Re: flock

On 2006-11-21 at 00:56:31 [+0100], Axel Dörfler <[email protected]> 
wrote:

I had a bit of trouble with receiving mail recently, so the reply's a bit 
late. But I had worse ones... ;-)

> "François Revol" <[email protected]> wrote:
> > > a while ago, I implemented advisory locking support to the VFS.
> > > However, I didn't add the non-POSIX flock() function; this locking
> > > functionality is only available using the POSIX defined fcntl(...,
> > > F_SETLK, ...) method.
> > > Should we add flock() as well even though it's not part of POSIX?
> > Beware, though fcntl(F_SETLK) uses a struct flock, this one has
> > nothing
> > to do and is not compatible with it, and has a different semantic
> > (for
> > ex one removes the lock on exec or a close, the other doesn't). I
> > scratched my head several days on them.
> > This one is a BSDism, but I think several apps wants it (samba ?).
> 
> We should collect the differences and implement flock() and F_SETLK
> correctly, then.

It's actually as simple as this: flock() locks are bound to file descriptors 
numbers, fcntl(F_SETLK) locks to file descriptors. Meaning, if you acquire a 
lock on FD number 4 via flock(), dup2() 4 to 8, you can't unlock 8. And after 
fork() the child process won't have a lock on FD 4 either. fcntl(F_SETLK) has 
the expected semantics with respect to dup()/fork().

> If possible, I would prefer to have flock() done via F_SETLK, and if
> it's only the COE behaviour, I'm sure we can fix that :)

I suppose it should be possible, I only wonder, if it's worth the trouble. At 
least the automake/autoconf infected packages using file locking I've seen so 
far usually support both methods, fcntl(F_SETLK) being the preferred one.

CU, Ingo

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