Need to be declared with a return type, e.g. int, and a type for the arguments, e.g. void.
Example function to return fibonacci number:
void fibonacci(int n){
int F_old_old = 0, F_old = 1, F_next = 0;
printf("Step 1:\t%d\n", 0);
printf("Step 2:\t%d\n", 1);
for (int i = 3; i <= n; i++)
{
F_next = F_old_old + F_old;
printf("Step %d:\t%d\n", i, F_next);
F_old_old = F_old;
F_old = F_next;
}
}
Often contain some kind of [[Control Flow]].