How to properly terminate a pthread?

Question

I have a tcp echo server that creates a pthread for each client that connects to it. For each connection, I have a variable nbOfClients that increments. When a client closes its connection, I detect it and decrease the number of clients. However the server keeps thinking that the client it alive and keeps on trying to read/write from the socket. I guessed that it was because of the thread that created the client and I tries to kill the thread with pthread_cancel all to non avail. I want to kill the pthread associated to a certain client that closes its connection. How can I go about it?

Here's my code :

static int nbOfClients = 0;

static  pthread_t tid;

int main (int argc, char *argv[]) {

    int bytes_to_read, arg, listen_sd, new_conn, sockfd, client_len, port;
    struct sockaddr_in server, client_addr;
    char *bp, buf[BUFLEN];
    ssize_t n;


    sockfd = 0;

    switch(argc) {
        case 1:
          port = SERVER_TCP_PORT;   // Use the default port
          break;
        case 2:
          port = atoi(argv[1]); // Get user specified port
          break;
        default:
          fprintf(stderr, "Usage: %s [port]\n", argv[0]);
          exit(1);
    }

    // Create a stream socket
    if ((listen_sd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
        error("Cannot Create Socket!");

    // set SO_REUSEADDR so port can be resused imemediately after exit, i.e., after CTRL-c
    arg = 1;
    if (setsockopt (listen_sd, SOL_SOCKET, SO_REUSEADDR, &arg, sizeof(arg)) == -1)
        error("setsockopt");

    // Bind an address to the socket
    bzero((char *)&server, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = htonl(INADDR_ANY); // Accept connections from any client

    if (bind(listen_sd, (struct sockaddr *)&server, sizeof(server)) == -1)
        error("bind error");

    listen(listen_sd, MAX_CONNECTIONS); ///put a define constant indicating the maximum number of clients #define NB_CLIENTS 3

    while (TRUE) {
        client_len = sizeof(client_addr);
        if ((new_conn = accept(listen_sd, (struct sockaddr *) &client_addr, (socklen_t *)&client_len)) == -1)
          error("accept error");

        if(new_conn > 0) {
            if(nbOfClients < MAX_CONNECTIONS) {
                printf("just here\n");
                printf(">> Initializing remote address: %s\n", inet_ntoa(client_addr.sin_addr));
                nbOfClients++;


                fclose(fp);

                printf("Connections to date: %u \n",nbOfClients);

                printf("make thread\n");
                pthread_create(&tid,NULL,&echo, (void *)new_conn);
                printf("had thread\n");
            }
            else {
                printf("connection limit reached\n");
                if(send(new_conn, "Server full!\n", 13, 0) == -1)
                    perror("send");
                close(new_conn);
            }
        }
    }

    return(0);
}

void * echo(void *arg) {
    char buf[BUFSIZE]; /* message buffer */
    int n, i = 0;

    bzero(buf, BUFSIZE);
    if(send((int)arg, "Welcome!!\n", 20, 0) == -1)
        perror("send");

    detect_closed_connection(arg);

    while(TRUE) {
        n = read((int)arg, buf, BUFSIZE);

        /**read: read input string from the client*/
        if(n < 0) {
            perror("error reading from socket");
        }

        printf("Server received from client, %d bytes: %s\n", n, buf);

        /**write: echo the input string in UPPERCASE back to the client*/

        int len = strlen(buf);
        for(i = 0; buf[i]; i++)
            buf[i] = toupper(buf[i]);

        n = write((int)arg, buf, len);
        if(n < 0) {
            error("ERROR writing to socket");
        }
    }
}

void detect_closed_connection(void * listenSocket) {
    struct pollfd pfd;
    pfd.fd = (int)listenSocket;
    pfd.events = POLLIN | POLLHUP | POLLRDNORM;
    pfd.revents = 0;
    while(pfd.revents == 0) {
        if(poll(&pfd, 1, 100) > 0) {
            // if result > 0, this means that there is either data available on the
            // socket, or the socket has been closed
            char buffer[32];
            if (recv((int)listenSocket, buffer, sizeof(buffer), MSG_PEEK | MSG_DONTWAIT) == 0) {
                // if recv returns zero, that means the connection has been closed:

                nbOfClients--;
                pthread_cancel(tid);

            }
        }
    }
}

Thanks.

Answer 1

You should check read() for returning 0 in the thread servering the client, as read() returns 0 in case the peer (client here) closed the connection.

After this line

n = read((int)arg, buf, BUFSIZE);

add

if (0 == n)
{
  fprintf(stderr, "The client closed the connection.\n");
  break;
}

Just before the thread function leave you could add the statement to decrement the number of running threads.

---

Also be aware that nbOfClients is accessed concurently by all the "client"-threads as well as by the main thread, so accessing it shall be protected, for example by using a mutex.

---

There is another issues, as the call to strlen() on the buffer read expects the buffer to be 0 -terminate, which does not necessarily needs ot be the case, even if you sent 0 -terminated "strings". read() might very well return the "string" the client sent in more then one part. So loop around read() until the 0 -terminator had been received.

---

Do not make the thread end itself by calling pthread_cancel() , use pthread_exit() instead.