This isn't a question we can really answer: it's going to depend to a large extent on what kind of physical interface you have between the two systems, and what software is running on both.
Android (for example) doesn't support FTP without additional software - I use
FTP Server[
^] and FileZilla, but FTP from C is also possible using libcurl.
But there are so many different ways to connect two devices, and they all require different methods to communicate across them that any answer we did give would almost certainly be inappropriate to exactly what you want to do - and only you know teh details of that.
So sit down, work out what communications interfaces you have available, and see how each of those fits into your task parameters. Then ask a more specific question (if you need to) based on that, and giving us plenty of information!
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