GDB: assign a breakpoint to change a character

Question

I am debugging a C application that has a segmentation error. I have already defined the variable that is causing the problem. However, I do not yet know when a variable is assigned so that it causes a segmentation error.

Is there a way to set a breakpoint in GDB if a new value is assigned to an existing variable?

Answer 1

You need a watchpoint :

 (gdb) watch my_var 

Answer 2

 #include <stdio.h> struct foo { int i[12]; int j; }; int main(void) { struct foo foo = {{0}}; int *p; p = foo.i; p[12] = 42; printf("j is %d\n", foo.j); return 0; } 

  gdb ./a.out
 [...]
 Reading symbols from a.out ... done.
 (gdb) break main
 Breakpoint 1 at 0x40052c: file 6469109.c, line 9.
 (gdb) run
 Starting program: a.out 

 Breakpoint 1, main () at 6469109.c: 9
 9 struct foo foo = {{0}};
 (gdb) watch foo.j
 Hardware watchpoint 2: foo.j
 (gdb) cont
 Continuing
 Hardware watchpoint 2: foo.j

 Old value = -7936
 New value = 0
 0x0000000000400545 in main () at 6469109.c: 9
 9 struct foo foo = {{0}};
 (gdb) cont
 Continuing
 Hardware watchpoint 2: foo.j

 Old value = 0
 New value = 42
 main () at 6469109.c: 14
 14 printf ("j is% d \ n", foo.j);
 (gdb) quit
 A debugging session is active.

         Inferior 1 [process 572] will be killed.

 Quit anyway?  (y or n) y