Hello, sorry for the title but I haven't find any more descriptive

That's the question: can a function be called safely multiple times at the same moment? I'm using the Win32 API function 'CreateThread' on a server program to handle the incoming requests. The threaded function call several other functions to perform operations, but I'm afraid that in some way (I don't know in what way), the data managed by those other functions could be mixed. Is that a real problem? For the moment I haven't get any datamix, but can it be happen?

Thank's in advance
Niara