Restricts access to a specified range of bytes in the stream. Supporting this functionality is optional since some file systems do not provide it.

HRESULT LockRegion(

//Specifies the byte offset for the beginning of the range
//Specifies the length of the range in bytes
DWORD dwLockType
//Specifies the restriction on accessing the specified range



[in]Specifies the byte offset for the beginning of the range.


[in]Specifies, in bytes, the length of the range to be restricted.


[in]Specifies the restrictions being requested on accessing the range.

Return Values


The specified range of bytes was locked.


Locking is not supported at all or the specific type of lock requested is not supported.


Requested lock is supported, but cannot be granted because of an existing lock.


The byte range can extend past the current end of the stream. Locking beyond the end of a stream is useful as a method of communication between different instances of the stream without changing data that is actually part of the stream.

Three types of locking can be supported: locking to exclude other writers, locking to exclude other readers or writers, and locking that allows only one requestor to obtain a lock on the given range, which is usually an alias for one of the other two lock types. A given stream instance might support either of the first two types, or both. The lock type is specified by dwLockType, using a value from the LOCKTYPE enumeration.

Any region locked with IStream::LockRegion must later be explicitly unlocked by calling IStream::UnlockRegion with exactly the same values for the libOffset, cb, and dwLockType parameters. The region must be unlocked before the stream is released. Two adjacent regions cannot be locked separately and then unlocked with a single unlock call.

Notes to Callers

Since range locking is not supported by the OLE-provided implementation of streams, calling IStream::LockRegion has no effect for this implementation.

Since the type of locking supported is optional and can vary in different implementations of IStream, you must provide code to deal with the STG_E_INVALIDFUNCTION error.

Notes to Implementors

Support for this method is optional for implementations of stream objects since it may not be supported by the underlying file system. The type of locking supported is also optional. The STG_E_INVALIDFUNCTION error is returned if the requested type of locking is not supported.

See Also

LOCKTYPE, IStream::UnlockRegion

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