Something

#include <stdio.h>

#define INCLUDE_LIMIT 81    /* char limit included for printing */


main(){

    char c;
    char buffer[INCLUDE_LIMIT];
    int i, j;

    for(j = 0;j < 81;j++)
        buffer[j] = 0;
    
    i = 0;
    while((c = getchar()) != EOF){
        i++;
        if(i < 81)
            buffer[i] = c;
        else if(i == 81)
            for(j = 0; j < 81; j++)
                putchar(buffer[j]);
            putchar(c);
        else
            putchar(c);
        if(c == '\n'){
            i = 0;
            for(j = 0;j < 81;j++)
                buffer[j] = 0;
        }
    }

}