Date: Wed, 21 Apr 1999 15:51:24 +0200 From: Graham Wheeler <gram@cdsec.com> To: Ladavac Marino <mladavac@metropolitan.at>, hackers@freebsd.org Subject: Re: File locking problem Message-ID: <371DD7DC.4B5356E@cdsec.com> References: <55586E7391ACD211B9730000C11002761795EF@r-lmh-wi-100.corpnet.at>
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Ladavac Marino wrote:
> [ML] Please disregard my second "solution". It won't work (the
> second open will create the file if you specify O_CREAT). Talking about
> that, the real solution might be to open the file read-write without
> O_CREAT flag--the open will fail if the file was unlinked between dir
> read and open.
That is the solution. I have a routine:
FILE *OpenExclusive(const char *path, const char *mode);
This routine maps normal fopen() style modes to open() style flags,
does the open() with locking, and then an fdopen().
The code that uses it was quite correctly passing "r+" through as the
mode, but the OpenExclusive routine was incorrectly setting the
O_CREAT flag in this case. After checking Stephens' APitUE fig 5.3
I realised that this was wrong. Removing the O_CREAT solved the
problem.
I've been using the OpenExclusive routine for a long time, and have
never had problems with it before (probably because this is the
first time I've used it with the "r+" mode, and also because the
problem actually occurs so intermittently and I couldn't reliably
replicate it), so I wasted a couple of days poring
over the (fairly complex) code of the process that uses it, and
couldn't find anything wrong (so I posted the problem). I figured
I was misunderstanding file locking in the context of multiple
processes. I'm glad that that wasn't the problem, or I would have had to
implement my locks using UUCP style `semafiles'.
Thanks to everyone for their suggestions.
--
Dr Graham Wheeler E-mail: gram@cdsec.com
Citadel Data Security Phone: +27(21)423-6065/6/7
Firewalls/Virtual Private Networks Fax: +27(21)24-3656
Internet/Intranet Network Specialists
Data Security Products WWW: http://www.cdsec.com/
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