No, but they are constrained to be less than or equal to 999,999.

See http://gitweb.dragonflybsd.org/dragonfly.git/blob/586c43085f...

When FreeBSD moved from 16 bit PIDs to 5 digit PIDs back in about 1999, I got the impression that one reason for not using the full 32 bit space was compatibility with tabular formatting in lots of userspace tools.

signed int, IIRC (cf. pid_t)