Myujth

My question is about client server communication in the following scenario.

A server listens on port 21 and is capable of serving multiple incoming connections simultaneously from clients (C1, C2 and so on ) without blocking them

So when a client C1 connects to server S on the port 21 the server accepts the connection on main thread and creates a new thread TH1 and let it handle the client request while the main thread starts listing to port 21 again to accept new clients.

TH1 sends some data back to client C1,
C1 reveives the data and send more data to the server on the same connection which was establisehd on port 21.

This data arrives at server and is received by thread Th1 while the main the main thread of Server continues to listen on port 21.

My question is how does the further data sent by client C1 on the same connection which was made on port 21 automatically goes to the thread Th1 ? even though the main thread of server is listening on port 21 for new connections ?

You need to provide any package which gets send between client a server with a unique client id. You need a distinguishable hand-shake packet.

When the server receives a hand-shake packet, it starts a thread, gives it the client id from the packet, and continues listening to port 21.

Thread then sends whatever is need, say at port 22 to the client and starts listening on a conditional variable.

When the server's main thread gets a data packet at port 21, it publishes the id and signals the conditional variable. One of the waiting threads will recognize the id and will start working. Alternatively the main thread can created multiple queues and conditional vars, one per thread and wake only a single thread which related to the id. Meanwhile the server continues listening.

Do not forget to kill the thread either at a good-bye packet or at a timeout.