Protecting Regions of Code with Mutexes

Protecting Regions of Code with Mutexes

Mutexes are kernel objects, which enables them to be shared between processes. This also means that mutex-protected sections are heavier weight than critical sections.

Mutexes are created with a call to CreateMutex() or CreateMutexEx(), which was introduced in Windows Vista. The call will return the handle to the newly created mutex object.

The first parameter to the CreateMutex() call is a pointer to the security attributes, or zero if the default security attributes should be used. The second parameter is a boolean that indicates if the mutex should be created in the state of being already acquired by the calling thread. The final parameter is an optional name for the mutex.

The CreateMutexEx call takes the security attributes; an optional name for the mutex; a flag that has either the value 0 or the value CREATE_MUTEX_INITIAL_OWNER, which indicates that the mutex should be created as owned by the calling thread; and a mask that sets the access permissions for the mutex (this can be left as zero).

Once the application has finished with the mutex, the kernel resources need to be freed by a call to CloseHandle(). Listing 6.18 shows the process of creating and releas-ing a mutex.

Listing 6.18   Creating and Disposing of Mutexes

HANDLE mutex;

...

mutex = CreateMutex( 0, 0, 0 );

...

CloseHandle( mutex );

To acquire the mutex, the application makes a call to WaitForSingleObject(), which either returns with the mutex acquired or returns after the specified timeout. Once the thread has completed, the section of code protected by the mutex can be released with a call to ReleaseMutex(). The code in Listing 6.19 shows how to acquire and release the mutex.

Listing 6.19   Acquiring and Releasing a Mutex

volatile int counter = 0;

HANDLE mutex;

unsigned int __stdcall test( void * )

{

while ( counter<100 )

{

WaitForSingleObject( mutex, INFINITE );

int number = counter++;

ReleaseMutex( mutex );

printf( "ThreadID %i; value = %i, is prime = %i\n", GetCurrentThreadId(), number, isprime(number) );

}

return 0;

}

int _tmain(int argc, _TCHAR* argv[])

{

HANDLE h1, h2;

mutex = CreateMutex( 0, 0, 0 );

h1 = (HANDLE)_beginthreadex( 0, 0, &test, (void*)0, 0, 0); h2 = (HANDLE)_beginthreadex( 0, 0, &test, (void*)1, 0, 0); WaitForSingleObject( h1, INFINITE );

WaitForSingleObject( h2, INFINITE );

CloseHandle( h1 );

CloseHandle( h2 ); getchar();

CloseHandle(mutex);

return 0;

}