Expand directory name of currently opened buffer in ex-mode

I want to invoke the cscope command with the current working directory as the argume开发者_StackOverflownt. I have set up Vim to change its CWD to the currently opened buffer. If I have an opened buffer of the file "C:\dir\subdir\file.d", I'd like to issue this call:

:cscope add C:\dir\subdir

Without having to manually type the directory name, of course.

How does :cscope behaves with :cscope %:p:h ? (some command accepts this form, some not)