To show in which functions differences occur for C and similar languages, you can use the --show-c-function (-p) option. This option automatically defaults to the context output format (see 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 --show-c-function (-p) option is equivalent to -F '^[[:alpha:]$_]' if the unified format is specified, otherwise -c -F '^[[:alpha:]$_]' (see Specified Headings). GNU diff provides this option for the sake of convenience.