Where string data is stored?

Question

I wrote a small c program:

#include <stdio.h>

int main()
{
    char s[] = "Hello, world!";
    printf("%s\n", s);
    return 0; 
}

which compiles to (on my linux machine):

    .file   "hello.c"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    subq    $32, %rsp
    movq    %fs:40, %rax
    movq    %rax, -8(%rbp)
    xorl    %eax, %eax
    movl    $1819043144, -32(%rbp)
    movl    $1998597231, -28(%rbp)
    movl    $1684828783, -24(%rbp)
    movw    $33, -20(%rbp)
    leaq    -32(%rbp), %rax
    movq    %rax, %rdi
    call    puts
    movl    $0, %eax
    movq    -8(%rbp), %rdx
    xorq    %fs:40, %rdx
    je  .L3
    call    __stack_chk_fail
.L3:
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2"
    .section    .note.GNU-stack,"",@progbits

I don't understand the assembly code, but I can't see anywhere the string message. So how the executable know what to print?

Answer 1

It's here:

movl    $1819043144, -32(%rbp) ; 1819043144 = 0x6C6C6548 = "lleH"
movl    $1998597231, -28(%rbp) ; 1998597231 = 0x77202C6F = "w ,o"
movl    $1684828783, -24(%rbp) ; 1684828783 = 0x646C726F = "dlro"
movw    $33, -20(%rbp)         ;         33 =     0x0021 = "\0!"

In this particular case the compiler is generating inline instructions to generate the literal string constant before calling printf. Of course in other situations it may not do this but may instead store a string constant in another section of memory. Bottom line: you can not make any assumptions about how or where the compiler will generate and store string literals.

Answer 2

The string is here:

movl    $1819043144, -32(%rbp)
movl    $1998597231, -28(%rbp)
movl    $1684828783, -24(%rbp)

This copies a bunch of values to the stack. Those values happen to be your string.