read() 在套接字编程中没有阻塞 [英] read() is not blocking in socket programming

问题描述

我有一个服务器每 5 秒向客户端发送一次数据.我希望客户端阻止 read() 直到服务器发送一些数据然后打印它.我知道 read () 默认是阻塞的.我的问题是我的客户没有阻止 read().这很奇怪，这似乎不是一个正常的问题.

我的代码在无限循环中打印Nothing come back".我在一台linux机器上，用c编程.我的代码片段如下.请指教.

while(1){n = 读取(sockfd，recvline，MAXLINE)；如果 (n > 0){recvline[n] = 0;if (fputs(recvline, stdout) == EOF)printf("fputs 错误");}否则如果(n == 0)printf("什么都没回来");否则，如果 (n <0)printf("读取错误");}返回;

解决方案

可能有多种原因，不同地方可能有几种异常:

1. 检查您创建的套接字:

   <块引用>

   sockfd=socket(AF_INET,SOCK_STREAM,0);如果 (sockfd==-1) {perror("创建套接字");}

2. 您还可以在使用之前明确启用阻塞模式:

   <块引用>

   //设置socket I/O模式:本例为FIONBIO//启用或禁用阻塞模式//基于 iMode 数值的套接字.//如果 iMode = 0，则启用阻塞；//如果 iMode != 0，则启用非阻塞模式.ioctl(sockfd, FIONBIO, &iMode);

   或者你可以使用 setsockopt 如下:

    struct timeval t;t.tv_sec = 0;tv_usec = 0;套索选择(sockfd,//套接字描述符SOL_SOCKET,//在套接字 API 级别操作选项SO_RCVTIMEO,//指定接收或发送超时const void *(&t),//选项值大小(t));

3. 检查读取函数调用(错误原因)

   <块引用>

   n = read(sockfd, recvline, MAXLINE);如果(n <0){perror("读取错误:");}

4. 另外检查服务器代码！:

   <块引用>

   1. 愿您的服务器发送一些空白(不可打印，空，输入)章程.而你对此一无所知.错误你的服务器代码.

   2. 或者您的服务器在您的客户端可以读取之前终止.

5. 还有一件有趣的事，试着理解:

   <块引用>

   当您在服务器上调用 N write() 时，不需要在另一端调用 N read().

I have a server that sends data to a client every 5 seconds. I want the client to block on read() until the server sends some data and then print it. I know read () is blocking by default. My problem is that my client is not blocking on read(). This is very odd and this does not seem to be a normal issue.

My code prints "Nothing came back" in an infinite loop. I am on a linux machine, programming in c. My code snippet is below. Please advice.

while(1)
{
    n = read(sockfd, recvline, MAXLINE);
    if ( n > 0) 
    {
        recvline[n] = 0;    
        if (fputs(recvline, stdout) == EOF)
            printf("fputs error");
    }
    else if(n == 0)
        printf("Nothing came back");
    else if (n < 0)
        printf("read error");
}
return; 

解决方案

There may be several cause and several exceptions are possible at different place:

1. check socket where you create:

   sockfd=socket(AF_INET,SOCK_STREAM,0);  
   if (sockfd==-1) {
       perror("Create socket");
   }
   

2. You and also enable blocking mode explicitly before use it:

   // Set the socket I/O mode: In this case FIONBIO  
   // enables or disables the blocking mode for the   
   // socket based on the numerical value of iMode.  
   // If iMode = 0, blocking is enabled;   
   // If iMode != 0, non-blocking mode is enabled.
   ioctl(sockfd, FIONBIO, &iMode);  
   

   or you can use setsockopt as below:

    struct timeval t;    
    t.tv_sec = 0;
    tv_usec = 0;
    setsockopt(
         sockfd,     // Socket descriptor
         SOL_SOCKET, // To manipulate options at the sockets API level
         SO_RCVTIMEO,// Specify the receiving or sending timeouts 
         const void *(&t), // option values
         sizeof(t) 
     );   
   

3. Check Read function call (Reason of bug)

   n = read(sockfd, recvline, MAXLINE);
   if(n < 0){  
       perror("Read Error:");
   }  
   

4. Also check server code!:

   1. May your server send some blank(non-printable, null, enter) charter(s). And your are unaware of this. Bug you server code too.

   2. Or your server terminated before your client can read.

5. One more interesting thing, Try to understand:

   When you call N write() at server its not necessary there should be N read() call at other side.

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