Secure Programming Cookbook for C and C++

2.10. Synchronizing Resource Access Across Processes on Windows

Problem

You want to ensure that two processes cannot simultaneously access the same resource.

Solution

Use a named mutex (mutually exclusive lock) to synchronize access to the resource.

Discussion

Coordinating access to a shared resource between multiple processes on Windows is much simpler and much more elegant than it is on Unix. For maximum portability on Unix, you must use a lock file and make sure to avoid a number of possible race conditions to make lock files work properly. On Windows, however, the use of named mutexes solves all the problems Unix has without introducing new ones.

A named mutex is a synchronization object that works by allowing only a single thread to acquire a lock at any given time. Mutexes can also exist without a name, in which case they are considered anonymous. Access to an anonymous mutex can only be obtained by somehow acquiring a handle to the object from the thread that created it. Anonymous mutexes are of no use to us in this recipe, so we won’t discuss them further.

Mutexes have a namespace much like that of a filesystem. The mutex namespace is separate from namespaces used by all other objects. If two or more applications agree on a name for a mutex, access to the mutex can always be obtained to use it for synchronizing access to a shared resource.

A mutex is created with a call to the CreateMutex( ) function. You will find it particularly useful in this recipe that the mutex is created ...