GDB Watchpoint

Stop when value of an expression changes

Command	Meaning
`watch p`	Consider p as expression. Watch value of the expression.
`watch *p`	Consider *p as expression. Watch value of the expression.
`watch -l p`	Find memory address of p. Watch content at the address (memory watchpoint)
`watch -l *p`	Find address p points to. Watch content at the address (memory watchpoint)
`watch ((int ) 0xffffffffec9c)`	Watch absolute memory.
`rwatch p`	Stop when p is read (read watchpoint).
`awatch p`	Stop when p is either read or written (read-write watchpoint) .

Sample 1: watch p

int main()
{
    int *p = 0; // watch p
    int a = 1;
    int b = 2;  
    p = &a;     // hit
    p = &b;     // hit
    return 0;
}

(gdb) start
(gdb) watch p

(gdb) continue
Hardware watchpoint 2: p
Old value = (int *) 0x0
New value = (int *) 0xffffffffec94
main () at demo.c:7

(gdb) continue
Hardware watchpoint 2: p
Old value = (int *) 0xffffffffec94
New value = (int *) 0xffffffffec90
main () at demo.c:8

Sample 2: watch *p

int main()
{
  int *p = 0; //watch *p
  int a = 1;
  int b = 2;
  p = &a;     //hit: *p = 1
  *p = 10;    //hit: *p = 10
  a = 100;    //hit: *p = a = 100
  p = &b;     //hit: *p = b = 2
  return 0;
}

(gdb) start
(gdb) watch *p

(gdb) continue
Hardware watchpoint 2: *p
Old value = <unreadable>
New value = 1
main () at demo.c:7

(gdb) continue
Hardware watchpoint 2: *p
Old value = 1
New value = 10
main () at demo.c:8

(gdb) continue
Hardware watchpoint 2: *p
Old value = 10
New value = 100
main () at demo.c:9

(gdb) continue
Hardware watchpoint 2: *p
Old value = 100
New value = 2
main () at demo.c:10

Sample 3: Memory Watchpoint

int main()
{  
  int a = 1, b = 2;
  int *p = &a;    // watch -l *p
  p = &b;
  b = 20;
  *p = 200;
  a = 10;         // hit
  a = 100;        // hit
  return 0;
}

Watch the memory p points to.

(gdb) start
(gdb) watch -l *p

(gdb) continue
Hardware watchpoint 2: -location *p
Old value = 1
New value = 10
main () at demo.c:9

(gdb) continue
Hardware watchpoint 2: -location *p
Old value = 10
New value = 100
main () at demo.c:10

Sample 4: Watch Absolute Memory.

int main()
{
  int n = 3;  // watch *((int*)address of n)
  n += 1;     // hit
  return 0;
}

(gdb) print &n
$1 = (int *) 0xffffffffec9c
(gdb) print *((int *) 0xffffffffec9c)
$2 = 3

# watch a 4-byte region at the address
(gdb) watch *((int *) 0xffffffffec9c)

(gdb) continue
Hardware watchpoint 2: *((int *) 0xffffffffec9c)
Old value = 3
New value = 4
main () at demo.c:5