[Gambas-user] How to lock a file

[Rolf-Werner Eilert]

For my apps in Gambas, I use lockfiles, not folders, to lock a certain 
file. Experience told me that "sometimes" when two users/processes want 
to access the same file, these may overlap. So I added an identification 
method and a time delay in case of locks. This is what it looks like 
(kinda prototyped):

WHILE Exist(theLockFile) THEN

   OPEN theLockFile FOR READ ...
     LINE INPUT firstLine
   CLOSE ...

   IF firstLine = myOwnName THEN 'old lock file from myself:
     KILL theLockFile              'may be deleted, then proceed

   ELSE                          'lock file from someone else:
     WAIT one second

WEND

'now first of all, write my own lockfile
'then begin reading the file I want
'finally delete lockfile

This is how this looks in practice:

   WHILE Exist(pfad &/ "klassenbuch.block")
     OPEN pfad &/ "klassenbuch.block" FOR READ AS #dtnr
       LINE INPUT #dtnr, t$
     CLOSE #dtnr
     IF t$ = system.User THEN
       KILL pfad &/ "klassenbuch.block"
     ELSE
       WAIT 1
     END IF
   WEND


   OPEN pfad &/ "klassenbuch.block" FOR WRITE CREATE DIRECT AS #dtnr
     PRINT #dtnr, system.User
   CLOSE #dtnr

   OPEN karteiVerz &/ jg &/ "klassenbuch" FOR READ AS #dtnr

and so on. While I am writing this email, I find that this has one big 
drawback: If the lock file was made by someone else and that app 
crashed, my program will wait forever for the lockfile to disappear. So 
one should add a counter, e. g. 5 seconds, to stop waiting.

Regards

Rolf


nando schrieb:
> In some applications, I create a folder as a lock.
> The existance of the folder indicates 'locked'
> It works very well and is used through Samba to share
> the lock on both the linux and windows side.
> -Fernando
> 
> 
> ---------- Original Message -----------
> From: Benoit Minisini <gambas at ...1...>
> To: mailing list for gambas users <gambas-user at lists.sourceforge.net>
> Sent: Fri, 26 Sep 2008 23:51:57 +0200
> Subject: Re: [Gambas-user] How to lock a file
> 
>> On vendredi 26 septembre 2008, Almanova Sistemi wrote:
>>> I need to lock the file to avoid writing by another user in multiuser
>>> environment
>>>
>> Sorry for that, the documentation is completely false. But who wrote it? :-)
>>
>> The LOCK instruction does not lock a specific stream not a stream, but instead 
>> use a specific path to create a global system lock.
>>
>> You use it this way:
>>
>> DIM hFile AS File
>>
>> TRY hFile = LOCK "~/lock"
>> IF ERROR THEN
>>   PRINT "Already locked by something else!"
>> ELSE
>>   PRINT "Got locked!"
>>   ...
>>   UNLOCK hFile
>> ENDIF
>>
>> Do not rely on the lock file contents. It is truncated to zero byte when the 
>> lock is acquired.
>>
>> Regards,
>>
>> -- 
>> Benoit Minisini
>>