UDP 概述

UDP 客户端方法

有两种方式构建 UDP 客户端:

使用 connect(), send()/recv()

在 TCP 类型的套接字上调用 connect(), 会通过网络向服务器发送数据包进行握手.
在 UDP 类型的套接字上调用 connect(), 只是将地址保存在本地.
调用 send() 会将数据发送至 connect() 指定的地址.
调用 recv() 只接收来自 connect() 指定地址的数据.

使用 sendto()/recvfrom()

调用 sendto() 会将数据发送至函数参数指定的地址.
调用 recvfrom() 会返回所有收到的数据, 无论发送者是谁.

UDP 服务器方法

TCP, 每个套接字服务一条连接.
UDP, 一个套接字可以和任意数量的节点通信.
TCP 服务器需要调用 listen() 监听新连接, 调用 accept() 建立新连接.
UDP 则不用, 绑定本地地址后, 就可以立即开始发送和接受数据.
TCP 服务器使用 select() 就像是在监测多个独立的套接字, 而 UDP 只需监测一个.
如果程序同时使用 TCP 和 UDP 套接字, 监测这些套接字只需调用一次 select().

一个简单的 UDP 客户端

/* sendto.udp.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include "network.h"
int main(int argc, char ** argv) {
printf("Configuring remote address ...\n");
struct addrinfo info_addr;
memset(&info_addr, 0, sizeof(info_addr));
info_addr.ai_family = AF_INET;
info_addr.ai_socktype = SOCK_DGRAM;
struct addrinfo * addr_peer;
if (getaddrinfo("127.0.0.1", "8080", &info_addr, &addr_peer)) {
fprintf(stderr, "getaddrinfo() failed, errno: %d, %s\n", errno, strerror(errno));
return EXIT_FAILURE;
}
char addr_buff[100];
char serv_buff[100];
getnameinfo(
addr_peer->ai_addr, addr_peer->ai_addrlen,
addr_buff, sizeof(addr_buff),
serv_buff, sizeof(serv_buff),
NI_NUMERICHOST | NI_NUMERICSERV
);
printf("Remote address: %s %s\n", addr_buff, serv_buff);
printf("Creating socket ...\n");
int sock_peer = socket(
addr_peer->ai_family, addr_peer->ai_socktype, addr_peer->ai_protocol
);
if (sock_peer < 0) {
fprintf(stderr, "socket() failed, errno: %d, %s\n", errno, strerror(errno));
return EXIT_FAILURE;
}
const char * hello_msg = "Hello World";
printf("Sending: %s\n", hello_msg);
int bytes_sent = sendto(
sock_peer,
hello_msg, strlen(hello_msg),
0,
addr_peer->ai_addr, addr_peer->ai_addrlen
);
printf("Sent %d bytes ...\n", bytes_sent);
freeaddrinfo(addr_peer);
close(sock_peer);
printf("Finished ...\n");
return EXIT_SUCCESS;
}

调用 sendto(), 向 127.0.0.1:8080 发送字符串 "Hello World".
无论 127.0.0.1:8080 是否有服务在运行, sendto.udp 都产生相同的输出.
即是说, sendto() 函数并不关心数据包是否真的被传输.

将字符串字符转换成大写形式(UDP 版本)

/* toupper.server.udp.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <stdbool.h>
#include "network.h"
int main(int argc, char ** argv) {
printf("Configuring local address ...\n");
struct addrinfo info_local;
memset(&info_local, 0, sizeof(info_local));
info_local.ai_family = AF_INET6;
info_local.ai_socktype = SOCK_DGRAM;
info_local.ai_flags = AI_PASSIVE;
struct addrinfo * addr_local;
getaddrinfo(NULL, "8080", &info_local, &addr_local);
printf("Creating socket ...\n");
int sock_listen = socket(
addr_local->ai_family, addr_local->ai_socktype, addr_local->ai_protocol
);
if (sock_listen < 0) {
fprintf(stderr, "socket() failed, errno: %d, %s\n", errno, strerror(errno));
return EXIT_FAILURE;
}
printf("Binding socket to local address ...\n");
if (bind(
sock_listen,
addr_local->ai_addr, addr_local->ai_addrlen
)) {
fprintf(stderr, "bind() failed, errno: %d, %s\n", errno, strerror(errno));
return EXIT_FAILURE;
}
freeaddrinfo(addr_local);
fd_set sset;
FD_ZERO(&sset);
FD_SET(sock_listen, &sset);
int max_sock = sock_listen;
int i;
printf("Waiting for clients ...\n");
while (true) {
fd_set reads = sset;
if (select(max_sock+1, &reads, 0, 0, NULL) < 0) {
fprintf(stderr, "select() failed, errno: %d, %s\n", errno, strerror(errno));
return EXIT_FAILURE;
}
if (FD_ISSET(sock_listen, &sset)) {
struct sockaddr_storage addr_client;
socklen_t client_len = sizeof(addr_client);
char data_buff[1024];
int bytes_recv = recvfrom(
sock_listen,
data_buff, sizeof(data_buff),
0,
(struct sockaddr *) &addr_client, &client_len
);
if (bytes_recv < 1) {
fprintf(stderr, "recvfrom() failed, errno: %d, %s\n", errno, strerror(errno));
return EXIT_FAILURE;
}
char addr_buff[100];
char serv_buff[100];
getnameinfo(
(struct sockaddr *) &addr_client, client_len,
addr_buff, sizeof(addr_buff),
serv_buff, sizeof(serv_buff),
NI_NUMERICHOST | NI_NUMERICSERV
);
printf("New client: %s %s\n", addr_buff, serv_buff);
for (i = 0; i < bytes_recv; ++i)
data_buff[i] = toupper(data_buff[i]);
sendto(
sock_listen,
data_buff, bytes_recv,
0,
(struct sockaddr *) &addr_client, client_len
);
}
}
printf("Closing listening socket ...\n");
close(sock_listen);
printf("Finished ...\n");
return EXIT_SUCCESS;
}

《C 语言网络编程实践》(《Hands-On Network Programming with C》)