Blocks concurrent access to shared resources from multiple threads
Mutex stands for "MUTual EXclusion". A mutex is a synchronization object, used when multiple threads are involved.
When you want to protect a part of the code from being accessed simultaneously by multiple threads, you typically use a mutex. When a thread is locked by a mutex, any other thread trying to lock it will be blocked until the mutex is released by the thread that locked it. This way, you can allow only one thread at a time to access a critical region of your code.
@database = Database.new # this is a critical resource that needs some protection @mutex = SF::Mutex.new def thread1() @mutex.lock() # this call will block the thread if the mutex is already locked by thread2 @database.write(...) @mutex.unlock() # if thread2 was waiting, it will now be unblocked end def thread2() @mutex.lock() # this call will block the thread if the mutex is already locked by thread1 @database.write(...) @mutex.unlock() # if thread1 was waiting, it will now be unblocked end
Be very careful with mutexes. A bad usage can lead to bad problems, like deadlocks (two threads are waiting for each other and the application is globally stuck).
To make the usage of mutexes more robust, particularly in
environments where exceptions can be thrown, you should
use the helper method
synchronize to lock/unlock mutexes.
SFML mutexes are recursive, which means that you can lock
a mutex multiple times in the same thread without creating
a deadlock. In this case, the first call to
as usual, and the following ones have no effect.
However, you must call
unlock() exactly as many times as you
lock(). If you don't, the mutex won't be released.
However, you must call unlock() exactly as many times as you
called lock(). If you don't, the mutex won't be released.
Lock the mutex
If the mutex is already locked in another thread, this call will block the execution until the mutex is released.
Lock the mutex, execute the block, then unlock the mutex (even if an exception is raised).