GDB Watchpoint

Stop when the value of the expression changes.

watch expression

expression: a variable, an address cast to an data type, a C/C++ expression.

Sample: stop when p changes.

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

(gdb) start

(gdb) p &a
$1 = (int *) 0xffffffffec94
(gdb) p &b
$2 = (int *) 0xffffffffec90

(gdb) watch p
Hardware watchpoint 2: 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: stop when *p changes.

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
Hardware watchpoint 2: *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

Memory Watchpoints

Sample: watch the memory p points to.

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;
}

(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: Watch an absolute memory.

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

Command	Explain
`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.
`watch -l *p`	Find address p points to. Watch content at the address.
`watch ((int)addr)`	Watch absolute memory.

(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

GDB Watchpoint

Expression Watchpoint

Memory Watchpoints

Read Watchpoints

Read-Write Watchpoints