Free() Function is used to free the memory pointed by the pointer back to the memory heap. This function should be called on a pointer that was used either with "calloc()" or "malloc()", otherwise the function will destroy the memory management making a system to crash.

    void *free(void *ptr);   

#include <stdlib.h>
#include <stdio.h>
int main ()
 {
    int * ptr;
    ptr = (int*) malloc (80*sizeof(int));
    free(ptr);
    return(0);
 }

In the above example the first the memory is allocated using the "malloc()" then using the free() function the memory is freed.