Initializing a Mutex
Normally, a mutex is initialized by its constructor before an acquire operation can occur. However, a mutex declared at global scope can be accessed before it is constructed, if threads are launched during the static initialization phase. Therefore, global static instances of
RWMutexLock can require a different style of initialization.
To account for this possibility, the mutex acquire operation is designed to check for proper initialization each time it is called. If the mutex has not been initialized, the acquire operation performs the initialization itself. This allows a thread to safely acquire a mutex prior to its actual construction.
If a global mutex is declared using the normal constructor and that mutex is accessed before construction, then the mutex is initialized a second time when it is finally constructed.
To avoid the possibility of an erroneous second initialization, you must use a special constructor that does not initialize the mutex. This static constructor is selected by initializing the mutex with a special enumerated constant, RW_STATIC_CTOR, as shown in this declaration:
// Declare a global static mutex instance
RWMutexLock mutex(RW_STATIC_CTOR);
The
RWMutexLock static constructor must only be used for global static instances. Use of this constructor with an automatically or dynamically allocated instance produces errors or other unpredictable behavior.
NOTE: The initialization of a global static mutex is not thread-safe. It is conceivable that two threads can attempt to acquire and initialize a mutex simultaneously, resulting in a race condition. When designing your application, you must take care to avoid such a possibility.
