11.7.1 All About Stack Frames

GDB needs to understand the stack on which local (automatic) variables are stored. The area of the stack containing all the local variables for a function invocation is known as the stack frame for that function (or colloquially just as the frame). In turn the function that called the function will have its stack frame, and so on back through the chain of functions that have been called.

Almost all architectures have one register dedicated to point to the end of the stack (the stack pointer). Many have a second register which points to the start of the currently active stack frame (the frame pointer). The specific arrangements for an architecture are a key part of the ABI.

A diagram helps to explain this. Here is a simple program to compute factorials:

#include <stdio.h>
int fact (int n)
{
  if (0 == n)
    {
      return 1;
    }
  else
    {
      return n * fact (n - 1);
    }
}

main ()
{
  int i;

  for (i = 0; i < 10; i++)
    {
      int   f = fact (i);
      printf ("%d! = %d\n", i, f);
    }
}

Consider the state of the stack when the code reaches line 6 after the main program has called fact (3). The chain of function calls will be main (), fact (3), fact (2), fact (1) and fact (0).

In this illustration the stack is falling (as used for example by the OpenRISC 1000 ABI). The stack pointer (SP) is at the end of the stack (lowest address) and the frame pointer (FP) is at the highest address in the current stack frame. The following diagram shows how the stack looks.

In each stack frame, offset 0 from the stack pointer is the frame pointer of the previous frame and offset 4 (this is illustrating a 32-bit architecture) from the stack pointer is the return address. Local variables are indexed from the frame pointer, with negative indexes. In the function fact, offset -4 from the frame pointer is the argument n. In the main function, offset -4 from the frame pointer is the local variable i and offset -8 from the frame pointer is the local variable f⁵.

It is very easy to get confused when examining stacks. GDB has terminology it uses rigorously throughout. The stack frame of the function currently executing, or where execution stopped is numbered zero. In this example frame #0 is the stack frame of the call to fact (0). The stack frame of its calling function (fact (1) in this case) is numbered #1 and so on back through the chain of calls.

The main GDB data structure describing frames is struct frame_info. It is not used directly, but only via its accessor functions. frame_info includes information about the registers in the frame and a pointer to the code of the function with which the frame is associated. The entire stack is represented as a linked list of frame_info structs.