UnlockFile function (fileapi.h)

Unlocks a region in an open file. Unlocking a region enables other processes to access the region.

For an alternate way to specify the region, use the UnlockFileEx function.


BOOL UnlockFile(
  [in] HANDLE hFile,
  [in] DWORD  dwFileOffsetLow,
  [in] DWORD  dwFileOffsetHigh,
  [in] DWORD  nNumberOfBytesToUnlockLow,
  [in] DWORD  nNumberOfBytesToUnlockHigh


[in] hFile

A handle to the file that contains a region locked with LockFile. The file handle must have been created with either the GENERIC_READ or GENERIC_WRITE access right. For more information, see File Security and Access Rights.

[in] dwFileOffsetLow

The low-order word of the starting byte offset in the file where the locked region begins.

[in] dwFileOffsetHigh

The high-order word of the starting byte offset in the file where the locked region begins.

[in] nNumberOfBytesToUnlockLow

The low-order word of the length of the byte range to be unlocked.

[in] nNumberOfBytesToUnlockHigh

The high-order word of the length of the byte range to be unlocked.

Return value

If the function succeeds, the return value is nonzero.

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


This function always operates synchronously, but may not queue a completion entry when a completion port is associated with the file handle.

Unlocking a region of a file releases a previously acquired lock on the file. The region to unlock must correspond exactly to an existing locked region. Two adjacent regions of a file cannot be locked separately and then unlocked using a single region that spans both locked regions.

If a process terminates with a portion of a file locked or closes a file that has outstanding locks, the locks are unlocked by the operating system. However, the time it takes for the operating system to unlock these locks depends upon available system resources. Therefore, it is recommended that your process explicitly unlock all files it has locked when it terminates. If this is not done, access to these files may be denied if the operating system has not yet unlocked them.

In Windows 8 and Windows Server 2012, this function is supported by the following technologies.

Technology Supported
Server Message Block (SMB) 3.0 protocol Yes
SMB 3.0 Transparent Failover (TFO) Yes
SMB 3.0 with Scale-out File Shares (SO) Yes
Cluster Shared Volume File System (CsvFS) Yes
Resilient File System (ReFS) Yes


Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header fileapi.h (include Windows.h)
Library Kernel32.lib
DLL Kernel32.dll

See also

Locking and Unlocking Byte Ranges in Files


File Management Functions