From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 13828 invoked by alias); 26 Oct 2003 21:35:25 -0000 Mailing-List: contact gdb-patches-help@sources.redhat.com; run by ezmlm Precedence: bulk List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sources.redhat.com Received: (qmail 13821 invoked from network); 26 Oct 2003 21:35:24 -0000 Received: from unknown (HELO concert.shout.net) (204.253.184.25) by sources.redhat.com with SMTP; 26 Oct 2003 21:35:24 -0000 Received: from duracef.shout.net (duracef.shout.net [204.253.184.12]) by concert.shout.net (8.12.10/8.12.10) with ESMTP id h9QLY3Ir026162; Sun, 26 Oct 2003 15:34:03 -0600 Received: from duracef.shout.net (localhost [127.0.0.1]) by duracef.shout.net (8.12.10/8.12.9) with ESMTP id h9QLY3fa027425; Sun, 26 Oct 2003 15:34:03 -0600 Received: (from mec@localhost) by duracef.shout.net (8.12.10/8.12.9/Submit) id h9QLY3IM027424; Sun, 26 Oct 2003 16:34:03 -0500 Date: Sun, 26 Oct 2003 21:35:00 -0000 From: Michael Elizabeth Chastain Message-Id: <200310262134.h9QLY3IM027424@duracef.shout.net> To: drow@mvista.com, gdb-patches@sources.redhat.com Subject: Re: [rfa/doc/threads] thread breakpoints and system calls Cc: eliz@gnu.org X-SW-Source: 2003-10/txt/msg00774.txt.bz2 Okay, how about this? This version eschews the words 'legal' and 'illegal'. It also adds a sentence about the root cause, which is the interaction between multiple threads and the signals used to implement breakpoints and other events that stop execution. Tested with makeinfo for syntax. Michael C 2003-10-26 Michael Chastain * gdb.texinfo (Thread Stops): Document the issue with premature return from system calls in multi-threaded programs. Index: gdb.texinfo =================================================================== RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v retrieving revision 1.183 diff -c -3 -p -r1.183 gdb.texinfo *** gdb.texinfo 23 Oct 2003 00:11:59 -0000 1.183 --- gdb.texinfo 26 Oct 2003 21:28:00 -0000 *************** allows you to examine the overall state *** 3761,3766 **** --- 3761,3807 ---- switching between threads, without worrying that things may change underfoot. + @cindex thread breakpoints and system calls + @cindex system calls and thread breakpoints + @cindex premature return from system calls + There is an unfortunate side effect. If one thread stops for a + breakpoint, or for some other reason, and another thread is blocked in a + system call, then the system call may return prematurely. This is a + consequence of the interaction between multiple threads and the signals + that @value{GDBN} uses to implement breakpoints and other events that + stop execution. + + To handle this problem, your program should check the return value of + each system call and react appropriately. This is good programming + style anyways. + + For example, do not write code like this: + + @smallexample + sleep (10); + @end smallexample + + The call to @code{sleep} will return early if a different thread stops + at a breakpoint or for some other reason. + + Instead, write this: + + @smallexample + int unslept = 10; + while (unslept > 0) + unslept = sleep (unslept); + @end smallexample + + A system call is allowed to return early, so the system is still + conforming to its specification. But @value{GDBN} does cause your + multi-threaded program to behave differently than it would without + @value{GDBN}. + + Also, @value{GDBN} uses internal breakpoints in the thread library to + monitor certain events such as thread creation and thread destruction. + When such an event happens, a system call in another thread may return + prematurely, even though your program does not appear to stop. + @cindex continuing threads @cindex threads, continuing Conversely, whenever you restart the program, @emph{all} threads start