socket--发送RST

 通过设置SO_LINGER套接字选项导致连接被关闭时在TCP套接字上发送一个RST。

 

/*client*/
int main(int argc, char **argv){
    int        sockfd;
    struct linger     ling;
    struct sockaddr_in    servaddr;
    if (argc != 2){
        printf("usage : tcpcli <IPAddress>");
        return 0;
    }

    sockfd = sockrt(AF_INET, SOCK_STREAM, 0);
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htone(SERV_PORT);
    inet_pton(AF_INET, argv[1], &servaddr.sin_addr);
    connect(sockfd, (SA*)&servaddr, sizeof(servaddr));
    
    ling.l_onoff = 1;
    ling.l_linger = 0;
    setsockopt(sockfd, SOL_SOCKET, SO_LINGER, &ling, sizeof(ling));
    close(sockfd);
    return 0;
        
}

/*server*/
int main(int argc, char **argv){
    int        i, maxi, maxfd, listenfd, connfd, sockfd;
    int        nreadt, client[FD_SETSIZE];
    ssize_t    n;
    fd_set     rset, allset;
    char       line[MAXLINE];
    socklen_t  clilen;
    struct sockaddr_in    cliaddr, servaddr;
    
    listenfd = socket(AF_INT, SOCK_STREAM, 0);
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(SERV_PORT);
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

    bind(listenfd, (SA*)&servaddr, sizeof(servaddr));
    listen(listenfd, LISTENQ);

    maxfd = listenfd;
    maxi = -1;
    for (i = 0; i < FD_SETSIZE; i++){
        client[i] = -1;
    }
    FD_ZERO(&allset);
    FD_SET(listenfd, &allset);

    for (;;){
        rset = allset;
        nready = select(maxfd+1, &rset, NULL, NULL, NULL);
        
        if (FD_ISSET(listenfd, &rset)){
            printf("listening socket readable\n");
            sleep(5);
            clilen = sizeof(cliaddr);
            connfd = accept(listenfd, (SA*)&cliaddr, &clilen);
            for (i = 0; i < FD_SETSIZE; i++){
                if (client[i] < 0){
                    client[i] = connfd;
                    break;
                }
            }
            if (i == FD_SETSIZE)
                return 1; //too many clients
            FD_SET(connfd, &allset);
            if (connfd > maxfd)
                maxfd = connfd;
            if (i > maxi)
                maxi = i;

            if (--nready <= 0)
                continue;
        }
        
        for (i = 0; i <= maxi; i++) {
            if ( (sockfd = client[i]) < 0 )
                continue;
            if (FD_ISSET(sockfd, &rset)) {
                if ( (n = readline(sockfd, line, MAXLINE)) ==0){
                    close(sockfd);
                    FD_CLR(sockfd, &allset);
                    client[i] = -1;
                }
                writen(sockfd, line, n);
                
                if (--nready <= 0)
                    break;
            }
        }
    }
}

考虑如上的客户端-服务器通信程序，流程如下：

• 客户建立一个连接并随后终止它
• select向服务器进程返回可读条件，不过服务器要过一小段时间才调用accept
• 在服务器从select返回到调用accept期间，服务器TCP收到来自客户的RST
• 这个已完成的连接被服务器TCP驱除队列，我们假设队列中没有其它已完成的连接
• 服务器调用accept 但是由于没有任何已完成的连接，服务器于是阻塞