Chapter 10

Page 216

  1. The program doesn’t change rsp after the first instruction pushes rbp, so this instruction doesn’t change the values in any registers. TUI only highlights changes.

    Changing the exit value to 123 gave me Inferior 1 (process 4891) exited with code 0173] when gdb ended execution of the program. A number beginning with 0 in gdb means that it’s in octal: 1738 = 12310

  2. It broke at the first {. I couldn’t see what was going on in either the prologue or the epilogue. gdb only shows the source code, which is in C.

  3. Minimum C program in assembly language.

    # f.s
# Minimum components of a C function, in assembly language.
.intel_syntax noprefix
.text
.globl  f
.type   f, @function
f:
      push    rbp         # save caller's frame pointer
      mov     rbp, rsp    # establish our frame pointer

      mov     eax, 0      # return 0;

      mov     rsp, rbp    # restore stack pointer
      pop     rbp         # restore caller's frame pointer
      ret                 # back to caller

    We also need a header fiile so we can call this function in C.

    /* f.h
 * Returns 0
 */

#ifndef F_H
#define F_H
int f(void);
#endif

    And here’s a simple C function to display the return value.

    /* test_f.c
 * Tests f() function.
 */

#include <stdio.h>
#include "f.h"

int main(void)
{
  int returnValue;
  returnValue = f();
  printf("f returned %i.\n", returnValue);

  return 0;
}

  4. Return ints

    /* test_ints.c
 * Tests three functions that return ints.
 */

#include <stdio.h>
#include "twelve.h"
#include "thirtyFour.h"
#include "fiftySix.h"

int main(void)
{
  int return1, return2, return3;

  return1 = twelve();
  return2 = thirtyFour();
  return3 = fiftySix();
  printf("The returned ints are: %i, %i, and %i.\n",
        return1, return2, return3);

  return 0;
}

    /* twelve.h
 * Returns 12
 */

#ifndef TWELVE_H
#define TWELVE_H
int twelve(void);
#endif

    # twelve.s
# Returns twelve.
        .intel_syntax noprefix
        .text
        .globl  twelve
        .type   twelve, @function
twelve:
        push    rbp         # save caller's frame pointer
        mov     rbp, rsp    # establish our frame pointer

        mov     eax, 12     # return 12;

        mov     rsp, rbp    # restore stack pointer
        pop     rbp         # restore caller's frame pointer
        ret                 # back to caller

    /* thirtyFour.h
 * Returns 34
 */

#ifndef THIRTYFOUR_H
#define THIRTYFOUR_H
int thirtyFour(void);
#endif

    # thirtyFour.s
# Returns 34.
      .intel_syntax noprefix
      .text
      .globl  thirtyFour
      .type   thirtyFour, @function
thirtyFour:
      push    rbp         # save caller's frame pointer
      mov     rbp, rsp    # establish our frame pointer

      mov     eax, 34     # return 34;

      mov     rsp, rbp    # restore stack pointer
      pop     rbp         # restore caller's frame pointer
      ret                 # back to caller

    /* fiftySix.h
 * Returns 56
 */

#ifndef FIFTYSIX_H
#define FIFTYSIX_H
int fiftySix(void);
#endif

    # fiftySix.s
# Returns 56.
        .intel_syntax noprefix
        .text
        .globl  fiftySix
        .type   fiftySix, @function
fiftySix:
        push    rbp         # save caller's frame pointer
        mov     rbp, rsp    # establish our frame pointer

        mov     eax, 56     # return 56;

        mov     rsp, rbp    # restore stack pointer
        pop     rbp         # restore caller's frame pointer
        ret                 # back to caller

  5. Return chars

    /* test_chars.c
 * Tests three functions that return chars.
 */

#include <stdio.h>
#include "exclaim.h"
#include "upperOh.h"
#include "tilde.h"

int main(void)
{
  char return1, return2, return3;

  return1 = exclaim();
  return2 = upperOh();
  return3 = tilde();
  printf("The returned chars are: %c, %c, and %c.\n",
        return1, return2, return3);

  return 0;
}

    /* exclaim.h
 * Returns '!'
 */

#ifndef EXCLAIM_H
#define EXCLAIM_H
char exclaim(void);
#endif

    # exclaim.s
# Returns '!'.
        .intel_syntax noprefix
        .text
        .globl  exclaim
        .type   exclaim, @function
exclaim:
        push    rbp         # save caller's frame pointer
        mov     rbp, rsp    # establish our frame pointer

        mov     eax, '!'    # return '!';

        mov     rsp, rbp    # restore stack pointer
        pop     rbp         # restore caller's frame pointer
        ret                 # back to caller

    /* upperOh.h
 * Returns 'O'
 */

#ifndef UPPEROH_H
#define UPPEROH_H
char upperOh(void);
#endif

    # upperOh.s
# Returns 'O'.
        .intel_syntax noprefix
        .text
        .globl  upperOh
        .type   upperOh, @function
upperOh:
        push    rbp         # save caller's frame pointer
        mov     rbp, rsp    # establish our frame pointer

        mov     eax, 'O'    # return 'O';

        mov     rsp, rbp    # restore stack pointer
        pop     rbp         # restore caller's frame pointer
        ret                 # back to caller

    /* tilde.h
 * Returns '~'
 */

#ifndef TILDE_H
#define TILDE_H
char tilde(void);
#endif


    # tilde.s
# Returns '~'.
        .intel_syntax noprefix
        .text
        .globl  tilde
        .type   tilde, @function
tilde:
        push    rbp         # save caller's frame pointer
        mov     rbp, rsp    # establish our frame pointer

        mov     eax, '~'    # return '~';

        mov     rsp, rbp    # restore stack pointer
        pop     rbp         # restore caller's frame pointer
        ret                 # back to caller