CODEWITHSUNDEEP
stringassemblyx86disassembly
Montana Burr
Montana Burr

Reputation: 287

How does this x86 Assembly code create a string?

I'm studying the x86 assembly language. In order to better understand what's going on behind the scenes of string creation, I have a sample program that just prints a string. GCC produced the following Assembly program, and I'm having trouble understanding the compiler's output:

Assembly Code:

Dump of assembler code for function main:
   0x0000000000400596 <+0>: push   %rbp
   0x0000000000400597 <+1>: mov    %rsp,%rbp
   0x000000000040059a <+4>: sub    $0x10,%rsp
   0x000000000040059e <+8>: movq   $0x400668,-0x8(%rbp)
   0x00000000004005a6 <+16>:    mov    -0x8(%rbp),%rax
   0x00000000004005aa <+20>:    mov    %rax,%rsi
=> 0x00000000004005ad <+23>:    mov    $0x400675,%edi
   0x00000000004005b2 <+28>:    mov    $0x0,%eax
   0x00000000004005b7 <+33>:    callq  0x4004a0 <printf@plt>
   0x00000000004005bc <+38>:    mov    $0x0,%eax
   0x00000000004005c1 <+43>:    leaveq 
   0x00000000004005c2 <+44>:    retq

C Code:

#include <stdio.h>
int main()
{
char *me = "abcdefghijkl";
printf("%s",me);
}

At the conceptual level, I understand that the stack pointer is being subtracted to allocate memory on the stack, and then somehow, and this is the part I'm having trouble understanding the mechanics of, the program creates the string.

Can someone please help? Thanks.

Upvotes: 3

Views: 1473

Answers (1)

pcarter
pcarter

Reputation: 1618

It's a lot clearer if you use the -S flag to gcc to create an assembly file for your program (gcc -S asm.c). This generates a asm.s file:

        .file   "asm.c"
        .section        .rodata
.LC0:
        .string "abcdefghijkl"
.LC1:
        .string "%s"
        .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    $16, %rsp
        movq    $.LC0, -8(%rbp)
        movq    -8(%rbp), %rax
        movq    %rax, %rsi
        movl    $.LC1, %edi
        movl    $0, %eax
        call    printf
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-36)"
        .section        .note.GNU-stack,"",@progbits

From this you can see that the string is just some initialized memory in the .rodata section assigned the label .LC0. Changing that memory changes the string.

Upvotes: 4

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