Windows: Named-Mutex Solution
In search of a better solution, I decided to look at how two open-source libraries—Boost.Threads (boost.org/doc/html/threads.html) and Pthreads-w32 (sourceware.org/pthreads-win32/)—deal with this problem. Although neither library is directly concerned with Singletons, each has code that implements once-routine support on Windows. Boost.Threads provides a portable set of handy MT-related primitives to C++ programmers. Naturally, the platforms supported by this library include Windows. Pthreads-w32, on the other hand, implements a pthread-compatible layer, including pthread_once, on top of the Windows API, thus simplifying the porting of MT programs from UNIX to Windows. I reasoned that whatever technique these libraries use to implement once routines on Windows, it should be possible to use that same technique to implement my Singleton.
The technique I discovered in Boost.Threads (libs/thread/src/once.cpp under the Boost source tree) relied on a special Windows feature that lets a name be associated with a mutex. If a thread (not necessarily within the same process) creates a mutex with a name that already exists, instead of creating a new mutex, the system returns a handle to the existing one. The system also makes sure that the mutex is not destroyed as long as there are any open handles to it. Finally, the system guarantees that creating a mutex and closing a mutex handle are atomic operations. Listing Eight (available electronically) shows how a named mutex can be used to implement our Singleton.
The name of the mutex is designed to be unique. If you don't think the string "Singleton::instance" provides sufficient guarantee from collisions, you can use a random string generated by your favorite method (the string actually used in the Boost code is "2AC1A572DB6944B0A65C38C4140AF2F4"). Also, note that this code uses the Double-Checked-Lock optimization (www.cs.wustl.edu/~schmidt/PDF/DC-Locking.pdf). To avoid the various problems with naïve implementations of this optimization (www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf), we make sure that all access to the flag is done through Windows interlocked routines, which incorporate all the necessary memory barriers. The InterlockedExchangeAdd(&flag, 0) call—a no-op incrementing the flag by 0—is just a way of using interlocked routines for reading the flag. (The same effect can be achieved more efficiently with some inlined assembler.)
Although a clever technique from the Boost.Threads library lets you improve upon the previous implementation by removing the inherent unpredictability of spinning, the solution suffers from several inefficiencies. To begin with, named mutexes are intended to be used for synchronization across process boundaries rather than among threads within the same process. Manipulating, and creating such "interprocess" mutexes is expensive—substantially more so than those used to synchronize threads. Moreover, with this heavy approach, a named mutex has to be created at least once, even when there is absolutely no contention—even if the application using this Singleton is singlethreaded! Finally, generating a unique mutex name from a pointer is artificial and rather inelegant. Given these drawbacks, this solution, although acceptable in many applications, did not satisfy the needs of my high-performance reusable infrastructure library.
