c - 无法理解 read() 系统调用

Question

有一个简单的客户端-服务器程序。

• 服务器已启动 $./server 5000
• 客户端也连接到服务器.. $./client 127.0.0.1 5000
• 客户端输入一个字符串
• 客户端的终端上显示消息“我收到您的消息”，服务器显示客户端输入的字符串

问题是，当我通过执行 $./client 127.0.0.1 5000 运行客户端时。我在服务器端终端中得到了这个。

Before Listen 
After Listen 
Before accept 
After accept 
Before read

然后仅当我在客户端屏幕上输入消息然后按 Enter 时。我得到这个显示..

Here is the message: This is a message

Why is that the code goes into wait state before read() call.. Shouldnt it just read nothing and return 0..

我在概念上有些地方是错误的。请帮助我..

客户端相关代码:-

if (connect(sockfd,&serv_addr,sizeof(serv_addr)) < 0) 
        error("ERROR connecting");
    printf("Please enter the message: ");
    bzero(buffer,256);
    fgets(buffer,255,stdin);
    n = write(sockfd,buffer,strlen(buffer));
    if (n < 0) 
        error("ERROR writing to socket");
    bzero(buffer,256);
    n = read(sockfd,buffer,255);
    printf("%d\n",n);
    if (n < 0) 
        error("ERROR reading from socket");
    printf("%s\n",buffer);

相关服务器代码:-

if (bind(sockfd, (struct sockaddr *) &serv_addr,
                sizeof(serv_addr)) < 0) 
        error("ERROR on binding");
    printf("Before Listen ");
    listen(sockfd,5);
    printf("After Listen ");
    clilen = sizeof(cli_addr);
    printf("Before accept ");
    newsockfd = accept(sockfd, 
            (struct sockaddr *) &cli_addr, 
            &clilen);
    printf("After accept");
    if (newsockfd < 0) 
        error("ERROR on accept");
    bzero(buffer,256);
    printf("Before read");
    n = read(newsockfd,buffer,255);
    if (n < 0) error("ERROR reading from socket");
    printf("Here is the message: %s\n",buffer);
    n = write(newsockfd,"I got your message",18);
    if (n < 0) error("ERROR writing to socket");

Answer 1

默认情况下，套接字被创建为阻塞套接字，并且读取调用将阻塞(等待)直到有数据要读取。