Function prototypes
In C, a function call can be appear before the function which can be defined later in the same file, or in another file, or it can come from a library. e.g.
#include <stdio.h> /* file cs_main.c */
int n = 3, sum, compute_sum(int);
printf("%d\n", n); /* 3 is printed */
printf("%d\n", n); /* 3 is printed */
printf("%d\n", sum); /* 6 is printed */