How to declare global arrays of unknown length?

I want to declare a global array of length p, which will be determined by the main function depending on the particular case. Is this possible? When I process it, the complier says:

variable-size type declared outside of any function

Any way round this?

I guess you could make an array with malloc and then realloc it to get the lenght you wish... (not sure if it's a good way though)

Global variables are those variables which will get initialized during compilation itself. Therefore you cannot dynamically allocate memory for global arrays. I think.
I dont know why you need this. Can you please tell me why you need this (In fact I never heard of question like this).

I have a maths problem to solve and it consists of two parts, 1) calculating a list of prime numbers which I am storing in an array, 2) using those prime numbers in a recursion formula with some starting values.

So instead of having to send an array of prime numbers of unknown length to a function, I have declared a global array to store them in, which my function to do the recursion can read.

Does that make sense?