This is command was introduced in the LAN Manager 1.0 dialect. The LAN Manager 1.0 version of this command is not compatible with files that have greater than 32-bit offsets. The support for files that have 64-bit offsets was introduced into this command in the NT LAN Manager dialect.

This command is used to explicitly lock and/or unlock a contiguous range of bytes in a regular file. More than one non-overlapping byte range can be locked and/or unlocked on an open file. Locks prevent attempts to lock, read, or write the locked portion of the file by other processes using a separate file handle (FID). Any process using the same FID specified in the request that obtained the lock has access to the locked bytes.

SMB_COM_LOCKING_ANDX (section is also used by the server to send OpLock break notification messages to the client, and by the client to acknowledge the OpLock break. This is the one instance in the CIFS Protocol in which the server sends a request.

The following are the commands that can follow an SMB_COM_LOCKING_ANDX (section in an AndX chain: