This appears to be an artifact of dtruss(1). Syscalls that aren't special-cased are assumed to have 3 arguments: https://github.com/joyent/dtruss-osx/blob/c3b23

This appears to be an artifact of dtruss(1). Syscalls that aren't special-cased are assumed to have 3 arguments: https://github.com/joyent/dtruss-osx/blob/c3b23e279187056e76c8ad02057d98b67f760290/dtruss#L237 https://github.com/joyent/dtruss-osx/blob/c3b23e279187056e76... There are special cases for 0..6 arguments (including one for close(2)) so I guess getpid(2) slipped through the cracks.