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LIBTHR(3)              FreeBSD Library Functions Manual              LIBTHR(3)

     libthr - 1:1 POSIX threads library

     1:1 Threading Library (libthr, -lthr)

     #include <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.

     The library is tightly integrated with the run-time link editor and Standard C Library (libc, -lc); all three components
     must be built from the same source tree.  Mixing libc and libthr
     libraries from different versions of FreeBSD 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

     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_op(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

     By default, the main thread's stack size is equal to the value of
     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 FreeBSD 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.

     The following sysctl MIBs affect the operation of the library:

     kern.ipc.umtx_vnode_persistent       By default, a shared lock backed by
                                          a mapped file in memory is
                                          automatically destroyed on the last
                                          unmap of the corresponding file's
                                          page, which is allowed by POSIX.
                                          Setting the sysctl to 1 makes such a
                                          shared lock object persist until the
                                          vnode is recycled by the Virtual
                                          File System.  Note that in case file
                                          is not opened and not mapped, the
                                          kernel might recycle it at any
                                          moment, making this sysctl less
                                          useful than it sounds.

     kern.ipc.umtx_max_robust             The maximal number of robust mutexes
                                          allowed for one thread.  The kernel
                                          will not unlock more mutexes than
                                          specified, see _umtx_op for more
                                          details.  The default value is large
                                          enough for most useful applications.

     debug.umtx.robust_faults_verbose     A non zero value makes kernel emit
                                          some diagnostic when the robust
                                          mutexes unlock was prematurely
                                          aborted after detecting some
                                          inconsistency, as a measure to
                                          prevent memory corruption.

     The RLIMIT_UMTXP limit (see getrlimit(2)) defines how many shared locks a
     given user may create simultaneously.

     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 libc, 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), errno(2), thr_exit(2),
     thr_kill(2), thr_kill2(2), thr_new(2), thr_self(2), thr_set_name(2),
     _umtx_op(2), dlclose(3), dlopen(3), getenv(3), pthread_attr(3),
     pthread_attr_setstacksize(3), pthread_create(3), signal(3), atomic(9)

     The libthr library was originally created by Jeff Roberson
     <[email protected]>, and enhanced by Jonathan Mini <[email protected]> and
     Mike Makonnen <[email protected]>.  It has been substantially rewritten and
     optimized by David Xu <[email protected]>.

FreeBSD 11.1-RELEASE-p4          May 17, 2016          FreeBSD 11.1-RELEASE-p4
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