c-buffer-is-cc-mode is a variable defined in `cc-defs.el'.
Its value is

  • Automatically becomes permanently buffer-local when set.

Non-nil for all buffers with a major mode derived from CC Mode.
Otherwise, this variable is nil. I.e. this variable is non-nil for
`c-mode', `c++-mode', `objc-mode', `java-mode', `idl-mode',
`pike-mode', `awk-mode', and any other non-CC Mode mode that calls
`c-initialize-cc-mode'. The value is the mode symbol itself
(i.e. `c-mode' etc) of the original CC Mode mode, or just t if it's
not known.