Why can't the size of a static array be made variable?

Question

Related but not quite duplicate as it discusses C++:
can we give size of static array a variable

I am definin

Answer 1

You are allocating the array at compile-time, so the compiler has to know the array's size in advance. You have to declare siz as a constant expression before you declare arr, for instance:

#define siz 5

or

enum ESizes
{
    siz = 5
};

Alternatively, if you need to determine its size in run-time, you can allocate it on the heap by using malloc:

static int* arr;
arr = (int*)malloc(siz * sizeof(int))

EDIT: as eddieantonio has mentioned, my answer is valid for C89. In C99 it is allowed to declare arrays of variable size.