In NOTES file, section "COMPATIBILITY OPTIONS": # # Implement system calls compatible with 4.3BSD and older versions of # FreeBSD. You probably do NOT want to remove this as much current code # still relies on the 4.3 emulation. Note that some architectures that # are supported by FreeBSD do not include support for certain important # aspects of this compatibility option, namely those related to the # signal delivery mechanism. # options COMPAT_43 # Old tty interface. options COMPAT_43TTY The comment seems outdated since COMPAT_43 has been removed from GENERIC about 10 years now ;) I'm also suggesting to disable those options by default now on NOTES.
It seems there is another outdated information here: # Enable 32-bit Linux ABI emulation (requires COMPAT_43 and COMPAT_FREEBSD32) #options COMPAT_LINUX32 But according to commit #159651: "For amd64 there's an explicit comment that it's needed for the linuxolator. This is not the case anymore."