很多socket編程的初學者可能會遇到這樣的問題:如果先ctrl+c結束服務器端程序的話,再次啓動服務器就會出現Address already in use這個錯誤,或者你的程序在正常關閉服務器端socket後還是有這個問題。正如下面的這段簡單的socket程序。
server.c
- #include <sys/types.h>
- #include <sys/socket.h>
- #include <stdio.h>
- #include <netinet/in.h>
- #include <arpa/inet.h>
- #include <unistd.h>
- #include <stdlib.h>
- #define BUFFER_SIZE 40
- int main()
- {
- char buf[BUFFER_SIZE];
- int server_sockfd, client_sockfd;
- int sin_size=sizeof(struct sockaddr_in);
- struct sockaddr_in server_address;
- struct sockaddr_in client_address;
- memset(&server_address,0,sizeof(server_address));
- server_address.sin_family = AF_INET;
- server_address.sin_addr.s_addr = INADDR_ANY;
- server_address.sin_port = htons(12000);
- // 建立服務器端socket
- if((server_sockfd = socket(AF_INET, SOCK_STREAM, 0))<0)
- {
- perror("server_sockfd creation failed");
- exit(EXIT_FAILURE);
- }
- // 將套接字綁定到服務器的網絡地址上
- if((bind(server_sockfd,(struct sockaddr *)&server_address,sizeof(struct sockaddr)))<0)
- {
- perror("server socket bind failed");
- exit(EXIT_FAILURE);
- }
- // 建立監聽隊列
- listen(server_sockfd,5);
- // 等待客戶端連接請求到達
- client_sockfd=accept(server_sockfd,(struct sockaddr *)&client_address,(socklen_t*)&sin_size);
- if(client_sockfd<0)
- {
- perror("accept client socket failed");
- exit(EXIT_FAILURE);
- }
- // 接收客戶端數據
- if(recv(client_sockfd,buf,BUFFER_SIZE,0)<0)
- {
- perror("recv client data failed");
- exit(EXIT_FAILURE);
- }
- printf("receive from client:%s/n",buf);
- // 發送數據到客戶端
- if(send(client_sockfd,"I have received your message.",BUFFER_SIZE,0)<0)
- {
- perror("send failed");
- exit(EXIT_FAILURE);
- }
- close(client_sockfd);
- close(server_sockfd);
- exit(EXIT_SUCCESS);
- }
client.c
- #include <sys/types.h>
- #include <sys/socket.h>
- #include <stdio.h>
- #include <netinet/in.h>
- #include <arpa/inet.h>
- #include <unistd.h>
- #include <stdlib.h>
- #define BUFFER_SIZE 40
- int main()
- {
- char buf[BUFFER_SIZE];
- int client_sockfd;
- int len;
- struct sockaddr_in address;// 服務器端網絡地址結構體
- int result;
- client_sockfd = socket(AF_INET, SOCK_STREAM, 0);// 建立客戶端socket
- address.sin_family = AF_INET;
- address.sin_addr.s_addr = inet_addr("127.0.0.1");
- address.sin_port = htons(12000);
- len = sizeof(address);
- // 與遠程服務器建立連接
- result = connect(client_sockfd, (struct sockaddr *)&address, len);
- if(result<0)
- {
- perror("connect failed");
- exit(EXIT_FAILURE);
- }
- printf("Please input the message:");
- scanf("%s",buf);
- send(client_sockfd,buf,BUFFER_SIZE,0);
- recv(client_sockfd,buf,BUFFER_SIZE,0);
- printf("receive data from server: %s/n",buf);
- close(client_sockfd);
- return 0;
- }
在成功的運行了第一次之後,當你再次啓動服務器端程序時,./server就變得邪惡起來,在bind()這個函數中居然出現了Address already in use這個錯誤。
然後你開始迷惑了,難道是忘記將socket給關閉了,或是關閉socket的順序不對?經過種種猜測與試驗,你發現問題毫無進展......過了一會,當你再次抱着試試看的態度重新在Linux的“黑色終端”中輸入./server時,程序居然運行了,什麼情況?究其原因,是socket選項在搗鬼。下面是IBM官網上對這一情況的具體解釋,參見http://www.ibm.com/developerworks/cn/linux/l-sockpit/。
bind 普遍遭遇的問題是試圖綁定一個已經在使用的端口。該陷阱是也許沒有活動的套接字存在,但仍然禁止綁定端口(bind 返回 EADDRINUSE),它由 TCP 套接字狀態 TIME_WAIT 引起。該狀態在套接字關閉後約保留 2 到 4 分鐘。在 TIME_WAIT 狀態退出之後,套接字被刪除,該地址才能被重新綁定而不出問題。
等待 TIME_WAIT 結束可能是令人惱火的一件事,特別是如果您正在開發一個套接字服務器,就需要停止服務器來做一些改動,然後重啓。幸運的是,有方法可以避開 TIME_WAIT 狀態。可以給套接字應用 SO_REUSEADDR 套接字選項,以便端口可以馬上重用。
考慮清單 3 的例子。在綁定地址之前,我以 SO_REUSEADDR 選項調用 setsockopt。爲了允許地址重用,我設置整型參數(on)爲 1 (不然,可以設爲 0 來禁止地址重用)。
按照IBM的做法,我重新改寫了server.c的代碼。
server.c
- #include <sys/types.h>
- #include <sys/socket.h>
- #include <stdio.h>
- #include <netinet/in.h>
- #include <arpa/inet.h>
- #include <unistd.h>
- #include <stdlib.h>
- #define BUFFER_SIZE 40
- int main()
- {
- char buf[BUFFER_SIZE];
- int server_sockfd, client_sockfd;
- int sin_size=sizeof(struct sockaddr_in);
- struct sockaddr_in server_address;
- struct sockaddr_in client_address;
- memset(&server_address,0,sizeof(server_address));
- server_address.sin_family = AF_INET;
- server_address.sin_addr.s_addr = INADDR_ANY;
- server_address.sin_port = htons(12000);
- // 建立服務器端socket
- if((server_sockfd = socket(AF_INET, SOCK_STREAM, 0))<0)
- {
- perror("server_sockfd creation failed");
- exit(EXIT_FAILURE);
- }
- // 設置套接字選項避免地址使用錯誤
- int on=1;
- if((setsockopt(server_sockfd,SOL_SOCKET,SO_REUSEADDR,&on,sizeof(on)))<0)
- {
- perror("setsockopt failed");
- exit(EXIT_FAILURE);
- }
- // 將套接字綁定到服務器的網絡地址上
- if((bind(server_sockfd,(struct sockaddr *)&server_address,sizeof(struct sockaddr)))<0)
- {
- perror("server socket bind failed");
- exit(EXIT_FAILURE);
- }
- // 建立監聽隊列
- listen(server_sockfd,5);
- // 等待客戶端連接請求到達
- client_sockfd=accept(server_sockfd,(struct sockaddr *)&client_address,(socklen_t*)&sin_size);
- if(client_sockfd<0)
- {
- perror("accept client socket failed");
- exit(EXIT_FAILURE);
- }
- // 接收客戶端數據
- if(recv(client_sockfd,buf,BUFFER_SIZE,0)<0)
- {
- perror("recv client data failed");
- exit(EXIT_FAILURE);
- }
- printf("receive from client:%s/n",buf);
- // 發送數據到客戶端
- if(send(client_sockfd,"I have received your message.",BUFFER_SIZE,0)<0)
- {
- perror("send failed");
- exit(EXIT_FAILURE);
- }
- close(client_sockfd);
- close(server_sockfd);
- exit(EXIT_SUCCESS);
- }
這次,讓我們再次反覆的啓動服務器,盡情的在“黑窗戶”裏面輸入./server ./server ./server ......服務器的程序好像突然間變乖了,呵呵,童鞋們,爲自己的成就慶祝吧!!!