本篇文章为总结使用C语言的api来完成UDP通信的基本功能，如果您对Socket不了解，请先阅读上一篇理论知识：iOS Socket理论知识
如果想学习TCP的，请阅读：iOS Socket之C语言版TCP
如果文章中有任何您认为不正确的或者有疑问的，请联系笔者！
先讲一讲UDP编程，因为比TCP要简单多了。首先，我们需要明白UDP是用户数据报协议，英文名为User Datagram Protocol，它是面向无连接的。
注意：Socket通信一定有要服务端和客户端。
客户端的工作流程：首先调用socket函数创建一个Socket，然后指定服务端的IP地址和端口号，就可以调用sendto将字符串传送给服务器端，并可以调用recvfrom接收服务器端返回的字符串，最后关闭该socket。
笔者这里分成了四步：
第一步：创建socket并配置socket，如服务端ip地址和端口号
第二步：调用sendto发送消息到服务器端
第三步：调用recvfrom接收来自服务器端的消息
第四步：调用close关闭socket
- (void)udpClient {
int clientSocketId;
ssize_t len;
socklen_t addrlen;
struct sockaddr_in client_sockaddr;
char buffer[256] = "Hello, server, how are you?";
// 第一步：创建Socket
clientSocketId = socket(AF_INET, SOCK_DGRAM, 0);
if(clientSocketId < 0) {
NSLog(@"creat client socket fail\n");
return;
}
addrlen = sizeof(struct sockaddr_in);
bzero(&client_sockaddr, addrlen);
client_sockaddr.sin_family = AF_INET;
client_sockaddr.sin_addr.s_addr = inet_addr("192.168.1.107");
client_sockaddr.sin_port = htons(1024);
int count = 10;
do {
bzero(buffer, sizeof(buffer));
sprintf(buffer, "%s", "Hello, server, how are you?");
// 第二步：发送消息到服务端
// 注意:UDP是面向无连接的，因此不用调用connect()
// 将字符串传送给server端
len = sendto(clientSocketId, buffer, sizeof(buffer), 0, (struct sockaddr *)&client_sockaddr, addrlen);
if (len > 0) {
NSLog(@"发送成功");
} else {
NSLog(@"发送失败");
}
// 第三步：接收来自服务端返回的消息
// 接收server端返回的字符串
bzero(buffer, sizeof(buffer));
len = recvfrom(clientSocketId, buffer, sizeof(buffer), 0, (struct sockaddr *)&client_sockaddr, &addrlen);
NSLog(@"receive message from server: %s", buffer);
count--;
} while (count >= 0);
// 第四步：关闭socket
// 由于是面向无连接的，消息发出处就可以了，不用管它收不收得到，发完就可以关闭了
close(clientSocketId);
}2015-12-06 15:38:36.095 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.286 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.286 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.291 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.291 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.296 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.296 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.316 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.317 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.324 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.324 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.328 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.329 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.339 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.339 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.355 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.356 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.366 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.366 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.372 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you? 2015-12-06 15:38:36.373 iOS-Socket-C-Version-Client[9709:4234848] 发送成功 2015-12-06 15:38:36.392 iOS-Socket-C-Version-Client[9709:4234848] receive message from server: Hello, server, how are you?
服务器端的工作流程：首先调用socket函数创建一个套接字，然后调用bind函数将其与本机地址以及一个本地端口号绑定，接收到一个客户端时，服务器显示该客户端的IP地址，并将字串返回给客户端。
笔者这里分成了五步：
第一步：创建socket并配置socket
第二步：调用bind绑定服务器本机ip及端口号
第三步：调用recvfrom接收来自客户端的消息
第四步：调用sendto将接收到服务器端的信息返回给客户端
第四步：调用close关闭socket
- (void)udpServer {
int serverSockerId = -1;
ssize_t len = -1;
socklen_t addrlen;
char buff[1024];
struct sockaddr_in ser_addr;
// 第一步：创建socket
// 注意，第二个参数是SOCK_DGRAM，因为udp是数据报格式的
serverSockerId = socket(AF_INET, SOCK_DGRAM, 0);
if(serverSockerId < 0) {
NSLog(@"Create server socket fail");
return;
}
addrlen = sizeof(struct sockaddr_in);
bzero(&ser_addr, addrlen);
ser_addr.sin_family = AF_INET;
ser_addr.sin_addr.s_addr = htonl(INADDR_ANY);
ser_addr.sin_port = htons(1024);
// 第二步：绑定端口号
if(bind(serverSockerId, (struct sockaddr *)&ser_addr, addrlen) < 0) {
NSLog(@"server connect socket fail");
return;
}
do {
bzero(buff, sizeof(buff));
// 第三步：接收客户端的消息
len = recvfrom(serverSockerId, buff, sizeof(buff), 0, (struct sockaddr *)&ser_addr, &addrlen);
// 显示client端的网络地址
NSLog(@"receive from %s\n", inet_ntoa(ser_addr.sin_addr));
// 显示客户端发来的字符串
NSLog(@"recevce:%s", buff);
// 第四步：将接收到的客户端发来的消息，发回客户端
// 将字串返回给client端
sendto(serverSockerId, buff, len, 0, (struct sockaddr *)&ser_addr, addrlen);
} while (strcmp(buff, "exit") != 0);
// 第五步：关闭socket
close(serverSockerId);
}2015-12-06 15:38:36.268 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.269 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.372 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.372 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.377 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.377 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.382 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.382 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.405 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.405 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.409 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.410 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.414 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.415 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.425 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.426 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.441 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.441 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.452 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.452 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you? 2015-12-06 15:38:36.472 iOS-Socket-C-Version-Server[39130:2473780] receive from 192.168.1.100 2015-12-06 15:38:36.473 iOS-Socket-C-Version-Server[39130:2473780] recevce:Hello, server, how are you?
我们这里打印出了客户端发来的消息，由于上面实现的代码中，只发10次，所以这里只有10条。
小伙伴们，可以到github下载了：iOS-Socket-C-Version
注意：这里面有两个工程，一个是客户端，一个是服务器端。运行时，先运行服务器端，然后再选择客户端。另外，客户端所指定的服务器端的ip地址一定要修改成您本机对应的ip，不然使用笔者这里的ip就会失败。
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