/*
 *  stack_pointer.c / ver 0x01 | (C)opyleft 2008 by oozie |  http://blog.ooz.ie/
 *  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *  Illustrates stack movement by reading ESP register on an x86_32 architecture.
 *
 *  The program is considered free software;  you can redistribute and/or modify
 *  it under the terms of the GNU  General Public License version 3 or any later
 *  version, as published by the  Free Software Foundation; If you do modify it, 
 *  please leave the  information  about the author unchanged.  The full text of 
 *  latest GPL licence is available at       http://www.gnu.org/licences/gpl.txt
 */


#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define MAX_FRAMES 0x10
#define MID_MSG1 " -> way to the bottom of the stack (via recursive calls)"
#define MID_MSG2 " <- way back to the top of the stack (via returns)"
char *spaces;

void r(int i) {
  int esp;
  char *space_ptr=spaces;
  asm ( "movl %%esp, %0;"
        : "=r"(esp) );
  
  space_ptr+=i*sizeof(char);

  printf("%s-> calling r(%i), esp = 0x%x\n", space_ptr, i,  esp);
  if (i) r(i-1); 
  else      
     printf("%s%s\n%s%s\n", space_ptr, MID_MSG1, space_ptr, MID_MSG2);

  printf("%s<- returning from r(%i), esp = 0x%x\n", space_ptr, i, esp);

  return;
}

int main(void) {

  /* Allocating a string of spaces and terminating with \0 */
  spaces=(char *)malloc(sizeof(char)*MAX_FRAMES); 
  memset(spaces, ' ', MAX_FRAMES*sizeof(char));
  spaces[MAX_FRAMES]=0;
  
  printf("Starting a recursive call of a function on the top of the stack...\n");
  
  /* Called function will iterate MAX_FRAMES times */
  r(MAX_FRAMES); 

  printf("Last return brought us back to the top of stack stack\n");

  return 0;

}