Question

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; 

Answer 1

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.