Something

void crivo() {
     int i,j;
     prime[0]=1;
     prime[1]=1;
     for (i=4;i<1000001;i+=2) prime[i]=1;
     for (i=3;i<=1000;i+=2) {
         if (prime[i]==0) {
            for (j=i*i;j<1000001;j+=i) prime[j]=1;
         }
     }
}