We have already discussed the x64 stack frame in one of our previous articles. In this article, we will learn how to construct C code from assembly code for functions. The article is divided into two parts. In the first part, we will write a small piece of C code and analyze the corresponding x86 assembly code. In the second part of the article, we will look into x86 assembly code and try to construct the corresponding C code that uses functions.

Let’s first write a small piece of C code that uses functions.

#include <stdio.h>
int func() {
        int i = 0;
        return i;
}
int main() {
        int n;
        n = func();
        return 0;
}

Now, let’s compile the above code and generate the corresponding object code.

$ gcc -c functions.c
$ objdump -M intel -d functions.o

Please note that the “-M intel” option will generate the assembly code in the Intel syntax.

The corresponding assembly code will look like the following:

0000000000000000 <func>:
   0:	push   rbp
   1:	mov    rbp,rsp
   4:	mov    DWORD PTR [rbp-0x4],0x0
   b:	mov    eax,DWORD PTR [rbp-0x4]
   e:	pop    rbp
   f:	ret    

0000000000000010 <main>:
  10:	push   rbp
  11:	mov    rbp,rsp
  14:	sub    rsp,0x10
  18:	mov    eax,0x0
  1d:	call   22 <main+0x12>
  22:	mov    DWORD PTR [rbp-0x4],eax
  25:	mov    eax,0x0
  2a:	leave  
  2b:	ret