LockFile function (fileapi.h)

Locks the specified file for exclusive access by the calling process.

To specify additional options, for example creating a shared lock or for block-on-fail operation, use the LockFileEx function.


BOOL LockFile(
  [in] HANDLE hFile,
  [in] DWORD  dwFileOffsetLow,
  [in] DWORD  dwFileOffsetHigh,
  [in] DWORD  nNumberOfBytesToLockLow,
  [in] DWORD  nNumberOfBytesToLockHigh


[in] hFile

A handle to the file. The file handle must have been created with the GENERIC_READ or GENERIC_WRITE access right. For more information, see File Security and Access Rights.

[in] dwFileOffsetLow

The low-order 32 bits of the starting byte offset in the file where the lock should begin.

[in] dwFileOffsetHigh

The high-order 32 bits of the starting byte offset in the file where the lock should begin.

[in] nNumberOfBytesToLockLow

The low-order 32 bits of the length of the byte range to be locked.

[in] nNumberOfBytesToLockHigh

The high-order 32 bits of the length of the byte range to be locked.

Return value

If the function succeeds, the return value is nonzero (TRUE).

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


If the call to LockFile completes synchronously, a completion entry may not be queued when a completion port is associated with the file handle.

The UnlockFile function unlocks a file region locked by LockFile.

Locking a region of a file gives the threads of the locking process exclusive access to the specified region using this file handle. If the file handle is inherited by a process created by the locking process, the child process is not granted access to the locked region. If the locking process opens the file a second time, it cannot access the specified region through this second handle until it unlocks the region.

Locking a region of a file does not prevent reading or writing from a mapped file view.

You can lock bytes that are beyond the end of the current file. This is useful to coordinate adding records to the end of a file.

Exclusive locks cannot overlap an existing locked region of a file. For more information, see LockFileEx.

If LockFile cannot lock a region of a file, it returns zero immediately. It does not block. To issue a file lock request that will block until the lock is acquired, use LockFileEx without the LOCKFILE_FAIL_IMMEDIATELY flag.

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


For an example, see Appending One File to Another File.


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