2. Stack vs. Heap

Stack size is usually limited, heap is not:

  $ ulimit -a | egrep 'stack|data'
  data seg size           (kbytes, -d) unlimited
  stack size              (kbytes, -s) 8192
  $

main program can allocate on stack and pass to functions,
but functions can not allocate on stack and pass back to main:

  char *f( int n)
  {
    char s[n]; // allocated on stack

    return s; // won't work, space for s goes away when function returns
  }

  char *g( int n)
  {
    char *s = malloc(n); // allocated on heap

    return s; // ok
  }