In this article, we will learn how to construct C code from x86 assembly for strings. The article is divided into two parts. In the first part, we will write C code that involves strings and analyze the corresponding x86 assembly code. In the second part, we will look into x86 assembly code and try to construct corresponding C code that involves strings.

Let’s write this small piece of C code first.

#include <stdio.h>
int main() {
        char str[2];
        str[0] = 'a';
        str[1] = 'b';
        return 0;
}

Now, let’s compile the code using the GCC compiler and use the objdump command to look into the corresponding x86 assembly code. Please note that the “-M intel” option will show the x86 assembly code in the Intel syntax.

$ gcc strings1.c
$ objdump -M intel -d a.out

The x86 assembly code will look like the following:

0000000000400546 <main>:
  400546:	push   rbp
  400547:	mov    rbp,rsp
  40054a:	sub    rsp,0x10
  40054e:	mov    rax,QWORD PTR fs:0x28
  400555:	00 00 
  400557:	mov    QWORD PTR [rbp-0x8],rax
  40055b:	xor    eax,eax
  40055d:	mov    BYTE PTR [rbp-0x10],0x61
  400561:	mov    BYTE PTR [rbp-0xf],0x62
  400565:	mov    eax,0x0
  40056a:	mov    rdx,QWORD PTR [rbp-0x8]
  40056e:	xor    rdx,QWORD PTR fs:0x28
  400575:	00 00 
  400577:	je     40057e <main+0x38>
  400579:	call   400420 <__stack_chk_fail@plt>
  40057e:	leave  
  40057f:	ret