Objdump hello world call instructions

Question

This simple hello world:

#include <stdio.h>
int main(void) {
  printf("Hello, world!\n");
  printf("Hello, world!\n");
  return 0;
}

Gives the following assembly in objdump:

    /helloworld.c:3
     804842c:       83 ec 0c                sub    $0xc,%esp
     804842f:       68 f0 84 04 08          push   $0x80484f0
     8048434:       e8 b7 fe ff ff          call   80482f0 
     8048439:       83 c4 10                add    $0x10,%esp
    /helloworld.c:4
     804843c:       83 ec 0c                sub    $0xc,%esp
     804843f:       68 f0 84 04 08          push   $0x80484f0
     8048444:       e8 a7 fe ff ff          call   80482f0 
     8048449:       83 c4 10                add    $0x10,%esp

• Why does the same call to puts have different hex codes (b7 vs a7)?
• e8 is the call part, but how is b7 fe ff ff translated to 80482f0?

Answer 1

call rel32 instruction: Call near, displacement relative to next instruction

The opcode for this instruction is E8, followed by the relative offset that is computed by the following equation: destination address - address of next instruction.

In this case, the relative offset of the first call is 80482f0 - 8048439 = FFFFFEB7, and the relative offset of the second call is 80482f0 - 8048449 = FFFFFEA7.