Yes, there are several. Here are a several URLs with their individual
detail. Please note that I have emailed the authors (if known) to get more
information about their libraries.
This is a very basic, bare-bones threading package which has been tested
on both single-CPU and SMP Linux boxes. Clone()-based & includes demo
program.
PCthreads (tm) is a multithreading library for Linux-based Intel systems
and is based on the POSIX 1003.1c standard. The kit contains the sources
for the library (libpthreads), a build environment for building both ELF
and A.OUT versions of the library, and a complete set of man pages for
all POSIX .1c functions.
Pthreads is a C library which implements POSIX threads for SunOS 4.1.x,
Solaris 2.x, SCO UNIX, FreeBSD and Linux. Used for GNU Ada Runtime; partial
libc support. User-space pthreads library; used for GNU Ada Runtime; partial
libc, POSIX 1003.4 support; pthreads documentation.
This is an experiment with the Linux 2.0 clone() call to implement
usable kernel threads in a user program. These jkthreads have an API that
has NOTHING to do with pthreads or Win32 threads or BeBox threads. Includes
C++ wrappers.
LinuxThreads is an implementation of the Posix 1003.1c thread package
for Linux. Unlike other implementations of Posix threads, LinuxThreads
provides kernel-space threads: threads are created with the new clone()
system call and all scheduling is done in the kernel. Includes partial
libc support & program examples.
User-space pthreads library with thread-blocking syscalls (read, write,
connect, accept, sleep, wait, etc.) and a thread-safe C library (stdio,
net utilities, etc.) A separate version is distributed with Linux libc
source but may not be built by default.
A toolkit for building user-space threads packages. Please note that
this is designed to facilitate building threads library packages; therefore,
it omits many higher-level functions. Documentation written in PostScript.