0 tests=AWL,BAYES_40,RP_MATCHES_RCVD autolearn=ham version=3.3.2 X-HELO: Ishtar.hs.tlinx.org Received: from ishtar.tlinx.org (HELO Ishtar.hs.tlinx.org) (173.164.175.65) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Sun, 17 Aug 2014 22:19:48 +0000 Received: from [192.168.4.12] (athenae [192.168.4.12]) by Ishtar.hs.tlinx.org (8.14.7/8.14.4/SuSE Linux 0.8) with ESMTP id s7HMJYZn025787 for ; Sun, 17 Aug 2014 15:19:38 -0700 Message-ID: <53F12A72.2090805@tlinx.org> Date: Sun, 17 Aug 2014 22:19:00 -0000 From: Linda Walsh User-Agent: Thunderbird MIME-Version: 1.0 To: cygwin@cygwin.com Subject: Re: Bash uses lseek while reading from serial device References: <20140816184739.3078D27C7E@mail.csclub.uwaterloo.ca> In-Reply-To: <20140816184739.3078D27C7E@mail.csclub.uwaterloo.ca> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-IsSubscribed: yes X-SW-Source: 2014-08/txt/msg00338.txt.bz2 Being a bit of a busybody... I forwarded this to the bash list and chet responded there... so forwarding it back here... not sure what isatty is supposed to do with a serial line, let alone one on windows... -------- Original Message -------- Subject: Re: Fwd: Bash uses lseek while reading from serial device Date: Sun, 17 Aug 2014 18:12:23 -0400 From: Chet Ramey Organization: ITS, Case Western Reserve University To: Linda Walsh, bug-bash CC: chet.ramey References: <53F041FD.3050509@tlinx.org> On 8/17/14, 1:47 AM, Linda Walsh wrote: > ?? Could this be a cygwin bug? It's hard to see why cygwin > would start using lseek calls when running bash unless bash called > them... but then tha's not to say something else entirely may be going on as > this is running on Windows... ;-/ The original poster's speculation is correct. Bash is not allowed to read more input from stdin than it actually consumes, so commands that it runs get the intended input. To that end, it tries to detect whether or not the fd it is using for standard input is seekable: if it is, bash assumes that it can correctly position the file pointer by seeking backwards; if it is not, bash reads a character at a time. Bash uses lseek to the current file position to check this. If the lseek returns -1/EPIPE, bash assumes the fd is not seekable. If it returns 0, bash assumes that it can move around freely. Since bash is trying to seek backwards in the file, stdin is either a regular file or a tty (in which case bash assumes that reads are newline-delimited by the device driver). I suspect what happens is that isatty() returns 1 for serial devices, but reads are not newline-delimited. Chet -- ``The lyf so short, the craft so long to lerne.'' - Chaucer ``Ars longa, vita brevis'' - Hippocrates Chet Ramey, ITS, CWRU chet@case.edu http://cnswww.cns.cwru.edu/~chet/ -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple