Compiled C Lab

Server used in this lab is Aarch64
$ ssh

Step 1 Write a basic C program which prints a message on the screen
#include <stdio.h>

int main() {
      printf ("Hello World!\n");

Step 2 Compile the program using the GCC compiler
$ gcc hello.c -o hello0 -g -O0 -fno-builtin && ll -h

-g # enable debugging information
-O0 # do not optimize (that's a capital letter and then the digit zero)
-fno-builtin # do not use builtin function optimization
ll -h # displays detail info of all files/directories  with names starting with letter h

Step 3 Using objdump
$ objdump hello0 -f -s -d | less

-f # display header information for the entire file
-s # display per-section summary information
-d # disassemble sections containing code
less # enable pageup and pagedown

Find the answers to these questions:
(i) Which section contains the code you wrote?   .text?
(ii) Which section contains the string to be printed?  .rodata

Step 4 Recompile the code with these changes
0: The original one

0000000000400594 <main>:
  400594:       a9bf7bfd         stp      x29, x30, [sp, #-16]!
  400598:       910003fd        mov     x29, sp
  40059c:       90000000        adrp    x0, 400000 <_init-0x418>
  4005a0:       9119c000        add     x0, x0, #0x670
  4005a4:       97ffffb7           bl        400480 <printf@plt>
  4005a8:       52800000        mov     w0, #0x0                        // #0
  4005ac:       a8c17bfd         ldp       x29, x30, [sp], #16
  4005b0:       d65f03c0         ret
  4005b4:       00000000        .inst     0x00000000 ; undefined

[explanation] This explanation below is written by Ryan Marzec. Click here to find the original blog.

push %rbp(register base pointer, start of stack) onto the stack
move data from %rsp(register stack pointer, current location in stack) to %rbp
– this is setting the current location of the stack the base, or start of the stack.
move $0x402010 (this is the address of the string containing “Hello World!\n”) to %edi (register destination index, destination of data copies)
move %0x0 (the value of zero) into %eax(register z extended)
– eax is basically storing a function return type
call the printf function.
– @plt here is for Procedure Linking Table
– <printf@plt> is essentially a small stub that will call the real printf function from an external shared library
this move is for the return for main
pop %rbp from the stack

1: Add the compiler option -static
$ gcc hello.c -o hello1 -g -O0 -fno-builtin -static && ll -h
This option disables the use of dynamic libraries, so what is compiled is generally large and doesn't need Dynamic connection library when run.

contain lots of things like every letter from alphebta ,every month... 
00000000004005e4 <main>:
  4005e4:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
  4005e8:       910003fd        mov     x29, sp

  4005ec:       f0000260        adrp    x0, 44f000 <free_mem+0x10>
  4005f0:       910f0000        add     x0, x0, #0x3c0
  4005f4:       9400182b        bl      4066a0 <_IO_printf>
  4005f8:       52800000        mov     w0, #0x0                        // #0
  4005fc:       a8c17bfd        ldp     x29, x30, [sp], #16
  400600:       d65f03c0        ret
  400604:       00000000        .inst   0x00000000 ; undefined


2: Remove the compiler option -fno-builtin
$ gcc hello.c -o hello2 -g -O0 && ll -h

Contents of section .debug_str:
0210 6465005f 494f5f72 6561645f 62617365    de._IO_read_base
 0220 00737973 5f6e6572 72005f76 7461626c  .sys_nerr._vtabl
 0230 655f6f66 66736574 005f494f 5f736176     e_offset._IO_sav
 0240 655f6261 73650073 79735f65 72726c69   e_base.sys_errli
 0250 7374005f 66696c65 6e6f0068 656c6c6f     st._fileno.hello
 0260 2e630073 74646f75 74005f49 4f5f325f      .c.stdout._IO_2_
 0270 315f7374 646f7574 5f005f49 4f5f6c6f        1_stdout_._IO_lo
 0280 636b5f74 00                                                 ck_t.

3: Remove the compiler option -g
$ gcc hello.c -o hello3 -O0 -fno-builtin && ll -h

no debug section

4: Add additional argument to the printf() function in your program

#include <stdio.h>

int main() {
      printf ("Hello World! %d\n", 18);

Contents of section .eh_frame_hdr:
Contents of section .debug_str:
new line: 40059c:       52800241        mov     w1, #0x12                       // #18
4005b4:       00000000        .inst   0x00000000 ; undefined

5: Move the printf() call to a separate function named output(), and call that function from main()
#include <stdio.h>

void message(char str[]) {

int main() {
      message("Hello World!\n");

Contents of section .text:
Contents of section .debug_info:

0000000000400594 <message>:
  400594:       a9be7bfd        stp     x29, x30, [sp, #-32]!
  400598:       910003fd        mov     x29, sp
  40059c:       f9000fe0        str     x0, [sp, #24]
  4005a0:       f9400fe0        ldr     x0, [sp, #24]
  4005a4:       97ffffb7         bl      400480 <printf@plt>
  4005a8:       d503201f        nop
  4005ac:       a8c27bfd        ldp     x29, x30, [sp], #32
  4005b0:       d65f03c0        ret

00000000004005b4 <main>:
  4005b4:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
  4005b8:       910003fd        mov     x29, sp
  4005bc:       90000000        adrp    x0, 400000 <_init-0x418>
  4005c0:       911a4000        add     x0, x0, #0x690
  4005c4:       97fffff4            bl      400594 <message>
  4005c8:       52800000        mov     w0, #0x0                        // #0
  4005cc:       a8c17bfd        ldp     x29, x30, [sp], #16
  4005d0:       d65f03c0        ret
  4005d4:       00000000        .inst   0x00000000 ; undefined

6: Remove -O0 and add -O3 to the gcc options
$ gcc hello.c -o hello0 -g -O3 -fno-builtin && ll -h
-O3  optimized, including the inline function.

Contents of section .eh_frame_hdr:
Contents of section .debug_info:
Contents of section .debug_str:
Contents of section .debug_ranges:
 0000 90044000 00000000 b0044000 00000000  ..@.......@.....
 0010 00000000 00000000 00000000 00000000  ................
<main> part 1st position


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