Thread: Question about sockets in C

I have a few questions with regards to socket programming over Telnet/TCP in C:

1. When I activate the client to access the program remotely it behaves normally however if I close down the terminal whilst the program is running, the last remote command issued decides to repeat itself until the program is terminated, how do is top this?
2. When my program reaches the socket code i.e. my declared function below i can no longer operate my program manually/locally even if I don't have the client open. I have had a Google with this issue and the top result seems to be to do with the accept socket command blocking access. Note: I have tried to use non blocking sockets but still no luck. Also note that I would like my program to be able to be operated both manually/locally simultaneously even when the remote terminal is activated, and if possible if the program is controlled manually when the remote terminal is up to reflect the change on the remote terminal.
3. And finally, I would like the ability to be able to type a word or phrase to activate a command rather than just a single character. If I enter a word instead of a character I get a multi character error and if I try double quotes it doesn't seem to respond to anything despite successfully compiling. From what I have heard you can use double quotes if you are programming in C++ but not C, however i have heard it is possible to use words to issue commands in C.

I have posted my socket code below, I know the while(1) loop is a issue but this is the only thing currently preventing it from closing the connection. Also note that the remotecommand function is declared in my main program in an infinite loop but when ran none of the code after the declared function is ran despite closing the connection.

Code:

void sigchld_handler(int s)
{
   while(waitpid(-1, NULL, WNOHANG) > 0);
}

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
   if (sa->sa_family == AF_INET) {
      return &(((struct sockaddr_in*)sa)->sin_addr);
   }

   return &(((struct sockaddr_in6*)sa)->sin6_addr);
}



int sockfd, new_fd;  // listen on sock_fd, new connection on new_fd
 int rc;
   struct addrinfo hints, *servinfo, *p;
   struct sockaddr_storage their_addr; // connector's address information
   socklen_t sin_size;
   struct sigaction sa;
   int yes=1;
   char s[INET6_ADDRSTRLEN];
   int rv;
   char recv_client_msg[100];


   memset(&hints, 0, sizeof hints);
   hints.ai_family = AF_UNSPEC;
   hints.ai_socktype = SOCK_STREAM;
   hints.ai_flags = AI_PASSIVE; // use my IP

   if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {
      fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
      return 1;
   }

   // loop throui_pgh all the results and bind to the first we can
   for(p = servinfo; p != NULL; p = p->ai_next) {
      if ((sockfd = socket(p->ai_family, p->ai_socktype,
            p->arotocol)) == -1) {
         perror("server: socket");
         continue;
      }

      if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,
            sizeof(int)) == -1) {
         perror("setsockopt");
         exit(1);
      }

      if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
         close(sockfd);
         perror("server: bind");
         continue;
      }

      break;
   }

   if (p == NULL)  {
      fprintf(stderr, "server: failed to bind\n");
      return 2;
   }

   freeaddrinfo(servinfo); // all done with this structure

   if (listen(sockfd, BACKLOG) == -1) {
      perror("listen");
      exit(1);
   }

   sa.sa_handler = sigchld_handler; // reap all dead processes
   sigemptyset(&sa.sa_mask);
   sa.sa_flags = SA_RESTART;
   if (sigaction(SIGCHLD, &sa, NULL) == -1) {
      perror("sigaction");
      exit(1);
   }

   int RemoteCommand(){  // main accept() loop
      sin_size = sizeof their_addr;
      new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
      if (new_fd == -1) {
         perror("accept");
      return(-1);    
      }
      //fcntl(sockfd, F_SETFL, O_NONBLOCK);
      //fcntl(new_fd, F_SETFL, O_NONBLOCK);

      inet_ntop(their_addr.ss_family,
         get_in_addr((struct sockaddr *)&their_addr),
         s, sizeof s);
      printf("server: got connection from %s\n", s);

      if (!fork()) { // this is the child process
      close(sockfd); // child doesn't need the listener
      if (send(new_fd, "Hello, world!", 13, 0) == -1)
      perror("send");       

   while(1) 
    {
      rc = read(new_fd, recv_client_msg, sizeof(recv_client_msg));
       if(write(new_fd,recv_client_msg, rc) <0 ) { printf("error writing to stream socket\n"); }
       if(recv_client_msg[0]=='1') {commandSwitch(1,1);}
       if(recv_client_msg[0]=='2') {commandSwitch(1,2);}
       if(recv_client_msg[0]=='3') {commandSwitch(2,1);}
       if(recv_client_msg[0]=='4') {commandSwitch(2,2);}   
     }
      close(new_fd);
         exit(0);
      }
      close(new_fd);  // parent doesn't need this
      return(0);
   }   
//end of socket setup 

This is my first use of sockets so please try and explain in detail what you mean should someone have a solution. I have posted this elsewhere and haven't found a solution.

Also may you have a solution for number 2). ?
This is my most important of the 3 questions and is having the greatest impact in letting me progress?

and have you any idea on number 2? I have tried to get rid of the while and avoid the closing of the connection but error messages get printed. i thought that would solve my issue when i added the non blocking sockets but no luck.
I assumed that no infinite while loop and a non blocking socket and the recv msg being in a function within a separate infinite loop would mean that the program would: start running into my infinite loop execute the beginning(which it does) then the declared function to check the connection, then exit the function and carry on the remainder of that infinite loop before looping back to the start again.
However as said above in slightly less detail, even if I get rid of the while loop use NB sockets, the program doesn't advance past the point at which the function is declared even if i haven't opened the client and issued any commands. I'm frankly stumped, spent hours on this, hope I can get a solution soon.


Thanks

In q2 i have something like this , in my main body:

Code:

for(;;) {  //infinite for loop repeat constantly
//code here (execute beginning) 
RemoteCommand();  //declared function above 
// code here for remainder of infinite function
}

What I aim to achieve:
1). when the program starts running the start of the infinite loop executes (does this already).
2). Regardless if I'm accessing my program with the client or not i would still like the infinite for loop to complete so the program can also be operated manually whilst the client is still being accessed.(this doesn't work, when the program reaches the declared function in the for loop even if there is no remote access with the ip, the remainder of the for loop will NEVER execute: it happens even if i remove the while(1) loop from the function).
3). So, if i start the program, i would like the program to operate as normal (as if the declared function wasn't there). Whilst being remotely accessed(accessing the port), I(if possible) would still like it to be operated manually as well. i.e. the function just checks for a connection and regardless if there is or isn't one it still continues the remainder of the for loop, NOT stay trapped inside that function.
4) And one final point: if possible, for example if the above can be achieved, say i'm accessing the system remotely using TCP and someone makes a local change, the system feeds back to me that there has been a change locally.

This is what i need to do for a project i am designing and i can't continue without it.
Thanks

I am using Linux I raspbian Debian on the raspberry pi to control a system without the declared function I can operate it locally (using the code after the declared function at the end of the infinite for loop)

I should add that I have implement the code already that's insufe the while(1) loop within the function into both a non blocking and select example. And on both occasions. No success, it still doesn't escape the function.

I have to enter the commands after the listen and accept for it to work at all. Difference when o try NM non blocking sockets: it behaves the same, doesn't run competeley until remotely accessed and once accessed, it still responds to commands but difference being, whenever I'm not giving it a command it flags up a warning repeatedly.