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Linux has two main implementations of the Pthreads API:
LinuxThreads: This is the original Linux threading implementation, developed by Xavier Leroy.
NPTL (Native POSIX Threads Library): This is the modern Linux threading implementation, developed by Ulrich Drepper and Ingo Molnar as a successor to LinuxThreads. NPTL provides performance that is superior to LinuxThreads, and it adheres more closely to the SUSv3 specification for Pthreads. Support for NPTL required changes to the kernel, and these changes appeared in Linux 2.6.