与TCP编程不同的是：

• 无需建立连接，在recvfrom()阻塞等待客户端的数据，收到数据后进入do something进行数据的处理。

1、sendto()，recvfrom()

• ssize_t sendto(int socket, void *message, size_t length, int flags, struct sockaddr *dest_addr, socklen_t dest_len);

  • struct sockaddr *dest_addr：接收数据方的IP地址和端口号
  • socklen_t *address_len：存放地址长度的变量

• size_t recvfrom(int socket, void *buffer, size_t length, int flags, struct sockaddr *dest_addr, socklen_t *address_len);

  • struct sockaddr *dest_addr：发送数据方的IP地址和端口号
  • socklen_t *address_len：存放地址长度的变量

• 这两个函数一般在使用UDP协议时使用

代码举例：net.h

#ifndef __MAKEU_NET_H__
#define __MAKEU_NET_H__#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <strings.h>
#include <sys/types.h>                  /* See NOTES */
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>                 /* superset of previous */#define SERV_PORT 5001
#define SERV_IP_ADDR "192.168.7.246"#define QUIT_STR "quit"#endif

server.c

#include "net.h"int main(void)
{int fd = -1;struct sockaddr_in sin;/* 1. 创建socket fd */if ((fd = socket (AF_INET, SOCK_DGRAM, 0)) < 0) { //udp程序perror ("socket");exit (1);}/* 2. 允许绑定地址快速重用 */int b_reuse = 1;setsockopt (fd, SOL_SOCKET, SO_REUSEADDR, &b_reuse, sizeof (int));/*2. 绑定 *//*2.1 填充struct sockaddr_in结构体变量 */bzero (&sin, sizeof (sin));sin.sin_family = AF_INET;sin.sin_port = htons (SERV_PORT);       //网络字节序的端口号/* 让服务器程序能绑定在任意的IP上 */
#if 1sin.sin_addr.s_addr = htonl (INADDR_ANY);
#elseif (inet_pton (AF_INET, SERV_IP_ADDR, (void *) &sin.sin_addr) != 1) {perror ("inet_pton");exit (1);}
#endif/*2.2 绑定 */if (bind (fd, (struct sockaddr *) &sin, sizeof (sin)) < 0) {perror ("bind");exit (1);}char buf[BUFSIZ];struct sockaddr_in cin;socklen_t addrlen = sizeof(cin);printf("\nUDP server started!\n");while(1) {bzero(buf, BUFSIZ);if( recvfrom(fd, buf, BUFSIZ-1, 0,(struct sockaddr *)&cin, &addrlen ) < 0) {perror("recvfrom");continue;}char ipv4_addr[16];if (!inet_ntop (AF_INET, (void *) &cin.sin_addr, ipv4_addr, sizeof (cin))) {perror ("inet_ntop");exit (1);}printf("Recived from(%s:%d), data:%s",ipv4_addr, ntohs(cin.sin_port), buf);if (!strncasecmp (buf, QUIT_STR, strlen (QUIT_STR))) {  //用户输入了quit字符printf ("Client(%s:%d) is exiting!\n", ipv4_addr, ntohs(cin.sin_port));}}close(fd);return 0;
}

client.c

/*udp demo */
/* usage:* ./client serv_ip serv_port 
*/
#include "net.h"
void usage(char *s)
{printf("\nThis is udp demo!\n");printf("\nUsage:\n\t %s serv_ip serv_port",s);printf("\n\t serv_ip: udp server ip address");printf("\n\t serv_port: udp server port(serv_port > 5000)\n\n");
}int main(int argc, char *argv[])
{int fd = -1;int port = SERV_PORT;port = atoi(argv[2]);if(port < 0 || (port >0 && port <= 5000)) {usage(argv[0]);exit(1);}struct sockaddr_in sin;if(argc !=3) {usage(argv[0]);exit(1);}        /* 1. 创建socket fd*/if( (fd = socket(AF_INET,SOCK_DGRAM, 0)) < 0) { //UDP编程perror("socket");exit(1);}/*2.1 填充struct sockaddr_in结构体变量 */bzero(&sin,sizeof(sin));sin.sin_family = AF_INET;sin.sin_port = htons(SERV_PORT); //网络字节序的端口号
#if 0sin.sin_addr.s_addr = inet_addr(argv[1]);
#elseif( inet_pton(AF_INET, argv[1], (void *)&sin.sin_addr) != 1) {perror("inet_pton");exit(1);}
#endif	printf("UDP client started!\n");char buf[BUFSIZ];while(1) {fprintf(stderr,"pls input string:");bzero(buf, BUFSIZ);if( fgets(buf, BUFSIZ-1, stdin) ==NULL) {perror("fgets");continue;}sendto(fd, buf, strlen(buf), 0, (struct sockaddr *)&sin, sizeof(sin)); if( !strncasecmp(buf, QUIT_STR, strlen(QUIT_STR))) {  //用户输入了quit字符printf("Client is exited!\n");break;}}close(fd);return 0;
}