To show in which functions differences occur for C and similar languages, you can use the `-p' or `--show-c-function' option. This option automatically defaults to the context output format (see section Context Format), with the default number of lines of context. You can override that number with `-C lines' elsewhere in the command line. You can override both the format and the number with `-U lines' elsewhere in the command line.
The `-p' and `--show-c-function' options are equivalent to
`-F'^[_a-zA-Z$]'' if the unified format is specified, otherwise
`-c -F'^[_a-zA-Z$]'' (see section Showing Lines That Match Regular Expressions). GNU diff
provides them for the sake of convenience.