udp socket通信

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client端：

    #include <stdio.h>      #include <stdlib.h>      #include <errno.h>      #include <string.h>      #include <netdb.h>      #include <sys/types.h>      #include <netinet/in.h>      #include <sys/socket.h>      #include <pthread.h>      #define PORT 7089      #define BUFFER_SIZE 1024      int sockfd,sendbytes,recvbytes;      char buf[BUFFER_SIZE];      char ip[36];      char message[72];      struct hostent *host;      struct sockaddr_in serv_addr;      void *thrd_recv()      {      int addr_len=sizeof(serv_addr);    while(1){          memset(buf , 0, sizeof(buf));      if ((recvbytes = recvfrom(sockfd, buf, BUFFER_SIZE, 0, (struct sockaddr *)&serv_addr, (socklen_t*)&addr_len)) == -1)                 {                    perror("recv");                     exit(1);                 }          printf("Received a message: %s\n", buf);            }      }      int main()      {          pthread_t thread[1];          printf("请输入对方 IP 或者主机名\n");          scanf("%s",&ip);          printf("对方 IP 或者主机名是:%s\n",ip);             if ((host = gethostbyname(ip) )== NULL)              {          perror("gethostbyname");          exit(1);              }      /*创建 socket*/      if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1)              {          perror("socket");          exit(1);              }      /*设置 sockaddr_in 结构体中相关参数*/          serv_addr.sin_family = AF_INET;          serv_addr.sin_port = htons(PORT);          serv_addr.sin_addr = *((struct in_addr *)host->h_addr);          bzero(&(serv_addr.sin_zero), 8);      /*调用 connect 函数主动发起对服务器端的连接*/        while(1){          pthread_create(&thread[0],NULL,thrd_recv,(void*)0);          memset(buf, 0, sizeof(buf));          printf("请输入 message:\n");          scanf("%s",&message);          sprintf(buf, "%s", message);      /*发送消息给服务器端*/      if ((sendbytes = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serv_addr, sizeof(serv_addr))) == -1)              {              perror("sendto");              exit(1);              }            }          close(sockfd);          exit(0);      }

要注意的是：udp客户端连接服务端时，服务端并不知道客户端的端口号，因为客户端的udp端口号是系统自动分配的，所以服务端不能先发送数据给客户端。因此只有当客户端先发送数据给服务端，服务端才能知道客户端的udp端口号。

为了解决此问题，客户端可自己设定udp端口号，也就是监听端口。添加bind函数即可：

    #include <stdio.h>      #include <stdlib.h>      #include <errno.h>      #include <string.h>      #include <netdb.h>      #include <sys/types.h>      #include <netinet/in.h>      #include <sys/socket.h>      #include <pthread.h>      #define PORT 7089      #define BUFFER_SIZE 1024      int sockfd,sendbytes,recvbytes;      char buf[BUFFER_SIZE];      char ip[36];      char message[72];      struct hostent *host;      struct sockaddr_in serv_addr;      struct sockaddr_in serverGet;    void *thrd_recv()      {      int addr_len=sizeof(serv_addr);     int getlen=sizeof(serverGet);    while(1){          memset(buf , 0, sizeof(buf));      if ((recvbytes = recvfrom(sockfd, buf, BUFFER_SIZE, 0, (struct sockaddr *)&serverGet, (socklen_t*)&getlen)) == -1)                 {                    perror("recv");                     exit(1);                 }          printf("Received a message: %s\n", buf);            }      }      int main()      {          pthread_t thread[1];          printf("请输入对方 IP 或者主机名\n");          scanf("%s",&ip);          printf("对方 IP 或者主机名是:%s\n",ip);             if ((host = gethostbyname(ip) )== NULL)              {          perror("gethostbyname");          exit(1);              }      /*创建 socket*/      if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1)              {          perror("socket");          exit(1);              }      /*设置 sockaddr_in 结构体中相关参数*/          serv_addr.sin_family = AF_INET;          serv_addr.sin_port = htons(PORT);          serv_addr.sin_addr = *((struct in_addr *)host->h_addr);          bzero(&(serv_addr.sin_zero), 8);      /*调用 bind 函数绑定本地监听端口，这里是7089*/        int getlen=sizeof(serverGet);         serverGet.sin_family=AF_INET;         serverGet.sin_port=htons(7089);         serverGet.sin_addr.s_addr=INADDR_ANY;            bind(sockfd,(struct sockaddr*)&serverGet,getlen);       while(1){          pthread_create(&thread[0],NULL,thrd_recv,(void*)0);          memset(buf, 0, sizeof(buf));          printf("请输入 message:\n");          scanf("%s",&message);          sprintf(buf, "%s", message);      /*发送消息给服务器端*/      if ((sendbytes = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serv_addr, sizeof(serv_addr))) == -1)              {              perror("sendto");              exit(1);              }            }          close(sockfd);          exit(0);      }

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