POSIX Threads Primitives POSIX thread (pthreads) is a standard that defines the APIs used to create and handle threads in software. POSIX threads had many (about 10) drafts during the 90s, and thus each operating system, and almost each revision, implements a different draft - hurting portability. Not all is lost - there are various common APIs that hardly changed, and there are various libraries that abstract threads, and allow for more portable multi-threaded programming.