There are two types of locking that need to be performed by an SMB server. The first is record locking that allows a client to lock a range of bytes in an open file. The second is the deny modes that are specified when a file is open.

Record locking semantics under UNIX are very different from record locking under Windows. Versions of Samba before 2.2 have tried to use the native fcntl() UNIX system call to implement proper record locking between different Samba clients. This cannot be fully correct for several reasons. The simplest is that a Windows client is allowed to lock a byte range up to 2^32 or 2^64, depending on the client OS. The UNIX locking only supports byte ranges up to 2^31. So it is not possible to correctly satisfy a lock request above 2^31. There are many more differences, too many to be listed here.

Samba 2.2 and above implement record locking completely independently of the underlying UNIX system. If a byte-range lock that the client requests happens to fall into the range of 0 to 2^31, Samba hands this request down to the UNIX system. No other locks can be seen by UNIX, anyway.

Strictly speaking, an SMB server should check for locks before every read and write call on a file. Unfortunately, with the way fcntl() works, this can be slow and may overstress the rpc.lockd . This is almost always unnecessary because clients are independently supposed to make locking calls before reads and writes if locking is important to them. By default, Samba only makes locking calls when explicitly asked to by a client, but if you set strict locking = yes, it will make lock checking calls on every read and write call.

You can also disable byte-range locking completely by using locking = no. This is useful for those shares that do not support locking or do not need it (such as CD-ROMs). In this case, Samba fakes the return codes of locking calls to tell clients that everything is okay.

The second class of locking is the deny modes . These are set by an application when it opens a file to determine what types of access should be allowed simultaneously with its open. A client may ask for DENY_NONE, DENY_READ, DENY_WRITE, or DENY_ALL. There are also special compatibility modes called DENY_FCB and DENY_DOS.