calling threads is blocked until it actually wakes someone. This mechanism ensures that wake-ups are not missed by making "release" operation blocking e.g. There are two operations: "wait" and "release" (wake). Just as futex(2), keyed events use table of wait queues hashed by address. They provide alternative undocumented API called "Keyed Events". I think OpenBSD partially implements futex(2) too.Įarlier versions of Windows (7, Vista, XP) are special._umtx_op(2) on OS X, FreeBSD (never used those, you can read more here).WaitOnAddress, WakeOnAddress on Windows 8 and later.You can find it here.Īnother decent article that shows usage of futex(2) is here, but this implementation is too "benchmark oriented".Īs for other operating systems, today most provide equivalent of Linux'es futex(2): It explains how futex(2) works and shows how it can be used to implemented common primitives. There is a good document "Futexes Are Tricky" about futexes written by Ulrich Drepper. Languages like Java and C# both use monitors instead of mutexes and condition variables.Īnother futex(2) trick is FUTEX_REQUEUE operation that can be used to move waiters from one futex to another and can be used to make condition varaible very effecient. Such combination of a mutex and a condition variable that only allows "notify" to be called from within critical section is usually called "monitor". If you allow "notify" to be only called from within critical section, additional mutex locking is not required, making implementation simpler and faster. State of condition variable might need to be protected by a separate mutex, so it is common for implementations to keep another mutex within a condition variable. Complexity and performance of implementation depends on how much you want to stick to a "classic" interface, mainly whether you allow "notify" to be called outside of critical section or not. * so it must not be touch - hence the extra CONTESTED state */Ĭondition variables are trickier. * The semaphore has been unlocked and could be deallocated, New = (int) ((unsigned int) val + n + (val ftx, &val, new)) Int new, waiters, val = atomic_load(&sem->ftx) Example below: struct mutex Ītomic_cmpxchg(&sem->ftx, LOCKED, CONTESTED) Using futex(2) it is possible to implement mutex that does not require any syscall if mutex is not contested, but is able to suspend calling thread otherwise. It is very important that FUTEX_WAIT verifies value before sleeping, as this ensures thread does not miss wake-ups. (e.g., inside FUTEX_WAIT) on the futex word at the address uaddr. This operation wakes at most val of the waiters that are waiting Then sleeps waiting for a FUTEX_WAKE operation on the futex word. The address uaddr still contains the expected value val, and if so, This operation tests that the value at the futex word pointed to by Two most commonly used operations are FUTEX_WAIT: I will try to focus on modern implementations.įirst, Linux uses futex syscall ( Fast Userspace mu TEX). When you go through the wrapper, it may not be immediately clear what the wrappers are actually doing underneath.I think your question is more about what primitives are used to implement those, and the answer is - it depends. The code will often be more straight forward. Consider using the API directly, without going through C++ std::thread and std::mutex wrappers. The choice is usually rather simple: if you are going to synchronize threads within the same address space, use mutexes and conditional variables if you need to synchronize processes, use semaphores. Semaphores can count to more than one, and any thread can decrement or increment the semaphore, whereas mutex can only be unlocked by the thread that locked it. The difference is subtle as discussed here. Mutexes are a special case of semaphores, namely they behave similarly to binary semaphores in case where there are exactly two threads.Mutexes can only be used within the same address space, whereas semaphores can be used to synchronize Linux processes.Semaphores have been invented by Dijkstra in 1962, whereas mutexes, in their current form, have been added to the Linux kernel in 2003. Mutexes are newer, and more light weight than semaphores.To summarize the differences are as follows: The Linux man pages for mutex, and semaphore, will give you all the information you need. As C++ threads are implemented on top of P-threads, all thread synchronization is implemented using P-thread library calls, or POSIX semaphores. Most importantly, both the C++ semaphore and mutex are actually wrappers on top of C libraries for POSIX threads and POSIX semaphores. To understand these differences, I would suggest the following. There are a number of differences between semaphores and mutexes, available via std::mutex. It is accessed via std::counting_semaphore and std::binary_semaphore. C++20 has introduced a new synchronization mechanism, namely the semaphore.
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