CreateMutex

The CreateMutex function creates a named or unnamed mutex object.

HANDLE CreateMutex(
  LPSECURITY_ATTRIBUTES lpMutexAttributes,
                       // pointer to security attributes
  BOOL bInitialOwner,  // flag for initial ownership
  LPCTSTR lpName       // pointer to mutex-object name
);
 

Parameters

lpMutexAttributes

Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpMutexAttributes is NULL, the handle cannot be inherited.

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.

bInitialOwner

Specifies the initial owner of the mutex object. If this value is TRUE and the caller created the mutex, the calling thread obtains ownership of the mutex object. Otherwise, the calling thread does not obtain ownership of the mutex. To determine if the caller created the mutex, see the Return Values section.

lpName

Pointer to a null-terminated string specifying the name of the mutex object. The name is limited to MAX_PATH characters and can contain any character except the backslash path-separator character (\). Name comparison is case sensitive.

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, waitable timer, job, or file-mapping object, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because these objects share the same name space.

Return Values

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 function returns a handle to the existing object and GetLastError returns ERROR_ALREADY_EXISTS. Otherwise, the caller created the mutex.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Remarks

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:

• A child process created by the CreateProcess function can inherit a handle to a mutex object if the lpMutexAttributes parameter of CreateMutex enabled inheritance.
• A process can specify the mutex-object handle in a call to the DuplicateHandle function to create a duplicate handle that can be used by another process.
• A process can specify the name of a mutex object in a call to the OpenMutex or CreateMutex function.

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.

Windows CE: The lpMutexAttributes parameter must be set to NULL.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winbase.h.
  Import Library: Use kernel32.lib.
  Unicode: Implemented as Unicode and ANSI versions on Windows NT.

See Also

Synchronization Overview, Synchronization Functions, CloseHandle, CreateProcess, DuplicateHandle, OpenMutex, ReleaseMutex, SECURITY_ATTRIBUTES