How to execute finish and then another command from inside commands?

Question

This is a reduced example of the structure of my code:

void increment(int j);

int main()
{
  int i = 0;

  while(1) {
    i = increment(i);
  }

  return 0;
}

int increment(int j)
{
  return j + 1;
}

And here is the corresponding GDB script:

b increment
command 1
finish
print i
continue
end

The problem is that the finish command prevents the commands that come after it (namely print i and continue) to not be called.

Is there a way to tell GDB to print i right after any increment call?

Answer 1

You can apparently work around this bug by wrapping all the commands in a single python invocation e.g.

(gdb) break doSomething
Breakpoint 1 at 0x400478: file iter.c, line 5.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>python gdb.execute("print i"); gdb.execute("finish"); gdb.execute("print i");
>end

Breakpoint 1, doSomething () at iter.c:5
5     while (i < 5)
$1 = 0
main (argc=1, argv=0x7fffffffe178) at iter.c:13
13    return 0;
$2 = 5

edit: a 2nd work around that doesn't require python appears to be defining a new gdb command and running that in commands:

define foo
print *i
set $addrOfI = i
finish
print *$addrOfI
end

break doSomething
commands
foo
end

Answer 2

The problem is that finish seems to stop abort the commands set for the first breakpoint after it.

This is expected behavior: any command that resumes the inferior (being debugged) process (as finish does) also stops the execution of canned command sequence.

Update:

See also this GDB bug report.

Is there a way to tell GDB to print i right after any increment call?

Yes:

1. Diassemble increment routine using disas command. Find ret instruction at the end of it (there will only be one).
2. Set a breakpoint on that instruction, using break *0xNNNNN syntax.

3. Attach a command to that breakpoint:

   command N
    print $rax    # or $eax if you are on 32-bit x86 platform
    continue
   end
   

Voila: you should get values being returned from increment() printed (just before being returned).