"We say that there is no reason to use --pedantic; it is just there to satisfy pedants who would otherwise say, "This isn't really an ANSI C compiler!"
Some GNU utilities such as df and du do not follow the POSIX spec unless you set the environment variable POSIXLY_CORRECT. Normally GNU df and du print disk space figures in units of k. POSIX says to print disk space figures in units of 512 bytes. If you set POSIXLY_CORRECT, GNU df and du do that. (My original plan was to name it POSIX_ME_HARDER.)"
interviewed by Federico Biancuzzi
link Danny Yee