how to send a file with multiple threads in a basic FTP project

Now , i am writing a basic FTP to check out the result of my seif-learning. But i get a problem about how to send a file with multiple threads both in server and client. i use the memmap to map the article to the memeriy , but unfortuniturily , i have no idea about it. i need a detial . Right now , i just know cut the article into several pieces , and use different thread to read and send it in server, but i donot know what i should do in client and the basic steps.I hope you can give me some tips . Thanks .FYI: i am using ubuntu in c/c++.