Thread's in FreeBSD

libkse :

kernel support for user threads

libc_r :

the old user space thread library.

libthr ( pthread ) :

library provides a 1:1 implementation of the pthread(3) library interfaces for application threading. It has been optimized for use by applications expecting system scope thread semantics, and can provide significant performance improvements compared to N:M Threading Library (libkse, -lkse).

pth :

GNU Portable Threads

linuxthreads:

/usr/ports/devel/linuxthreads
LinuxThreads is an POSIX pthreads implementation using "kernel threads". In this FreeBSD port, a kernel thread is started using rfork (whereas in the original Linux implementation a kernel thread is started using the Linux clone call).