My intention was to implement multiple  file transfer in the Socket File transfer post : 

https://www.arupsarker.com/blog/file-transfer-using-bsd-socket  If I did that, the whole post would be very long. The whole idea is to implement multiple file transfer with BSD socket, by using the previous code of file read/write mechanism at the server and client code. I will try to keep it short.

I have discussed basic socket operation by sending a simple message in the post: https://www.arupsarker.com/blog/file-transfer-using-bsd-socket  . 

Now, with that socket, I am going to send a single file from the client to the server. Primarily, I am gonna send a text file by reading each line from that file to make it simple. Later, we will discuss about sending multiple files or other types. Here are the basic steps:

How Socket Works?

When two network nodes are trying to communicate with each other, at layers 3 and 4, they can use a socket to connect and exchange data using the read/write method. It is a server-client model where the server creates a listener socket and the client tries to connect.

What is a Thread?

• The concept of a "procedure" that runs independently from its main program may best describe a thread.

• To go one step further, imagine a main program (a.out) that contains several procedures. Then imagine all of these procedures being able to be scheduled to run simultaneously and/or independently by the operating system. That would describe a "multi-threaded" program.

History

I have discussed multiple file transfers using the BSD socket in that post :

https://www.arupsarker.com/blog/multiple-file-transfer-using-bsd-socket 

On the server side, multiple connections are established for each file. accept function runs in an infinite while loop. Whenever a new connection comes from the client, it will be handled accordingly. But, if the client creates multiple connections concurrently, the server is unable to handle them.

In this article, I will explain, how concurrency works and how we can implement it.