From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gdb-return-44733-listarch-gdb=sources.redhat.com@sourceware.org>
Received: (qmail 57304 invoked by alias); 12 Oct 2015 14:14:22 -0000
Mailing-List: contact gdb-help@sourceware.org; run by ezmlm
Precedence: bulk
List-Id: <gdb.sourceware.org>
List-Subscribe: <mailto:gdb-subscribe@sourceware.org>
List-Archive: <http://sourceware.org/ml/gdb/>
List-Post: <mailto:gdb@sourceware.org>
List-Help: <mailto:gdb-help@sourceware.org>, <http://sourceware.org/ml/#faqs>
Sender: gdb-owner@sourceware.org
Received: (qmail 57289 invoked by uid 89); 12 Oct 2015 14:14:21 -0000
Authentication-Results: sourceware.org; auth=none
X-Virus-Found: No
X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=AWL,BAYES_00,SPF_HELO_PASS,T_RP_MATCHES_RCVD autolearn=ham version=3.3.2
X-HELO: mx1.redhat.com
Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Mon, 12 Oct 2015 14:14:21 +0000
Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24])	by mx1.redhat.com (Postfix) with ESMTPS id E5D8936B1F4;	Mon, 12 Oct 2015 14:14:19 +0000 (UTC)
Received: from [127.0.0.1] (ovpn01.gateway.prod.ext.ams2.redhat.com [10.39.146.11])	by int-mx11.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id t9CEEIbN007062;	Mon, 12 Oct 2015 10:14:19 -0400
Message-ID: <561BC039.9050708@redhat.com>
Date: Mon, 12 Oct 2015 14:14:00 -0000
From: Pedro Alves <palves@redhat.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0
MIME-Version: 1.0
To: Nancy <nancydreaming@gmail.com>, gdb@sourceware.org
Subject: Re: Is that a GDB bug?
References: <CADdyJmk-gKUVEUYRwqmXPknUL8QiT=x+J6ZA2BL8WjK1Nf+Z3A@mail.gmail.com>
In-Reply-To: <CADdyJmk-gKUVEUYRwqmXPknUL8QiT=x+J6ZA2BL8WjK1Nf+Z3A@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
X-SW-Source: 2015-10/txt/msg00055.txt.bz2

On 10/12/2015 01:46 PM, Nancy wrote:
> Hi,
> 
> Why line 5 execute twice?   Is that a GDB bug?
> 
> debug.c :
>      1    int main()
>      2    {
>      3        int x;
>      4        x=0;
>      5        L1: switch(x) { case 0: x=1; goto L1;  case 1: if(x==0)
> goto L1; else break; }
>      6        x=2;
>      7    }
> 
> $ gcc -O0 -g debug.c -o debug
> $ gdb debug
> GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
> Copyright (C) 2014 Free Software Foundation, Inc.
> ....................
> Reading symbols from debug...done.
> (gdb) b 5
> Breakpoint 1 at 0x80483fa: file debug.c, line 5.
> (gdb) r
> Starting program: /mnt/hgfs/cygwin/tmp/debug
> 
> Breakpoint 1, main () at debug.c:5
> 5        L1: switch(x) { case 0: x=1; goto L1;  case 1: if(x==0) goto
> L1; else break; }
> (gdb) n

x starts as 0.  So the fist time, "goto L1" is executed.  That makes execution
flow back to the address where the breakpoint was inserted, so you get another
breakpoint hit:

> 
> Breakpoint 1, main () at debug.c:5
> 5        L1: switch(x) { case 0: x=1; goto L1;  case 1: if(x==0) goto
> L1; else break; }

It doesn't matter that "next" was in progress.  The breakpoint hit takes
priority.

> (gdb) n
> 6        x=2;
> 
> 

Thanks,
Pedro Alves