How to avoid buffer overflow in C / C ++

The program below allows the occurrence of memory overflow, because it is possible to override the variable zero by placing a "large" value in the variable buffer . How to make a secure program by avoiding buffer overflow ?

#include <stdio.h>

main(){
    char buffer[8];
    int zero = 0;

    gets(buffer);
    puts(buffer);

    if(zero == 0){
        printf("Zero continua sendo zero");
    }else{
        printf("A variavel zero foi modificada");
    }


    return 0;
}