GDB: break if variable equal value

Question

I like to make GDB set a break point when a variable equal some value I set, I tried this example:

#include <stdio.h> main() {       int i = 0;      for(i=0;i<7;++i)         printf("%d\n", i);       return 0; } 

Output from GDB:

(gdb) break if ((int)i == 5) No default breakpoint address now. (gdb) run Starting program: /home/SIFE/run  0 1 2 3 4 5 6  Program exited normally. (gdb) 

Like you see, GDB didn't make any break point, is this possible with GDB?

Answer 1

in addition to a watchpoint nested inside a breakpoint you can also set a single breakpoint on the 'filename:line_number' and use a condition. I find it sometimes easier.

(gdb) break iter.c:6 if i == 5 Breakpoint 2 at 0x4004dc: file iter.c, line 6. (gdb) c Continuing. 0 1 2 3 4  Breakpoint 2, main () at iter.c:6 6           printf("%d\n", i); 

If like me you get tired of line numbers changing, you can add a label then set the breakpoint on the label like so:

#include <stdio.h> main() {       int i = 0;      for(i=0;i<7;++i) {        looping:         printf("%d\n", i);      }      return 0; }  (gdb) break main:looping if i == 5 

Answer 2

You can use a watchpoint for this (A breakpoint on data instead of code).

You can start by using watch i.
Then set a condition for it using condition <breakpoint num> i == 5

You can get the breakpoint number by using info watch