如何在汇编代码中调用C++函数

Question

我需要从程序集中调用PrintResult来显示结果.我知道我必须在某处使用extrn _PrintResult,我应该使用call _PrintResult调用该函数,但我不确定如何使用它.有什么建议

public _Square

.386

.model flat

.code

_Square proc

mov eax, [esp+4]

imul eax

ret

_Square endp

.............这是我的C++代码.........

#include <iostream>

using namespace std;

enum ResultCode {ShowSquare};
enum SuccessCode {Failure, Success};

extern "C" long Square (long);

void main ()
 {
 long Num1; 
         long Num2;

 do
  {
  cout << "Enter Number to Square" << endl;
  cin >> Num1;
  Result = Square (Num1);
  cout << "Square is: " << Result << endl;
  } while (Result > 0);
 }

void PrintResult (ResultCode PrintCode, long Value) //PrintCode, long Value)
 {
 switch (PrintCode)
  {
  case ShowSquare:
    cout << "Display of square is: " << Value << endl;
    break;

  default:
    cout << "Error in assembly routines" << endl;
  }
}

Answer 1

我通常不喜欢发布完整的代码,但尝试一下:

32位汇编

.386
.model flat
.code

_Square proc
mov eax, [esp+4]
imul eax

push eax ; Save the calculated result

; Call PrintResult here
push eax ; value
push 0 ; ShowSquare
call _PrintResult
add esp, 8 ; Clear the stack

pop eax ; Return the calculated result

ret
_Square endp

C++

#include <iostream>

using namespace std;

enum ResultCode {ShowSquare};
enum SuccessCode {Failure, Success};

extern "C" long Square(long);

int main(int argc, char* argv[])
{
    long Num1, Num2;

    do
    {
        cout << "Enter number to square" << endl;
        cin >> Num1;
        Num2 = Square(Num1);
        cout << "Square returned: " << Num2 << endl;
    }
    while (Num2);

    return 0; 
}

extern "C"
void PrintResult(ResultCode result, long value)
{
    switch (result)
    {
        case ShowSquare:
            cout << "Square is: " << value << endl;
            break;

        default:
            cout << "Error calculating square" << endl;
            break;
    }
}

因为您正在编写C程序,所以默认调用机制是cdecl,这意味着所有参数都在堆栈上传递,返回值被传回eax,然后调用者负责清理堆栈.

因此,为了调用PrintResult,您必须在调用过程之前将所有参数都压入堆栈.在程序返回后,我们必须清理stack(add esp, 8).

因为cdecl调用约定允许eax在调用期间进行修改eax,所以在PrintResult返回时可能不会保留,因此我们在调用PrintResult之前保存计算结果,然后在调用返回后恢复它.

我没有尝试过上面的代码,但我希望它能帮助你走上正确的道路.

注意:因为您使用的是C++编译器,所以extern "C"需要在PrintResult之前.