GNU Pth - GNU Portable Threads
Copyright (c) 1999 Ralf S. Engelschall <rse@engelschall.com>

Pth is a very portable POSIX/ANSI-C based library for Unix platforms
which provides non-preemptive priority-based scheduling for multiple
threads of execution ("multithreading") inside server applications. All
threads run in the same address space of the server application, but
each thread has it's own individual program-counter, run-time stack,
signal mask and errno variable.

The thread scheduling itself is done in a cooperative way, i.e. the
threads are managed by a priority- and event-based non-preemptive
scheduler. The intention is that this way one can achieve better
portability and run-time performance than with preemptive scheduling.
The event facility allows threads to wait until various types of events
occur, including pending I/O on filedescriptors, asynchronous signals,
elapsed timers, pending I/O on message ports, thread and process
termination, and even customized callback functions.

WWW: http://www.gnu.org/software/pth/