说明：  Linux网络编程用户数据报发送,sockfd,buf,len的意义和read,write一样,分别表示套接字描述符,发送或接收的缓冲区及大小. recvfrom负责从 sockfd接收数据,如果from不是NULL,那么在from里面存储了信息来源的情况,如果对信息的来源不感兴趣, 可以将from和fromlen 设置为NULL.sendto负责向to发送信息.此时在to里面存储了收信息方的详细资料.
(Linux network programming user datagram sockfd, buf, len sense and read, write, respectively, said socket descriptor, send or receive buffer size. Recvfrom responsible for from sockfd receive data, if from is not NULL, then stored in from inside sources of information, if the source of the information is not interested in, you can from and fromlen settings responsible for NULL.sendto to to send a message. storing the received information the details of the party to inside.)