Re: Is that a GDB bug?

[Pedro Alves <palves@redhat.com>]

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