The CreateMutex function creates a named or unnamed mutex object.
HANDLE CreateMutex(
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LPSECURITY_ATTRIBUTES lpMutexAttributes, |
// pointer to security attributes |
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BOOL bInitialOwner, |
// flag for initial ownership |
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LPCTSTR lpName |
// pointer to mutex-object name |
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); |
Windows NT: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new mutex. If lpMutexAttributes is NULL, the mutex gets a default security descriptor.
Windows 95: The lpSecurityDescriptor member of the structure is
ignored.
If lpName matches the name of an existing named mutex object, this function requests MUTEX_ALL_ACCESS access to the existing object. In this case, the bInitialOwner parameter is ignored because it has already been set by the creating process. If the lpMutexAttributes parameter is not NULL, it determines whether the handle can be inherited, but its security-descriptor member is ignored.
If lpName is NULL, the mutex object is created without a name.
If lpName matches the name of an existing event, semaphore, or file-mapping object, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because event, mutex, semaphore, and file-mapping objects share the same name space.
If the function succeeds, the return value is a handle to the mutex object. If the named mutex object existed before the function call, the GetLastError function returns ERROR_ALREADY_EXISTS. Otherwise, GetLastError returns zero.
If the function fails, the return value is NULL. To get extended error information, call GetLastError.
The handle returned by CreateMutex has MUTEX_ALL_ACCESS access to the new mutex object and can be used in any function that requires a handle to a mutex object.
Any thread of the calling process can specify the mutex-object handle in a call to one of the wait functions. The single-object wait functions return when the state of the specified object is signaled. The multiple-object wait functions can be instructed to return either when any one or when all of the specified objects are signaled. When a wait function returns, the waiting thread is released to continue its execution.
The state of a mutex object is signaled when it is not owned by any thread. The creating thread can use the bInitialOwner flag to request immediate ownership of the mutex. Otherwise, a thread must use one of the wait functions to request ownership. When the mutex’s state is signaled, one waiting thread is granted ownership, the mutex’s state changes to nonsignaled, and the wait function returns. Only one thread can own a mutex at any given time. The owning thread uses the ReleaseMutex function to release its ownership.
The thread that owns a mutex can specify the same mutex in repeated wait function calls without blocking its execution. Typically, you would not wait repeatedly for the same mutex, but this mechanism prevents a thread from deadlocking itself while waiting for a mutex that it already owns. However, to release its ownership, the thread must call ReleaseMutex once for each time that the mutex satisfied a wait.
Two or more processes can call CreateMutex to create the same named mutex. The first process actually creates the mutex, and subsequent processes open a handle to the existing mutex. This enables multiple processes to get handles of the same mutex, while relieving the user of the responsibility of ensuring that the creating process is started first. When using this technique, you should set the bInitialOwner flag to FALSE; otherwise, it can be difficult to be certain which process has initial ownership.
Multiple processes can have handles of the same mutex object, enabling use of the object for interprocess synchronization. The following object-sharing mechanisms are available:
Use the CloseHandle function to close the handle. The system closes the handle automatically when the process terminates. The mutex object is destroyed when its last handle has been closed.
CloseHandle, CreateProcess, DuplicateHandle, OpenMutex, ReleaseMutex, SECURITY_ATTRIBUTES