Question

I’m trying to develop an application in C to receive a message and send it in TCP. For that, I’m using a 2 threads : the server thread to listen and a client thread to send it.

Here the server and the main function :

void dostuff(int sock)
{
  int n;
  char buffer[256];

  bzero(buffer,256);
  n = read(sock,buffer,255);

  if (n < 0) error("ERROR reading from socket");
  else {
    pthread_t t_tcp_client;
    pthread_create(&t_tcp_client, NULL, tcp_client, buffer);
  }

  n = write(sock,"Message received",18);

  if (n < 0) error("ERROR writing to socket");
}

static void *tcp_server(void *p_data)
{
  int sockfd, newsockfd, pid;
  socklen_t clilen;
  struct sockaddr_in serv_addr, cli_addr;

  sockfd = socket(AF_INET, SOCK_STREAM, 0);

  if (sockfd < 0) error("ERROR opening socket");

  bzero((char *) &serv_addr, sizeof(serv_addr));
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_addr.s_addr = INADDR_ANY;
  serv_addr.sin_port = htons(TCP_PORT);

  if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) error("ERROR on binding");

  listen(sockfd,5);
  clilen = sizeof(cli_addr);

  while (1) {
    newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);

    if (newsockfd < 0) error("ERROR on accept");

    pid = fork();

    if (pid < 0) error("ERROR on fork");

    if (pid == 0) {
      close(sockfd);
      dostuff(newsockfd);
      exit(0);
    } else {
      close(newsockfd);
    }
  }
  close(sockfd);
  return 0;
}


int main (void)
{
  pthread_t t_tcp_server;
  pthread_create(&t_tcp_server, NULL, tcp_server, NULL);

  pthread_join(t_tcp_server, NULL);

  return 0;
}

And the client thread :

static void *tcp_client(void *p_data)
{
  if (p_data != NULL)
  {
    char const *message = p_data;

    printf("Message transmitted : %s\n", message);
  }

  return 0;
}

The problem is that the client thread doesn’t receive the good char, the result is :

Message transmitted : ???~?

I think the problem comes from the line : n = read(sock,buffer,255); but I don’t really understand why.

Thank you for your help

Answer 1

static char buffer[256];

void dostuff(int sock)
{
   int n;

   bzero(buffer,256);
   n = read(sock,buffer,255);
   ...
}

Be careful when using sockets, one read may don’t give you the totality of the message you wait.

In case of structured messages, the protocol you define (you should define it well) have to give a message identifier at first, and you read until you receive sizeof( theSpecificMessage ), theSpecificMessage is identified by the identifier.