c declaring function with array you dont know size code example

Example 1: size of an array c

int a[20];
int length;
length = sizeof(a) / sizeof(int);

Example 2: size of an array c

int a[]= { 1, 2, 3, 4, 5, 6, 7 };
int length = sizeof(a) / sizeof(a[0]); //return 7