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<chapter id="locking">
<chapterinfo>
&author.jeremy;
&author.jelmer;
</chapterinfo>
<title>Locking</title>
<para>One area which sometimes causes trouble is locking.</para>
<para>There are two types of locking which need to be
performed by a SMB server. The first is "record locking"
which allows a client to lock a range of bytes in a open file.
The second is the "deny modes" that are specified when a file
is open.</para>
<para>Record locking semantics under Unix is 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 can not
be fully correct due to several reasons. The simplest
is the fact 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.</para>
<para>Samba 2.2 and above implements record locking
completely independent of the underlying unix
system. If a byte range lock that the client requests
happens to fall into the range 0-2^31, Samba hands
this request down to the Unix system. All other locks
can not be seen by unix anyway.</para>
<para>Strictly a 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. It is also almost always unnecessary as clients
are supposed to independently make locking calls before reads
and writes anyway 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" then it will
make lock checking calls on every read and write. </para>
<para>You can also disable by range locking completely
using "locking = no". This is useful for those shares that
don't support locking or don't need it (such as cdroms). In
this case Samba fakes the return codes of locking calls to
tell clients that everything is OK.</para>
<para>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.</para>
</chapter>
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