objdump gives the same output for object files generated with and without -fPIC

Question

I have two files, a.h and a.cpp:

// a.h
extern "C" void a();

// a.cpp
#include "a.h"
#include <stdio.h>

void a()
{
    printf("a\n");
}

I compiled this both with and without -fPIC, and then objdumped both. Weirdly, I got the same output for both files. For a(), I get this in both cases:

callq 15 <a+0x15>

I also tried to compile object files with -no-pie, still no luck.

Answer 1

Compile your code (or anything) in verbose mode (-v), inspect the output, and you will find:

Configured with:  ... --enable-default-pie ...

which, since GCC 6, means the toolchain is built to compile PIC code and link PIE executables by default.

To insist on a non-PIC compilation, run e.g.

g++ -Wall -c -fno-PIC -o anopic.o a.cpp

And to insist on a PIC compilation, run e.g.

g++ -Wall -c -fPIC -o apic.o a.cpp

Then run:

$ objdump -d anopic.o 

anopic.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <a>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   bf 00 00 00 00          mov    $0x0,%edi
   9:   e8 00 00 00 00          callq  e <a+0xe>
   e:   90                      nop
   f:   5d                      pop    %rbp
  10:   c3                      retq     

and:

$ objdump -d apic.o 

apic.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <a>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # b <a+0xb>
   b:   e8 00 00 00 00          callq  10 <a+0x10>
  10:   90                      nop
  11:   5d                      pop    %rbp
  12:   c3                      retq

and you will see the difference.

You can interleave the relocations with the assembly by:

$ objdump --reloc -d anopic.o 

anopic.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <a>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   bf 00 00 00 00          mov    $0x0,%edi
            5: R_X86_64_32  .rodata
   9:   e8 00 00 00 00          callq  e <a+0xe>
            a: R_X86_64_PC32    puts-0x4
   e:   90                      nop
   f:   5d                      pop    %rbp
  10:   c3                      retq

and:

$ objdump --reloc -d apic.o 

apic.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <a>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # b <a+0xb>
            7: R_X86_64_PC32    .rodata-0x4
   b:   e8 00 00 00 00          callq  10 <a+0x10>
            c: R_X86_64_PLT32   puts-0x4
  10:   90                      nop
  11:   5d                      pop    %rbp
  12:   c3                      retq