Question

Here is the program I am trying to debug:

#include <stdio.h>
int i = 5;

int main(void)
{
    int x = 3;

    display(x);
    return 0;
}


void display(int x)
{
for ( i=0; i<x; ++i ) {
    printf("i is %d.\n", i);
}
}

This code is coming from here http://www.dirac.org/linux/gdb/05-Stepping_And_Resuming.php#breakpointsandwatchpoints. Here is the problem:

(gdb) break display 
Breakpoint 1 at 0x40051e: file try5.c, line 15.
(gdb) run
Starting program: /home/ja/gdb/learning/try5 

Breakpoint 1, display (x=3) at try5.c:15
(gdb) frame 1
#1  0x000000000040050c in main () at try5.c:8
(gdb) break 
Breakpoint 2 at 0x40050c: file try5.c, line 8.
(gdb) c
Continuing.
i is 0.
i is 1.
i is 2.

Breakpoint 2, main () at try5.c:9
(gdb) i b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x000000000040051e in display at try5.c:15
    breakpoint already hit 1 time
2       breakpoint     keep y   0x000000000040050c in main at try5.c:8
    breakpoint already hit 1 time
(gdb) c
Continuing.

Program exited normally.
(gdb) q

Debugger finished

It was supposed to stop at line 8 in main() but it stopped at line 9 it main(). For me it’s misleading. I think it should stop at line 9, because this is what ‘break’ commands does – sets a break point at the very next instruction. But why “info breakpoints” said that the break point was set at line 8?

Answer 1

The breakpoint is placed at the right place, as you saw, because it did break after returning from the function. If you do a disassemble, you’ll also see that the breakpoint is placed at the right instruction (in this example at 0x00401192):

b display
r
f 1
b
disassemble $pc
...
   0x0040118d <+29>:    call   0x401199 <display>
=> 0x00401192 <+34>:    mov    $0x0,%eax
   0x00401197 <+39>:    leave

i b
...
2       breakpoint     keep y   0x00401192 in main at try5.c:8

But it shows the wrong line number. First I thought it might be related to function return, so I added extra instructions after the display call, but it still shows the wrong line.

This looks a bug to me.