gdb watchpoint won't work when variable changes from going off end of array

Question

#include <stdio.h>

typedef struct ThingStruct {
    int arr[8];
    int after;
} Thing;

void foo(int i) {
    Thing thing;
    int* ip = &thing.after;
    thing.after = 12345;
    printf("beforehand\n");
    thing.arr[i] = 55;
    printf("done\n");
}

int main() {
    foo(8);
}

This code changes thing.after by accidentally going off the end of the array. I want to try to find the line where where thing.after is changing by using gdb. So I compile with -g , put a breakpoint on line 12, then put a watchpoint on thing.after , but the watchpoint doesn't trigger, even though putting a breakpoint on line 14 does show that thing.after did change.

I even tried taking the address of thing.after and setting a watchpoint on that, but it still does not trigger.

Answer 1

Watch point needs to be re-added each time the foo function is entered (Note that, as you are watching the local variable, it will not be valid after the stack frame exits and will be automatically deleted after the foo returns). Also, if the watched variable changes on the current line to be executed, then the watch point is not getting triggered (not sure why). For me it works when I add the watch point watch thing.after just after entering foo when on line int* ip = &thing.after; . When I continue, the watch point hits 2 times.

Answer 2

You didn't say which platform, what version of GDB, or what command you used to set the watchpoint.

Using gdb 7.9 on Ubuntu/x86_64, things work as I expect them to work:

(gdb) b foo
Breakpoint 1 at 0x400538: file t.c, line 10.
(gdb) r
Starting program: /tmp/a.out 

Breakpoint 1, foo (i=8) at t.c:10
10          int* ip = &thing.after;
(gdb) watch thing.after
Hardware watchpoint 2: thing.after
(gdb) c
Continuing.
Hardware watchpoint 2: thing.after

Old value = 4195712
New value = 12345
foo (i=8) at t.c:12
12          printf("beforehand\n");
(gdb) c
Continuing.
beforehand
Hardware watchpoint 2: thing.after

Old value = 12345
New value = 55
foo (i=8) at t.c:14
14          printf("done\n");

(gdb) q