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name Punditsdkoslkdosdkoskdo

How to Using GCC to produce readable assembly?

I was wondering how to use GCC on my C source file to dump a mnemonic version of the machine code so I could see what my code was being compiled into. You can do this with Java but I haven't been able to find a way with GCC.

I am trying to re-write a C method in assembly and seeing how GCC does it would be a big help.

If you compile with debug symbols, you can use objdump to produce a more readable disassembly.

>objdump --help
[...]
-S, --source             Intermix source code with disassembly
-l, --line-numbers       Include line numbers and filenames in output

objdump -drwC -Mintel is nice:

  • -r shows symbol names on relocations (so you'd see puts in the call instruction below)
  • -R shows dynamic-linking relocations / symbol names (useful on shared libraries)
  • -C demangles C++ symbol names
  • -w is "wide" mode: it doesn't line-wrap the machine-code bytes
  • -Mintel: use GAS/binutils MASM-like .intel_syntax noprefix syntax instead of AT&T
  • -S: interleave source lines with disassembly.

You could put something like alias disas="objdump -drwCS -Mintel" in your ~/.bashrc


Example:

> gcc -g -c test.c
> objdump -d -M intel -S test.o

test.o:     file format elf32-i386


Disassembly of section .text:

00000000 <main>:
#include <stdio.h>

int main(void)
{
   0:   55                      push   ebp
   1:   89 e5                   mov    ebp,esp
   3:   83 e4 f0                and    esp,0xfffffff0
   6:   83 ec 10                sub    esp,0x10
    puts("test");
   9:   c7 04 24 00 00 00 00    mov    DWORD PTR [esp],0x0
  10:   e8 fc ff ff ff          call   11 <main+0x11>

    return 0;
  15:   b8 00 00 00 00          mov    eax,0x0
}
  1a:   c9                      leave  
  1b:   c3                      ret
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