So far, I was able to call member functions which belong to a class for a struct. CEDET can give the suggestion nicely. However, for non-member functions, such as functions in C, which usually does not belong to anything i.e. functions like sort, qsort.... I can only auto-complete with CEDET if I have an instance of a class/struct. Without it, I have to type the function manually.
Is there a way to have code suggestion for such functions?
CEDET works fine for such functions - you need to be sure, that all necessary headers are included into your source code. You can also force CEDET to re-parse buffer with M-x bovinate command. But it's better to use CEDET from bzr repository, because fixes for C & C++ mode are made very often.
If it still don't working look to output of M-x semantic-analyze-debug-assist command, and post results to cedet-devel list