问关于recvfrom()疑点的用C语言实现的Ping程序

EN

        // Send
        if (sendto(sockfd, &sendicmp, ICMP_SIZE, 0, (struct sockaddr *) &to, sizeof(to)) == -1) {
            printf("sendto() error \n");
            continue;
        }
        // Receive
        struct timeval timeout = {3, 0};//3s
        setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
        if ((n = recvfrom(sockfd, buf, BUF_SIZE, 0, (struct sockaddr *) &from, &fromlen)) < 0) {

            printf("Time out! \n");
            continue;
        }
        nreceived++;
        if (unpack(buf, n) == -1) {
            printf("unpack() error \n");
        }