Recently, there was some idle discussion on #esoteric on the topic of reserved identifiers in C and POSIX. The discussion further progressed to a thought experiment on whether it would be possible to write useful programs in C if the standard (or POSIX) reserved all identifiers, i.e., using only identifiers defined by the standard, in a legal manner.

The following program is a rudimentary brainfuck (sans input) interpreter that is at least mostly valid C, yet uses no user-defined identifiers (after preprocessing).