atexit: causes the function pointed to by func to be called upon normal program termination
//Declaration: int atexit(void (*func)(void));
//Return: returns zero if the function is successfully registered as a termination function and nonzero otherwise.
#include <stdlib.h>
#include <stdio.h>
void done(void);
int main(void)
{ if(atexit(done)){
printf("Error in atexit().");
} return 0;
}