How to get diff of modified C functions between two versions of a file?

Question

Data Comparison

There is a task in the project to collect the list of the changed functions for C code.
The project is written in pure C, build for Unix, VCS ClearCase.
It was planned to use a regular file-by-file diff with the --show-c-function
type option
diff --show-c-function oldfile newfile
, but, as it turned out, sometimes it "bugs" and gives out extra functions that were not affected by the changes.
Tried a bunch of extras. options from diff with removing spaces, tabs, etc. - does not help.
Writing "bicycle" for these purposes is also not inspiring, and a quick google on the net did not give any ideas. What else can you try?
Does anyone know if there are any free tools that can solve this problem?