How can I get my client to wait to open a file until my server has made it?

I have a client/server program. My Client process spawns my server process using a `fork()` and `system()` call. The client and the server are meant to communicate to each other through named piped, that is a requirement of my homework. The client and the server are supposed to initially send a message to the other one, through a pipe, to confirm that a connection is made. My issue is that my client is trying to open one of the named pipes before the server has a chance to make it. This doesn't always happen though. Occasionally, it will run fine. Is there some way to synchronize this?

My overall goal is to get a filename and search target from the user, send those to the server through a named pipe. And the server searches for the target and sends info back.

OUTPUT:

Code:

---

    Client: PID 2986 - Running on Tue Sep 30 20:59:28 2014
    Client: PID 2986 - Enter File Name: largeSampleText.txt
    Server PID 2988 - Running on Tue Sep 30 20:59:32 2014
    Server: PID 2988 - Synchronized to Client.//This means the client-to-server fifo is working
    There was an error opening serverToClientfifo//This means client couldn't open the server-to-client fifo
    error is: 2//This means the fifo doesn't exist yet
    Client: PID 2986 - Enter Target:
    s
    Client: PID 2986 - Input File >>>largeSampleText.txt<<<
    Client: PID 2986 - Target >>>s<<<
    Client received unexpected connection message
    Message was: D(Ã*ì4
    Terminating

---

CORRECT OUTPUT:

Code:

---

    Client: PID 2542 - Running on Tue Sep 30 20:58:44 2014
    Client: PID 2542 - Enter File Name: largeSampleText.txt
    Server PID 2557 - Running on Tue Sep 30 20:58:54 2014
    Server: PID 2557 - Synchronized to Client.
    Client: PID 2542 - Enter Target:
    was
    Client: PID 2542 - Input File >>>largeSampleText.txt<<<
    Client: PID 2542 - Target >>>was<<<
    Client: PID 2542 - Synchronized to Server
    //The results of the search follows

---

RELEVANT CLIENT CODE:

Code:

---

  int main()
    {
   
            char requestedFileName[1024];
   
            //Get initial information on the client
            pid_t clientPID = getpid();
            time_t currentTime = time(0);
            char* stringTime= ctime(&currentTime);
            cout << "Client: PID " << clientPID <<" - Running on "
                << stringTime;
   
        //Get file name from user
        cout << "Client: PID " << clientPID <<" - Enter File Name: ";
            cin >> requestedFileName;
           
        //Launch the server process
        int status = 0;
            pid_t childPID;
            if(fork() == 0)
            {       
                    childPID = getpid();
                    status = system("pipedServer");
                    return 0;
            }
   
           
            //Create the client -> server pipe and send connection message
            mkfifo ("clientToServerfifo", 0666);
            mkfifo("serverToClientfifo",0666);
            int fd = open("clientToServerfifo", O_WRONLY);
            char connectionMessage[] = "client attempting to connect";
           
            write(fd, connectionMessage, sizeof(connectionMessage));       
   
            //Connect to fifo as reader
          int fd2, maxBufferSize = 1024;
          char buffer[maxBufferSize];
          char expectedConnectionMessage[]="server attempting to connect";
          fd2 = open("serverToClientfifo", O_RDONLY);
              if(fd2 == -1) {cout <<"There was an error opening serverToClientfifo" <<endl;
                                              cout << "error is: " <<errno <<endl;}
         
          // Recieve connection message
          read(fd2, buffer, maxBufferSize);
             
             
              //Get target search term
            char requestedTarget[1024];
            cout << "Client: PID " << clientPID <<" - Enter Target: " <<endl;
            cin >> requestedTarget;
   
            cout << "Client: PID " << clientPID <<" - Input File " << ">>>"
                    <<requestedFileName<<"<<<"<<endl;
            cout <<"Client: PID " << clientPID <<" - Target " << ">>>"
                << requestedTarget << "<<<" <<  endl;
   
   
        if(strcmp(buffer,expectedConnectionMessage) == 0)
            {
            cout <<"Client: PID " << clientPID <<" - Synchronized to Server" << endl;
            }
            else
            {
                    cout <<"Client received unexpected connection message" << endl;
                    cout << "Message was: " << buffer << endl;
                    cout << "Terminating" << endl;
                    kill(childPID, 0);
                        return 0;
            }
   
         
            //Send file name and search term to server
            write(fd, requestedFileName, sizeof(requestedFileName));
   
        write(fd, requestedTarget, sizeof(requestedTarget));
      //Other code follows

---

RELEVANT SERVER CODE:

Code:

---

  int main()
    {
            //Get basic server information
            pid_t serverPID = getpid();
            time_t currentTime = time(0);
            char* stringTime = ctime(&currentTime);
            cout << "Server PID " << serverPID << " - Running on "
                << stringTime;
   
           
        //Connect to fifo as reader
        int fd, maxBufferSize = 1024;
            char buffer[maxBufferSize];
        char expectedConnectionMessage[]="client attempting to connect";
        fd = open("clientToServerfifo", O_RDONLY);
            if(fd == -1) cout <<"There was an error opening clientToServerfifo" <<endl;
           
        //Recieve connection message
        read(fd, buffer,maxBufferSize);
        if(strcmp(buffer,expectedConnectionMessage) == 0)
              {
                    cout <<"Server: PID " << serverPID << " - Synchronized to Client." <<endl;
              }
        else
            {
            cout << "Server received unexpected connection message " << endl;
                    cout<<"Message: " << buffer <<endl;
            cout << "Terminating" <<endl;
            return 0;
              }
   
           
   
            //Create server -> client fifo and send connection message
            mkfifo ("serverToClientfifo", 0666);
           
            int fd2 = open("serverToClientfifo", O_WRONLY);
            char connectionMessage[] = "server attempting to connect";
           
            write(fd2, connectionMessage, sizeof(connectionMessage));
        //other code follows

---

EDIT: I tried creating both of my fifos in my client and my output changed to this

Code:

---

    Client: PID 3150 - Running on Tue Sep 30 21:23:17 2014
    Client: PID 3150 - Enter File Name: largeSampleText.txt
    Client: PID 3150 - Enter Target:
    Server PID 3153 - Running on Tue Sep 30 21:23:21 2014
    was
    Client: PID 3150 - Input File >>>largeSampleText.txt<<<
    Client: PID 3150 - Target >>>was<<<
    Client: PID 3150 - Synchronized to Server
    Server received unexpected connection message
    Message: was
    Terminating

---

So It appears like the client went ahead and put `requestedFileName` and`requestedTarget`into the pipe and the server read out `requestedTarget` instead of the connection message. Not sure why thoughand the server read out `requestedTarget` instead of the connection message. Not sure why though

I'm not a unix/linux programmer so there may be a better way of doing it on your system - but an 'easy' way would be that if the open failed to then sleep for say a second and then retry the open. Only if the open retries exceeds a limit (eg 3) is an error reported.