Question

I compile the following program with gcc and receive an output executable file a.out.:

#include <stdio.h>
int main () {
  printf("hello, world\n");
}

When I execute cat a.out, why is the file in “gibberish” (what is this called?) and not machine language of 0s and 1s:

??????? H__PAGEZERO(__TEXT__text__TEXT?`??__stubs__TEXT 
P__unwind_info__TEXT]P]__eh_frame__TEXT?H??__DATA__program_vars [continued]

Answer 1

The file is in 0 and 1, but when you open it with text editor those bits are grouped in bytes and then treated as text 😉 In Linux you could try to disassemble the output file to ensure that it contains machine instructions (x86 architecture):

objdump -D -mi386 a.out

Example output:

1:  83 ec 08                sub    $0x8,%esp
4:  be 01 00 00 00          mov    $0x1,%esi
9:  bf 00 00 00 00          mov    $0x0,%edi 

The second column contains that 0’s and 1’s in hexadecimal notation and the third column contains mnemonic assembler instructions.

If you want to display those 0’s and 1’s simply type:

xxd -b a.out

Example output:

 0000000: 01111111 01000101 01001100 01000110 00000010 00000001  .ELF..
 0000006: 00000001 00000000 00000000 00000000 00000000 00000000  ......