How to use va_start()?

Question

In a function with variable arguments, we initialize an object of type va_list ,'ap' with the function va_start() as:

void va_start(va_list ap, parmN);

I don't understand

1.what type of objects can be passed as parMN(last known parameter). I've done with examples of passing integers, strings with format specifiers, structures etc.

2. How the parMN describes the following optional parameters.

Answer 1

The C standard says that va_start() is actually a macro, not a function, so it can do things a function couldn't. The details are highly implementation dependent but you could imagine that it takes the address of parmN to determine the stack address of the next parameter. There's no need for va_start() to know about the types of the following parameters because that information is passed to va_arg(), which is another macro.