Why does this movq instruction work on linux and not osx?

Question

The following assembly code gives an error when running as on OSX 10.9.4, but works successfully on Linux (Debian 7.6). In particular, the movq instruction does

Answer 1

You are correct about the movq instruction not being able to reference the absolute address. This is in part due to the OS X ABI Mach-O format which uses relocatable addressing for symbols.

A Program that is compiled as a position-independent executable (PIE) generally cannot reference an absolute virtual address in the same manner as movq $_main, %rax does. Instead, Global Offset Tables are called upon, which allow position relative code (PC-rel) and position-independent code (PIC) to extract global symbols at their current absolute address location. Demonstrated below, GOTPCREL(%rip)creates an interpretation of lea rdi, _msg:

PC-rel code referencing it's Global Offset Table:

.globl _main
_main:

    movq     _main@GOTPCREL(%rip), %rax
    sub      $8, %rsp
    mov      $0, %rax
    movq     _msg@GOTPCREL(%rip), %rdi
    call     _printf
    add      $8, %rsp
    ret

.cstring
_msg:

    .ascii  "Hello, world\n"

Mac OS X Mach-O Assembler:

$ as -o test.o test.asm

Apple's version of GCC:

$ gcc -o test.out test.o

Output:

$ ./test.out
Hello, world