Identifying size of structure arrays

I would like to receive an unknown array of structures as a parameter in a function and identify how many arrays of that structure have been passed? How do I do it?

For e.g,

struct tag1 {
char fld1 [10];
char fld2[10];
};

struct tag1 st[10];

main()
{
func1(st);
}

func1(struct tag1 *ptr)
{
// how do I identify here that there are 10 arrays of this
// structure???
}

Add one more element to the structure to mark it as the end of the array. Then just loop through the array and test for that marker. :-)

Thanks. And yes; this is one of the solutions that I'm considering, the other being having the caller pass another parameter telling me how many arrays are being passed. But, I just want to make sure that there is no other way to handle this situation.