How can I use "watch" GDB?

Question

I tried to watch the change of the "int a" by the command "watch a". However, the program does not stop, when it changes to 12. Why?

 /* FILE: test.c */
 #include <stdio.h>
 #include <stdlib.h>

 int main(int argc, char** argv){
  printf("Hello world\n");

  int a = 12;
  a = 10;                                                                                                                                                                      
  return 0; 
 }

Answer 1

It may help to specify your platform, version of GDB, and exact sequence of GDB commands you used.

Here is what I see (GDB appears to work just fine):

$ gcc -g test.c

$ gdb a.out
GNU gdb (GDB) 6.8.50.20090430-cvs
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
(gdb) list
1       #include <stdio.h>
2       #include <stdlib.h>
3
4       int main(int argc, char** argv){
5         printf("Hello world\n");
6
7         int a = 12;
8         a = 10;                                                                                                                                                                      
9         return 0; 
10      }
11
(gdb) b 5
Breakpoint 1 at 0x4004a7: file test.c, line 5.
(gdb) r

Breakpoint 1, main (argc=1, argv=0x7fffffffdb28) at test.c:5
5         printf("Hello world\n");
(gdb) watch a
Hardware watchpoint 2: a
(gdb) c
Hello world
Hardware watchpoint 2: a

Old value = 0
New value = 12
main (argc=1, argv=0x7fffffffdb28) at test.c:8
8         a = 10;                                                                                                                                                                      
(gdb) c
Hardware watchpoint 2: a

Old value = 12
New value = 10
main (argc=1, argv=0x7fffffffdb28) at test.c:9
9         return 0; 
(gdb) c

Watchpoint 2 deleted because the program has left the block in
which its expression is valid.
0x00007ffff7ab3033 in exit () from /lib/libc.so.6
(gdb) c

Program exited normally.
(gdb) q