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Semaphore

Inherits: RefCounted < Object

A synchronization mechanism used to control access to a shared resource by Threads.

Description

A synchronization semaphore that can be used to synchronize multiple Threads. Initialized to zero on creation. For a binary version, see Mutex.

Warning: Semaphores must be used carefully to avoid deadlocks.

Warning: To guarantee that the operating system is able to perform proper cleanup (no crashes, no deadlocks), these conditions must be met:

• When a Semaphore‘s reference count reaches zero and it is therefore destroyed, no threads must be waiting on it.

• When a Thread‘s reference count reaches zero and it is therefore destroyed, it must not be waiting on any semaphore.

Tutorials

• Using multiple threads

• Thread-safe APIs

Methods

Method Descriptions

void post ( )

Lowers the Semaphore, allowing one more thread in.

bool try_wait ( )

Like wait, but won’t block, so if the value is zero, fails immediately and returns false. If non-zero, it returns true to report success.

void wait ( )

Waits for the Semaphore, if its value is zero, blocks until non-zero.