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In computer science, a semaphore is a variable or abstract data type used to control access to a common resource by multiple threads and avoid critical section problems in a concurrent system such as a multitasking operating system. Semaphores are a type of synchronization primitive. A trivial semaphore is a plain variable that is changed (for ...
Visual C++ provides the synchronize attribute of methods to be synchronized, but this is specific to COM objects in the Windows architecture and Visual C++ compiler. [10] C and C++ can easily access any native operating system locking features. C# provides the lock keyword on a thread to ensure its exclusive access to a resource.
In software engineering, double-checked locking (also known as "double-checked locking optimization" [1]) is a software design pattern used to reduce the overhead of acquiring a lock by testing the locking criterion (the "lock hint") before acquiring the lock.
For example, a useful pair of contracts, allowing occupancy to be passed without establishing the invariant, is: wait c: precondition I modifies the state of the monitor postcondition P c signal c precondition (not empty(c) and P c) or (empty(c) and I) modifies the state of the monitor postcondition I (See Howard [4] and Buhr et al. [5] for more.)
Poco::RWLock in POCO C++ Libraries; mse::recursive_shared_timed_mutex in the SaferCPlusPlus library is a version of std::shared_timed_mutex that supports the recursive ownership semantics of std::recursive_mutex. txrwlock.ReadersWriterDeferredLock Readers/Writer Lock for Twisted [19] rw_semaphore in the Linux kernel [20]
With priority ceiling protocol, the shared mutex process (that runs the operating system code) has a characteristic (high) priority of its own, which is assigned to the task of locking the mutex. This works well, provided the other high-priority task(s) that tries to access the mutex does not have a priority higher than the ceiling priority.
To enter a critical section, a thread must obtain a semaphore, which it releases on leaving the section. Other threads are prevented from entering the critical section at the same time as the original thread, but are free to gain control of the CPU and execute other code, including other critical sections that are protected by different semaphores.
In C and C++, the volatile keyword was intended to allow C and C++ programs to directly access memory-mapped I/O. Memory-mapped I/O generally requires that the reads and writes specified in source code happen in the exact order specified with no omissions. Omissions or reorderings of reads and writes by the compiler would break the ...