如何在变量参数函数中在堆栈上传递参数？

Question

I thought that the function removes the parameters from the stack after it's done, but a function like printf removes a variable number of parameters from the stack when it's called.

How does it know how many parameters to remove from the stack? Is there a secret argument to specify how many arguments are passed?

Thanks

Answer 1

The C calling convention specifies that is the caller and not the callee the one responsible from popping the parameters from the stack. That's why functions with a variable argument list must be cdecl . So,

I thought that the function removes the parameters from the stack after it's done.

That's only true for certain calling conventions, it isn't true for the C calling convention .

How does it know how many parameters to remove from the stack? Is there a secret argument to specify how many arguments are passed?

It doesn't, and no there is no secret argument.

Answer 2

The caller function will clean the stack (in the correct calling convention). The compiler will generate the code for that. The compiler is the one knowing exactly how many arguments you passed on the arguments list, because, well, it compiled it..

Answer 3

The calling code cleans up the stack, and it's up to the called function to correctly determine that there is "enough" arguments have been passed for whatever it wants to do. This doesn't have to be an argument as such, it could be something like this:

int sum(int first, ...)
{
    int s = first;
    int v;
    va_list va;
    va_start(va, first);

    while (v = va_arg(va, int) != -1)
    {
         sum += v;
    }

    va_end(va);
    return sum;
 }



 x = sum(1, 2, 3, -1);
 y = sum(1, 2, 3, 4, 5, 6, 7, 8, 9, -1);        

Link to how many arguments in varargs function