what is mean by machine level code and how to view it? – Debian

Given a simple C program like

#include <stdio.h>

int main( void )
{
  printf( "This is a testn" );
  return 0;
}

if you want to look at actual machine code (not just the assembler, but the actual opcodes and operands), you have a couple of options:

• Build the file as normal:

  gcc -o simple -std=c11 -pedantic -Wall -Werror simple.c

  Then use the objdump command with the -d option (objdump -d simple), and you’ll get a listing similar to this:

  00000000004004c7 <main>:
    4004c7:   55                      push   %rbp
    4004c8:   48 89 e5                mov    %rsp,%rbp
    4004cb:   bf 60 05 40 00          mov    $0x400560,%edi
    4004d0:   e8 1b ff ff ff          callq  4003f0 
    4004d5:   b8 00 00 00 00          mov    $0x0,%eax
    4004da:   5d                      pop    %rbp
    4004db:   c3                      retq   
    4004dc:   0f 1f 40 00             nopl   0x0(%rax)
    ^         ^                       ^
    |         |                       |
    |         |                       +---- assembler
    |         +---------------------------- machine code
    +-------------------------------------- instruction address
  

• Build the file with the -Wa,-aldh=listing-file option to generate a listing of assembly and machine code:

  gcc -o simple -std=c11 -pedantic -Wall -Werror -Wa,-aldh=simple.lst simple.c

  which will create a listing file similar to this:

  GAS LISTING /tmp/ccAMmy8W.s             page 1
  
  
     1                    .file   "simple.c"
     2                    .text
     3                    .section    .rodata
     4                .LC0:
     5 0000 54686973      .string "This is a test"
     5      20697320 
     5      61207465 
     5      737400
     6                    .text
     7                    .globl  main
     9                main:
    10                .LFB0:
    11                    .cfi_startproc
    12 0000 55            pushq   %rbp
    13                    .cfi_def_cfa_offset 16
    14                    .cfi_offset 6, -16
    15 0001 4889E5        movq    %rsp, %rbp
    16                    .cfi_def_cfa_register 6
    17 0004 BF000000      movl    $.LC0, %edi
    17      00
    18 0009 E8000000      call    puts
    18      00
    19 000e B8000000      movl    $0, %eax
    19      00
    20 0013 5D            popq    %rbp
    21                    .cfi_def_cfa 7, 8
    22 0014 C3            ret
    23                    .cfi_endproc
    24                .LFE0:
    26                    .ident  "GCC: (GNU) 7.3.1 20180712 (Red Hat 7.3.1-6)"
    27                    .section    .note.GNU-stack,"",@progbits
    ^  ^    ^             ^
    |  |    |             |
    |  |    |             +---- assembler
    |  |    +------------------ machine code
    |  +----------------------- instruction offset
    +-------------------------- instruction number
  

  If you build with the -g option, you’ll get the original source code interleaved with the generated output:

  GAS LISTING /tmp/ccJ7L1rJ.s          page 1
  
  
     1                    .file   "simple.c"
     2                    .text
     3                .Ltext0:
     4                    .section    .rodata
     5                .LC0:
     6 0000 54686973      .string "This is a test"
     6      20697320 
     6      61207465 
     6      737400
     7                    .text
     8                    .globl  main
    10                main:
    11                .LFB0:
    12                    .file 1 "simple.c"
     1:simple.c      **** #include <stdio.h>
     2:simple.c      **** 
     3:simple.c      **** int main( void )
     4:simple.c      **** {
    13                    .loc 1 4 0
    14                    .cfi_startproc
    15 0000 55            pushq   %rbp
    16                    .cfi_def_cfa_offset 16
    17                    .cfi_offset 6, -16
    18 0001 4889E5        movq    %rsp, %rbp
    19                    .cfi_def_cfa_register 6
     5:simple.c      ****   printf( "This is a testn" );
    20                    .loc 1 5 0
    21 0004 BF000000      movl    $.LC0, %edi
    21      00
    22 0009 E8000000      call    puts
    22      00
     6:simple.c      ****   return 0;
    23                    .loc 1 6 0
    24 000e B8000000      movl    $0, %eax
    24      00
     7:simple.c      **** }
    25                    .loc 1 7 0
    26 0013 5D            popq    %rbp
    27                    .cfi_def_cfa 7, 8
    28 0014 C3            ret
    29                    .cfi_endproc
    30                .LFE0:
    32                .Letext0:
    33                    .file 2 "/usr/lib/gcc/x86_64-redhat-linux/7/include/stddef.h"
    34                    .file 3 "/usr/include/bits/types.h"
    35                    .file 4 "/usr/include/libio.h"
    36                    .file 5 "/usr/include/stdio.h"