Bryand and O'Hallaron
                             Chapter 3 Section 4.3
                               Pointer Example

------------------------------------------------------
colosus:~/bando/chap3/sec4>more exchange.c
#include <stdio.h>
int exchange(int *xp, int y);
main() 
{
    int a = 4;
    int b = exchange(&a, 3);
    printf("a = %d, b = %d\n", a, b);
}
int exchange(int *xp, int y)
{
    int x = *xp;
    *xp = y;
    return x;
}
colosus:~/bando/chap3/sec4>gcc exchange.c
colosus:~/bando/chap3/sec4>./a.out
a = 3, b = 4
-------------------------------------------------------
colosus:~/bando/chap3/sec4>gcc -O2 -S exchange.c
colosus:~/bando/chap3/sec4>more exchange.s
        .file   "exchange.c"
        .text
        .p2align 4,,15
.globl exchange
        .type   exchange, @function
exchange:
        pushl   %ebp               # save old frame pointer
        movl    %esp, %ebp         # create new frame pointer
-------------------------------------------------------------
        movl    8(%ebp), %edx      # get xp (an address)
        movl    12(%ebp), %ecx     # get y
        movl    (%edx), %eax       # get x at *xp (and return x)
        movl    %ecx, (%edx)       # store y at *xp
-------------------------------------------------------------
        popl    %ebp               # restore old frame pointer
        ret                        # return value in %eax
        .size   exchange, .-exchange
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "a = %d, b = %d\n"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx      # save value of %esp (%esp+4)
        andl    $-16, %esp         # skip bytes so %esp = 0x???????0
        pushl   -4(%ecx)           # push original value of %esp on stack
        pushl   %ebp               # save old frame pointer
        movl    %esp, %ebp         # create new frame pointer
        pushl   %ecx               # push original %esp+4 on stack
        subl    $36, %esp          # reserve 36 bytes on stack
------------------------------------------------------------------------
        leal    -8(%ebp), %eax     # %eax = address where a is stored
        movl    $4, -8(%ebp)       # a = 4
        movl    $3, 4(%esp)        # constant 3 on the stack
        movl    %eax, (%esp)       # address of a on the stack
        call    exchange           # return address on stack
        movl    %eax, 8(%esp)      # return value on stack
        movl    -8(%ebp), %eax     # value of a in %eax
        movl    $.LC0, (%esp)      # address of string on stack
        movl    %eax, 4(%esp)      # value of a on stack
        call    printf
-----------------------------------------------------------------------
        addl    $36, %esp          # undo "reserve 36 bytes"
        popl    %ecx               # %ecx = original %esp+4
        popl    %ebp               # restore old frame pointer
        leal    -4(%ecx), %esp     # restore original value of %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.1.2 20070502 (Red Hat 4.1.2-12)"
        .section        .note.GNU-stack,"",@progbits
colosus:~/bando/chap3/sec4>
-----------------------------------------------------------------------

When the call exchange instruction is executed, the stack contains

          memory (the stack)             
         
         
         +-----------------+ 
 +------>|                 |<------  40(%esp)   main's frame pointer, &ebp
 |       +-----------------+ 
 |       |                 |<------  36(%esp)
 |       +-----------------+ 
 |   +-->|      0x00000004 |<------  32(%esp) or -8(%ebp)
 |   |   +-----------------+ 
 |   |   |                 |<------  28(%esp)
 |   |   +-----------------+ 
 |   |   |                 |<------  24(%esp)
 |   |   +-----------------+ 
 |   |   |                 |<------  20(%esp)
 |   |   +-----------------+ 
 |   |   |                 |<------  16(%esp)
 |   |   +-----------------+ 
 |   |   |                 |<------  12(%esp)
 |   |   +-----------------+ 
 |   |   |      0x00000003 |<------   8(%esp)
 |   |   +-----------------+ 
 |   +---|         xp = &a |<------   4(%esp
 |       +-----------------+        +-----------------+ 
 |       |  return address |<------ |                 | %esp 
 |       +-----------------+        +-----------------+
 +-------| old %ebp here   |<------  -4(%esp)  <--- new %ebp points here
         +-----------------+ 
         |                 |<------  -8(%esp)
         +-----------------+ 
		 |                 |<------ -12(%esp)
         +-----------------+