디스어셈블된 내용중에
lea edi,[ebp-48h]
이러한 부분이 있었는데 []의 의미를 몰랐다가 이제서야 찾아본다.
[]는 C언어의 []와 유사하게 그 변수의 내용을 메모리 번지로 해석해서 그 번지의 내용을 받아오는 연산자이다.
그러니까 ebp의 내용에서 0x48을 뺀 주소를 edi에 저정하라는 의미이다. (LEA)
#include "stdio.h"
int main(int argc, char *argv[])
{
int x = 1;
printf("%d %d %d\n", ++x, x, x++);
return 0;
}
You're running into two issues:
1. Undefined behavior -- you are attempting to modify the value of an
object more than once between sequence points, and the Standard imposes
no requirement on how to handle that behavior. Basically, any
expression of the forms:
i = i++;
j = k++ * k++;
foo(i,i++,--i);
invoke undefined behavior. Read up on the semantics of the "++" and
"--" operators in an *authoritative* reference (the Standard would
obviously be one, but also K&R2 or H&S5); they don't work the
way most
people think they should.
2. Order of evaluation -- AFAIK, there's no requirement that
expressions in a function parameter list be evaluated in any particular
order. At first blush, it *looks* like the arguments were evaluated
from right to left (if x = 1, then x++ evaluates to 1, with the side
effect that x == 2, and ++x evaluates to 3, with the side effect that x
== 3), but given that you've invoked undefined behavior the actual
reason may be something else entirely.
$ gcc -Wall cc_test.c
cc_test.c: In function ‘main’:
cc_test.c:6: warning: operation on ‘x’ may be undefined
cc_test.c:6: warning: operation on ‘x’ may be undefined
이런 경고가 발생한다.
분명 x는 변수인데, 그에 대한 operation이 정의되지 않았다는게 무슨 말인지 모르겠지만,
아무튼 실행을 하면
$ ./a.out
3 3 1
요런 희한한 결과가 나온다.
아무튼, calling convention과도 연관이 있어 보이는데,
c언어 특성상 right-left 로 push 하므로(가장 위에는 왼쪽 값이 오도록)
가장 먼저들어가는, 오른쪽 x++ 은 1이 들어가고
1을 더해준다음, 다음 명령어를 수행하면서(++x) 한번에 2가 증가하게 되고
그럼으로 인해 x, ++x 순으로 3이 들어가는게 아닐까 생각된다.
이런 의미를 가지는데, 굳이 해석하자면, 호출 규약 혹은 관용적인 호출방법 이라고 하면 되려나?
아무튼 문득 Iczelion 의 강좌를 읽다보니 어셈블리 기본 구조에
STDCALL 이라는 용어가 나오고, calling convention 이 나오길래 자세히 읽어 봤더니
'함수를 호출시에 인자를 넘겨주는 데이터를 stack에 넣는 방법'을 의미한다.
.MODEL FLAT,
STDCALL .MODEL
is an assembler directive that specifies memory model of your program.
Under
Win32, there's only on model, FLAT
model. STDCALL tells
MASM about
parameter passing convention. Parameter passing convention specifies
the order
of parameter passing, left-to-right or right-to-left, and also who
will
balance the stack frame after the function call. Under Win16, there are two types of calling
convention,
C and PASCAL C calling
convention passes
parameters from right to left, that is , the rightmost parameter is
pushed first.
The caller is responsible for balancing the stack frame after the
call. For
example, in order to call a function named foo(int first_param, int
second_param,
int third_param) in C calling convention the asm codes will look like
this:
push
[third_param]
; Push the third parameter push [second_param]
; Followed by the second push [first_param]
; And the first call foo add sp, 12
; The caller balances the stack frame
PASCAL
calling convention is the reverse of C calling convention. It passes
parameters
from left to right and the callee is responsible for the stack balancing
after
the call. Win16 adopts PASCAL
convention because it produces smaller codes. C convention is useful
when you
don't know how many parameters will be passed to the function as in the
case of
wsprintf(). In the case of wsprintf(), the function has no way to
determine beforehand
how many parameters will be pushed on the stack, so it cannot do the
stack balancing. STDCALL is
the hybrid of C and PASCAL convention. It passes parameter from right to
left
but the callee is responsible for stack balancing after the call.Win32
platform
use STDCALL
exclusively.
Except in one case: wsprintf(). You must use C calling convention with
wsprintf().
