socket programming using timers

Question

I need help with socket programming in C. I need to write a server-client program in C where two IPs communicate with each other on the sockets created. I was successful doing this. But now I want to add timer to the sockets. I've researched a bit and found out that it can be achieved with select() wherein we give the timeout value. I tried doing it but in vain. In the code shown below, I try to connect to a client and read on the new socket created for 10secs. If the client sends some message within those 10secs, the server should take in that message and send an acknowledgement back. If not, an error message must be given out saying that it timed out.

My server code is as follows:

    #include<stdio.h>
    #include<unistd.h>
    #include<sys/time.h>
    #include<sys/types.h>
    #include<sys/socket.h>
    #include<netinet/in.h>
    #include<stdlib.h>
    void error(char *msg)
    {
        perror(msg);
        exit(1);
    }

    int main(int argc, char *argv[])
    {
        int a=1;
        fd_set readfds,writefds;
        int ready_for_reading,reading;
        struct timeval time_out;
        FD_ZERO(&readfds);
  //    FD_ZERO(&writefds);
        FD_SET(a,&readfds);

        time_out.tv_sec = 10;    // 10 seconds
        time_out.tv_usec = 0;    // 0 milliseconds

  //    ready_for_reading=select(1,&readfds,NULL,NULL,&time_out);


        int sockfd, newsockfd, portno,n,clilen;;
        struct sockaddr_in serv_addr,cli_addr;
        char buffer[256];
        portno=atoi(argv[1]);
        sockfd=socket(AF_INET,SOCK_STREAM,0);
        if(sockfd==-1)
            error("\nError creating socket");
        serv_addr.sin_family=AF_INET;
        serv_addr.sin_port=htons(portno);
        serv_addr.sin_addr.s_addr=INADDR_ANY;

        n=bind(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr));
        if(n<0)
            error("\nError binding socket");
        listen(sockfd,5);
        clilen=sizeof(cli_addr);
        printf("\nAccept call reached");
        newsockfd=accept(sockfd,(struct sockaddr *)&cli_addr,&clilen);
        printf("\nnewSockfd=%d",newsockfd);
        printf("\ntesting value:%d",sizeof(readfds));
        if(newsockfd==-1)
            error("\nError on accept");
    //  FD_SET(a,&readfds);
        ready_for_reading=select(sizeof(readfds)*8,&readfds,NULL,NULL,&time_out);
        printf("\nready_for_reading=%d",ready_for_reading);
        if (ready_for_reading == -1) 
        {
            /* Some error has occured in input */
             printf("Unable to read your input\n");
             return -1;
        }
         else 
        {
                  if (ready_for_reading) 
              {
                     reading = read(newsockfd, buffer, 255);
                     printf("Read, %d bytes from input : %s \n", reading,buffer);
                  } 
              else 
              {
                     printf(" 10 Seconds are over - no data input \n");
             return 0;
                  }
            }

    /*  n=read(newsockfd,buffer,255);
        if(n<0)
            error("\nError in reading client's msg");
    */
        printf("\nClient's msg is:%s\n",buffer);
        n=write(newsockfd,"I got your message",25);
        if(n<0)
            error("\nError on writing");

        return 0;
    }

The select() function always returns '0'. I need help with this. Thanks in advance, Richard.

David Schwartz David Schwartz · Accepted Answer · 2012-09-05T23:23:23

      int a=1;
      fd_set readfds,writefds;
      int ready_for_reading,reading;
      struct timeval time_out;
      FD_ZERO(&readfds);
//    FD_ZERO(&writefds);
      FD_SET(a,&readfds); // a?!

You're selecting on file descriptor 1, stdout. You should be selecting on the file descriptor you got back from accept.

socket programming using timers

2 Answers