Hello!
I want to create a simple file server and client.
What's the best way to handle the data (and the size of the data) between the sockets?

best way is not simple

Depends on your needs and usage... are you going to have a lot of simultaneous clients? Yes, then I/O Completion Ports. Are you only going to work in a LAN? If yes, UDP instead of TCP might be an option for slightly higher throughput. If you're only going to have very few simultaneous connections, you could even use a thread per connection and blocking sockets.

TransmitFile api efficiently sends files across a socket connection.

TransmitFile has a limit on number of concurrent transfers, though, if you aren't running a server version of NT... and how would you keep a progress indicator updated with TransmitFile?

The SERVER end of a file server doesn't need to show progress information (i think a log with Tranfer Begin and Transfer End time stamps would probably be sufficient), just the clients, which would need to be sent the file size in advance, would show transfer progress.

As for the number of concurrent transfers, a limit on them would probably be best, you'd start bottlenecking on your HD with two or three concurrent reads on larger files.

I would have to agree with the TransmitFile suggestion. It's what i used in a small httpd i made for windows, and performance wise it seems to do the job damn well.

It can be useful enough to be able to check completion status server-side - especially when dealing with large files. But again, it all really comes down to what you need.

TransmitFile has the obvious advantage of making "as few user<>kernel switches as possible", those are sorta expensive. Especially if you have a busy server with lots of clients. If I was coding a httpd server, I'd use it too - but possibly using something else for larger files, to have a bit more control.

TransmitFile does not allow the server to implement byte ranges and hence, the server will not support resuming downloads.

LocoDelAssembly: I think byte ranges could be implemented if you use the OVERLAPPED structure that TransmitFile takes as an arg?

Yes