Qt 4.2.3 with C code

[maird]

So, I have to do a client - server application and I use Qt for the interface and C for the implementation.

That's an ambiguous requirement. It's a client server application so I assume it is two programs, a client and a server. For client server applications one could argue that the implementation of the application is the server program and the client is an interface. If your requirement is to use "Qt for the interface and C for the implementation" then you could assume that only the server program has to be in C. Write the server in C using no Qt and write the client in C++ and use bare Qt in the client program. Or, get less ambiguous requirements.