File locking problems

malcolm malcolm at
Thu May 19 12:13:33 GMT 2005

I have been writing some code to perform portable file locking between Linux 
and Windows. On Windows I use UnlockFileEx() and Linux fcntl(). To get over 
the incompatibilites between these calls, I use the calls to lock the first 
two bytes of the file with a read lock. The first byte is regarded as meaning 
that the file is read locked and the second byte that the file is write 
locked. The Linux side is routine but the Windows side is somewhat more 
difficult. If I want a read lock I test if I can get a write lock on the 
second byte. If successful then I release the lock obtained and set a read 
lock on the first byte. To obtain a write lock, I test whether I can set a 
write lock on both bytes, release this write lock and set a read lock on the 
second byte.

This works correctly for a samba share for the cases when client 1 and client 
2 are either both Linux or both Windows, or client 1 is Linux and client 2 is 
Windows. But if the order is changed and client 1 is Windows and client 2 is 
Linux, the open() call on Linux fails with EAGAIN. So I tried putting the 
open() call in a "for ( tries = 0 ; tries  < 100 : tries++ )" loop with a 
wait of usleep( 100 ) after each open() attempt. And bingo, this works with 
about 4 or 5 loops. Untill I discovered that when the delay between starting 
client 1 (Windows) and client 2 (Linux) is more than a couple of minutes 
(rather than seconds) then the open loop usually fails even after 1000 

In smb.conf I have:
	locking = Yes
        posix locking = Yes
        strict locking = No
        kernel oplocks = Yes
        blocking locks = No

Changing any of these means that nothing works.

The version of SAMBA is:
	Version 3.0.13-1.1-SUSE
Windows is XP Proffesional

Any help gratefully received.

Malcolm Agnew

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