Code? Ftp is generally not done by writing code; there are plenty of existing applications that can do file transfers using ftp. The most common application is (appropriately named) ftp. It has enough built-in help that most people can make it work, but the man page has enough to fill in the gaps.
If you are trying to write an ftp server &/or ftp client, then I would ask two questions: why?, and what language? But mostly just, why? In fact these days, there is little good reason to use the ftp protocol at all. sftp is better in virtually every way. Same for scp.
If you really want code, I suggest tracking down the open sources for the standard ftp that has been part of Unix/Linux since before there was dirt. Shouldn't be hard to find.