Locking and Unlocking Files

Although the system allows more than one application to open a file and write to it, applications must not write over each other's work. An application can prevent this problem by temporarily locking a region in a file. The LockFile and LockFileEx functions lock a specified range of bytes in a file. The range may extend beyond the current end of the file. Locking part of a file prevents all other processes from reading or writing anywhere in the specified area. Attempts to read from or write to a region locked by another process always fail.

The LockFileEx function allows an application to specify either a shared lock or an exclusive lock. An exclusive lock denies all other processes both read and write access to the specified region of the file. A shared lock denies all processes write access to the specified region of the file, including the process that first locks the region. This can be used to create a read-only region in a file.

The application unlocks the region by using the UnlockFile or UnlockFileEx function. An application should unlock all locked areas before closing a file.