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Man Pages

Manual Reference Pages  -  LIBTHR (3)


libthr - 1:1 POSIX threads library


Mutex Acquisition
Thread Stacks
Run-time Settings
Interaction With Run-time Linker
See Also


.Lb libthr


.In pthread.h


The libthr library provides a 1:1 implementation of the pthread(3) library interfaces for application threading. It has been optimized for use by applications expecting system scope thread semantics, and can provide significant performance improvements compared to
.Lb libkse .

The library is tightly integrated with the run-time link editor and
.Lb libc ; all three components must be built from the same source tree. Mixing libc and libthr libraries from different versions of
.Fx is not supported. The run-time linker has some code to ensure backward-compatibility with older versions of libthr.

The man page documents the quirks and tunables of the libthr. When linking with -lpthread, the run-time dependency is recorded in the produced object.


A locked mutex (see pthread_mutex_lock(3)) is represented by a volatile variable of type lwpid_t, which records the global system identifier of the thread owning the lock. libthr performs a contested mutex acquisition in three stages, each of which is more resource-consuming than the previous. The first two stages are only applied for a mutex of PTHREAD_MUTEX_ADAPTIVE_NP type and PTHREAD_PRIO_NONE protocol (see pthread_mutexattr(3)).

First, on SMP systems, a spin loop is performed, where the library attempts to acquire the lock by atomic(9) operations. The loop count is controlled by the LIBPTHREAD_SPINLOOPS environment variable, with a default value of 2000.

If the spin loop was unable to acquire the mutex, a yield loop is executed, performing the same atomic(9) acquisition attempts as the spin loop, but each attempt is followed by a yield of the CPU time of the thread using the sched_yield(2) syscall. By default, the yield loop is not executed. This is controlled by the LIBPTHREAD_YIELDLOOPS environment variable.

If both the spin and yield loops failed to acquire the lock, the thread is taken off the CPU and put to sleep in the kernel with the umtx(2) syscall. The kernel wakes up a thread and hands the ownership of the lock to the woken thread when the lock becomes available.


Each thread is provided with a private user-mode stack area used by the C runtime. The size of the main (initial) thread stack is set by the kernel, and is controlled by the RLIMIT_STACK process resource limit (see getrlimit(2)).

By default, the main thread’s stack size is equal to the value of RLIMIT_STACK for the process. If the LIBPTHREAD_SPLITSTACK_MAIN environment variable is present in the process environment (its value does not matter), the main thread’s stack is reduced to 4MB on 64bit architectures, and to 2MB on 32bit architectures, when the threading library is initialized. The rest of the address space area which has been reserved by the kernel for the initial process stack is used for non-initial thread stacks in this case. The presence of the LIBPTHREAD_BIGSTACK_MAIN environment variable overrides LIBPTHREAD_SPLITSTACK_MAIN; it is kept for backward-compatibility.

The size of stacks for threads created by the process at run-time with the pthread_create(3) call is controlled by thread attributes: see pthread_attr(3), in particular, the pthread_attr_setstacksize(3), pthread_attr_setguardsize(3) and pthread_attr_setstackaddr(3) functions. If no attributes for the thread stack size are specified, the default non-initial thread stack size is 2MB for 64bit architectures, and 1MB for 32bit architectures.


The following environment variables are recognized by libthr and adjust the operation of the library at run-time:
LIBPTHREAD_BIGSTACK_MAIN Disables the reduction of the initial thread stack enabled by LIBPTHREAD_SPLITSTACK_MAIN.
  Causes a reduction of the initial thread stack, as described in the section THREAD STACKS. This was the default behaviour of libthr before
.Fx 11.0 .
LIBPTHREAD_SPINLOOPS The integer value of the variable overrides the default count of iterations in the spin loop of the mutex acquisition. The default count is 2000, set by the MUTEX_ADAPTIVE_SPINS constant in the libthr sources.
LIBPTHREAD_YIELDLOOPS A non-zero integer value enables the yield loop in the process of the mutex acquisition. The value is the count of loop operations.
LIBPTHREAD_QUEUE_FIFO The integer value of the variable specifies how often blocked threads are inserted at the head of the sleep queue, instead of its tail. Bigger values reduce the frequency of the FIFO discipline. The value must be between 0 and 255.


On load, libthr installs interposing handlers into the hooks exported by libc. The interposers provide real locking implementation instead of the stubs for single-threaded processes in , cancellation support and some modifications to the signal operations.

libthr cannot be unloaded; the dlclose(3) function does not perform any action when called with a handle for libthr. One of the reasons is that the internal interposing of libc functions cannot be undone.


The implementation interposes the user-installed signal(3) handlers. This interposing is done to postpone signal delivery to threads which entered (libthr-internal) critical sections, where the calling of the user-provided signal handler is unsafe. An example of such a situation is owning the internal library lock. When a signal is delivered while the signal handler cannot be safely called, the call is postponed and performed until after the exit from the critical section. This should be taken into account when interpreting ktrace(1) logs.


ktrace(1),, getrlimit(2), umtx(2), dlclose(3), dlopen(3), errno(3), getenv(3), libc(3), pthread_attr(3), pthread_attr_setstacksize(3), pthread_create(3), signal(3), atomic(9)


.An -nosplit The libthr library was originally created by
.An Jeff Roberson Aq , and enhanced by
.An Jonathan Mini Aq and
.An Mike Makonnen Aq . It has been substantially rewritten and optimized by
.An David Xu Aq .
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