You are using the 'prime_number' like a variable, but it is a function.
int next_prime_number()
{
for(int k; ; k++)
{
if(prime_number(k)) return k;
break;
}
}
Looking at the code, the function devisible() should be renamed to 'prime_number(int z)',
because the function checks if a given number is prime or not.