How to disassemble a binary executable in Linux to get the assembly code?

Question

I was told to use a disassembler. Does gcc have anything built in? What is the easiest way to do this?

Answer 1

I don't think gcc has a flag for it, since it's primarily a compiler, but another of the GNU development tools does. objdump takes a -d/--disassemble flag:

$ objdump -d /path/to/binary 

The disassembly looks like this:

080483b4 <main>:  80483b4:   8d 4c 24 04             lea    0x4(%esp),%ecx  80483b8:   83 e4 f0                and    $0xfffffff0,%esp  80483bb:   ff 71 fc                pushl  -0x4(%ecx)  80483be:   55                      push   %ebp  80483bf:   89 e5                   mov    %esp,%ebp  80483c1:   51                      push   %ecx  80483c2:   b8 00 00 00 00          mov    $0x0,%eax  80483c7:   59                      pop    %ecx  80483c8:   5d                      pop    %ebp  80483c9:   8d 61 fc                lea    -0x4(%ecx),%esp  80483cc:   c3                      ret      80483cd:   90                      nop  80483ce:   90                      nop  80483cf:   90                      nop 

Answer 2

An interesting alternative to objdump is gdb. You don't have to run the binary or have debuginfo.

$ gdb -q ./a.out  Reading symbols from ./a.out...(no debugging symbols found)...done. (gdb) info functions  All defined functions:  Non-debugging symbols: 0x00000000004003a8  _init 0x00000000004003e0  __libc_start_main@plt 0x00000000004003f0  __gmon_start__@plt 0x0000000000400400  _start 0x0000000000400430  deregister_tm_clones 0x0000000000400460  register_tm_clones 0x00000000004004a0  __do_global_dtors_aux 0x00000000004004c0  frame_dummy 0x00000000004004f0  fce 0x00000000004004fb  main 0x0000000000400510  __libc_csu_init 0x0000000000400580  __libc_csu_fini 0x0000000000400584  _fini (gdb) disassemble main Dump of assembler code for function main:    0x00000000004004fb <+0>:     push   %rbp    0x00000000004004fc <+1>:     mov    %rsp,%rbp    0x00000000004004ff <+4>:     sub    $0x10,%rsp    0x0000000000400503 <+8>:     callq  0x4004f0 <fce>    0x0000000000400508 <+13>:    mov    %eax,-0x4(%rbp)    0x000000000040050b <+16>:    mov    -0x4(%rbp),%eax    0x000000000040050e <+19>:    leaveq     0x000000000040050f <+20>:    retq    End of assembler dump. (gdb) disassemble fce Dump of assembler code for function fce:    0x00000000004004f0 <+0>:     push   %rbp    0x00000000004004f1 <+1>:     mov    %rsp,%rbp    0x00000000004004f4 <+4>:     mov    $0x2a,%eax    0x00000000004004f9 <+9>:     pop    %rbp    0x00000000004004fa <+10>:    retq    End of assembler dump. (gdb) 

With full debugging info it's even better.

(gdb) disassemble /m main Dump of assembler code for function main: 9       {    0x00000000004004fb <+0>:     push   %rbp    0x00000000004004fc <+1>:     mov    %rsp,%rbp    0x00000000004004ff <+4>:     sub    $0x10,%rsp  10        int x = fce ();    0x0000000000400503 <+8>:     callq  0x4004f0 <fce>    0x0000000000400508 <+13>:    mov    %eax,-0x4(%rbp)  11        return x;    0x000000000040050b <+16>:    mov    -0x4(%rbp),%eax  12      }    0x000000000040050e <+19>:    leaveq     0x000000000040050f <+20>:    retq     End of assembler dump. (gdb) 

objdump has a similar option (-S)