GNU Pth is a portable library providing non-preemptive, priority-based scheduling for multiple execution threads. Each thread has its own program-counter, run-time stack, signal mask and errno variable. Threads are scheduled in a cooperative way, rather than in the standard preemptive way, such that they are managed according to priority and events. However, Pth also features emulation of POSIX.1c threads ("pthreads") for backwards compatibility.