TCP and UDP server client application for message management

For data transmission between client and server, I have defined several different structures for all of message types, they are in the common.h header. The functions used in the code are added in functions.c file.

First, I opened a TCP and UDP socket, I made a bind to the specified address and port, and I used listen for connections. For IO multiplexing, I used poll and added the TCP and UDP sockets to the list of file descriptors. I used poll to wait for new connections and to receive messages from clients.

After in a loop I checked if an event occurred on one of the TCP, UDP or stdin sockets and I have treated each case separately. For TCP sockets, I accepted the connection and added the socket to the list of files descriptors. I've disabled Nagle's algorithm so I can send messages faster. To be able to receive data efficiently, I created two functions receive_data and send_data. send_data first sends the size of the message and then the message, as well as receive_data, first receives the size of the message and then sends the data and check if everything has been sent.

After receiving the tcp data in a buffer, I cast to the corresponding structure and treated each one individual case. To check if a client is already connected, I implemented the function connect_client in which I also displayed the messages for clients that were disconnected and had sf set to 1. If the client does not exist I added the client to the list of clients and updated the list of sockets.

For the UDP sockets, I received the data in a buffer and checked the corresponding structure. I created a convert_message function that converts the received message to the appropriate type and adds them to the tcp_data structure to be sent to customers. Further, in the send_data_to_mathing_clients function, I went through the list of clients and I checked the topic to which they are subscribed if the client was connected and the topic coincided with that of message, send the message to the client. Otherwise, I checked if it has active sf and saved the message in the list of messages for that client.

For the stdin socket I read from the keyboard and checked if the command is exit, if so I freed the memory and I closed the server together with all the clients. Then I received subscribe or unsubscribe messages, for this I implemented two functions, subscribe and unsubscribe in which I added or deleted from the clients topic.

I used message structures over TCP. These are udp_message, tcp_message, tcp_data. Each one was used for a message type. I used a data structure to store the clients (struct client), in which I saved the socket, the id, the topics to which it is subscribed and the messages that must be sent when sf = 1 if the client is not logged in.

In the subscribe.c file, I also opened the necessary socket and made the connection. Likewise, I used poll to wait for messages from the server and from stdin. First I sent a message with the client id. After I implemented the functions subscribe_to_topic and unsubscribe_from_topic in which I sent the messages with the necessary topic to the server. Then I received the messages from the server and displayed them.