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LIBTHR(3) Library Functions Manual LIBTHR(3) NAME libthr -- 1:1 POSIX threads library LIBRARY 1:1 Threading Library (libthr, -lthr) SYNOPSIS #include <pthread.h> DESCRIPTION The libthr library provides a 1:1 implementation of the pthread(3) li- brary 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 ld-elf.so.1(1) and Standard C Library (libc, -lc); all three components must be built from the same source tree. Mixing libc and libthr li- braries from different versions of FreeBSD is not supported. The run- time linker ld-elf.so.1(1) has some code to ensure backward-compatibil- ity with older versions of libthr. The man page documents the quirks and tunables of the libthr. When linking with -lpthread, the run-time dependency libthr.so.3 is recorded in the produced object. MUTEX ACQUISITION 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 acquisi- tion 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 at- tempts to acquire the lock by atomic(9) operations. The loop count is controlled by the LIBPTHREAD_SPINLOOPS environment variable, with a de- fault value of 2000. If the spin loop was unable to acquire the mutex, a yield loop is exe- cuted, 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 environ- ment 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 owner- ship of the lock to the woken thread when the lock becomes available. THREAD STACKS 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 envi- ronment variable is present in the process environment (its value does not matter), the main thread's stack is reduced to 4MB on 64bit archi- tectures, and to 2MB on 32bit architectures, when the threading library is initialized. The rest of the address space area which has been re- served by the kernel for the initial process stack is used for non-ini- tial 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) func- tions. 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. RUN-TIME SETTINGS 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. 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 over- rides 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 speci- fies how often blocked threads are in- serted at the head of the sleep queue, instead of its tail. Bigger values re- duce the frequency of the FIFO disci- pline. The value must be between 0 and 255. LIBPTHREAD_UMTX_MIN_TIMEOUT The minimal amount of time, in nanosec- onds, the thread is required to sleep for pthread operations specifying a timeout. If the operation requests a timeout less than the value provided, it is silently increased to the value. The value of zero means no minimum (de- fault). 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 au- tomatically 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 Vir- tual 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 mu- texes allowed for one thread. The kernel will not unlock more mu- texes 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 in- consistency, as a measure to pre- vent memory corruption. The RLIMIT_UMTXP limit (see getrlimit(2)) defines how many shared locks a given user may create simultaneously. INTERACTION WITH RUN-TIME LINKER 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 sup- port 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. SIGNALS 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. PROCESS-SHARED SYNCHRONIZATION OBJECTS In the libthr implementation, user-visible types for all synchroniza- tion objects (e.g. pthread_mutex_t) are pointers to internal struc- tures, allocated either by the corresponding pthread_<objtype>_init() method call, or implicitly on first use when a static initializer was specified. The initial implementation of process-private locking ob- ject used this model with internal allocation, and the addition of process-shared objects was done in a way that did not break the appli- cation binary interface. For process-private objects, the internal structure is allocated using either malloc(3) or, for pthread_mutex_init(3), an internal memory al- locator implemented in libthr. The internal allocator for mutexes is used to avoid bootstrap issues with many malloc(3) implementations which need working mutexes to function. The same allocator is used for thread-specific data, see pthread_setspecific(3), for the same reason. For process-shared objects, the internal structure is created by first allocating a shared memory segment using _umtx_op(2) operation UMTX_OP_SHM, and then mapping it into process address space with mmap(2) with the MAP_SHARED flag. The POSIX standard requires that: only the process-shared synchronization object itself can be used for performing synchronization. It need not be referenced at the address used to initialize it (that is, another mapping of the same object can be used). With the FreeBSD implementation, process-shared objects require ini- tialization in each process that use them. In particular, if you map the shared memory containing the user portion of a process-shared ob- ject already initialized in different process, locking functions do not work on it. Another broken case is a forked child creating the object in memory shared with the parent, which cannot be used from parent. Note that processes should not use non-async-signal safe functions after fork(2) anyway. SEE ALSO ktrace(1), ld-elf.so.1(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) HISTORY The libthr library first appeared in FreeBSD 5.2. AUTHORS The libthr library was originally created by Jeff Roberson <jeff@FreeBSD.org>, and enhanced by Jonathan Mini <mini@FreeBSD.org> and Mike Makonnen <mtm@FreeBSD.org>. It has been substantially rewrit- ten and optimized by David Xu <davidxu@FreeBSD.org>. FreeBSD 13.2 October 1, 2021 LIBTHR(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | MUTEX ACQUISITION | THREAD STACKS | RUN-TIME SETTINGS | INTERACTION WITH RUN-TIME LINKER | SIGNALS | PROCESS-SHARED SYNCHRONIZATION OBJECTS | SEE ALSO | HISTORY | AUTHORS
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