How to detect NULL in VarChar (C function)

I try to write a function that combines two varchar columns.

Now the problem arose that (either) one of te columns may contain a NULL 
value, in which case the return value seems to become NULL.

I have not been able to find a description of how to detect NULL values 
in VarChar variables passed to C-functions to catch those cases.

Any help will be appreciated
   Benno