Hi, It seems so in v1.39 and 1.40, without the macro BOOST_ASIO_DISABLE_KQUEUE defined. I've found it's due to a line in the file [boost/trunk/boost/asio/detail/kqueue_reactor_fwd.hpp]: ------------------------------------------------------------- #if defined(__MACH__) && defined(__APPLE__) // Define this to indicate that epoll is supported on the target platform. #define BOOST_ASIO_HAS_KQUEUE 1 ------------------------------------------------------------- That only checks whether it is a MacOS system to enable kqueue, but doesn't care any of FreeBSD. One said it's because some issues are not yet fixed on use of kqueue on FreeBSD, but this seems to exist for quite long. Is there any more information? Thanks