If I write a server, how can I implement the receive function to get all the data sent by a specific client if I don't know how that client sends the data?
I am using a TCP/IP protocol.
If you really have no protocol defined, then all you can do is accept groups of bytes from the client as they arrive. Without a defined protocol, there is no way to know that you have received "all the bytes" that the client sent, since there is always the possibility that a network failure occurred somewhere between the client and your server during transmission, causing the last part of the stream not to arrive at the server. In that case, you would get the usual end-of-stream indication from the TCP socket (e.g. recv() returning 0, or EWOULDBLOCK if you are using non-blocking sockets), so you would know that you aren't going to receive any more data from the client (because the TCP connection is now disconnected)... but that isn't quite the same thing as knowing you have received all of the data the client meant for you receive.
Depending on your application, that might be good enough. If not, then you'll have to work out a protocol, and trust that your clients will abide by the rules of that protocol. Having the client send a header first saying how many bytes it plans to send is a good approach; or having it send some special "Okay, that's all I meant to send" indicator is also possible (although if you do it that way, you have to watch out for false positives if the special indicator could appear by chance inside the data itself)