Select c: wfds is always turned on, causing block

Question

For some reason FD_ISSET always returns true for &wfds, even when there is nothing to send. Here is the code snippet (same on both client and server). Both client and server get same issue with select saying wfds is on. Shouldn't it only activate when i type a message on my keyboard and press enter?

while (1) {
            //trying select..
            tv.tv_sec = 29;
            tv.tv_usec = 500000;
            FD_ZERO(&rfds);
            FD_ZERO(&wfds);
            FD_SET(new_sockfd, &rfds);
            FD_SET(new_sockfd, &wfds);

            n = select(new_sockfd + 1, &rfds, &wfds, NULL, &tv);

            if (n > 0) {
                if (FD_ISSET(new_sockfd, &rfds)) {

                    while (1) {
                        if ((num = recv(new_sockfd, buffer, 10240, 0)) == -1) {
                            //fprintf(stderr,"Error in receiving message!!\n");
                            perror("recv");
                            exit(1);
                        } else if (num == 0) {
                            printf("Connection closed\n");
                            return 0;
                        }
                        buffer[num] = '\0';
                        printf("Message received: %s\n", buffer);
                        break;
                    }
                }
                //this always returns true on client and host
                if (FD_ISSET(new_sockfd, &wfds)) {

                    while (1) {
                        fgets(buffer, MAXDATASIZE - 1, stdin);
                        if ((send(new_sockfd, buffer, strlen(buffer), 0)) == -1) {
                            fprintf(stderr, "Failure Sending Message\n");
                            close(new_sockfd);
                            exit(1);
                        } else {
                            printf("Message being sent: %s\n", buffer);
                            break;
                        }
                    }
                } 
            }
        }

Answer 1

You probably misunderstood how writefds parameter works for select().

You should set the flag in writefds for your file descriptor before calling select() if and only if you have something to send.

Then select() returns with the flag left set in writefds, when the socket has enough space in the socket buffers to accept data for sending. Then you check for that flag, and realize that the socket is available for sending, and you also know that you have something to send, since originally it was you, who set the flag before calling select(). Therefore you can proceed with sending the data over the socket. Then, if you have sent all data you have, and your to-be-sent buffers are empty, you keep the flag for writefds cleared when next time you call select().