了解一些汇编代码

Question

I am trying to learn some assembly code, so I read in some tutorial that the assembly code for

int proc(void)
  {
      int x,y;
       scanf("%x %x", &y, &x);
       return x-y;
   }

is

1   proc:
2     pushl  %ebp
3     movl  %esp, %ebp
4     subl   $40, %esp
5     leal  -4(%ebp), %eax
6     movl  %eax, 8(%esp)
7     leal  -8(%ebp), %eax
8     movl  %eax, 4(%esp)
9     movl  $.LC0, (%esp)
10    call  scanf
 Diagram stack frame at this point
11    movl  -4(%ebp), %eax
12    subl  -8(%ebp), %eax
13    leave
14    ret

If I well understood, the instructions of line 5 to 8 store some addresses that will be used to store the values of scanf 's input. So is it right to say that scanf uses systematically the address %esp plus a certain number of bytes (depending on the sizeof the input) to fetch the address at which is the data will be stored ?

Answer 1

What's happening here is that a stack frame is built up to pass arguments to scanf . subl is used to allocate space for the new stack frame and the movl is used with offsets from the stack pointer, %esp , to write values for the arguments on the freshly allocated stack frame.

A more thorough explanation on x86 calling conventions and cdecl in particular can be found here . Understanding the high-level structure of the stack frame and the cdecl convention will help you make sense of the intent of this code snippet.

Answer 2

Calling convention of scanf is cdecl . It passes its arguments to stack pointed by esp .