Re: Problem using mandatory locks (other apps can read/delete etc)

["E. Abbink" <esger@bumblebeast.com>]

> On 10 April 2002 15:08, E. Abbink wrote:
> > I'm trying to solve a problem using mandatory locks but am having some
> > difficulty in doing so. (if there's a more appropriate place for
> > discussing this please ignore the rest of this post. pointers to that
> > place would be appreciated ;) )
> >
> > my problem:
> >
> > when I lock a file with a mandatory write lock (ie. fcntl, +s-x bits and
> > mand mount option. for code see below) it is still possible:
> >
> > - for me to rm the file in question
> > - for the file to be read by an other process
> 
> [snip]
> 
> >     lock.l_type = F_WRLCK ;   <================
> >     lock.l_whence = SEEK_SET ;
> >     lock.l_start = 0 ;
> >     lock.l_len = 0 ;
> >     lock.l_pid = 0 ; // ignored
> >
> >     int err = fcntl (fd, F_SETLK, &lock) ;
> 
> I know nothing about file locking in Unix, but it looks like you
> requested write lock, i.e. forbid writing to a file. Why are you
> surprised that reads are allowed?
> 
> Probably someone else would comment on why rm is working, though...
> --
> vda

as i understand it what is called a "write" lock is actually an
exclusive lock (ie both read/write). Also, afaik, you cant set both a
read & write lock through fcntl which supports that assumption.
 
Esger

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