Re: Transitioning from existence-based lockfiles and /var/lock to flock
Russ Allbery wrote:
> Way back in the day, it used to matter whether you used flock or fcntl and
> one type of lock was potentially invisible to the other type of lock. Has
> this been fixed, or is that still a concern?
It's still a concern, in that those are two different kinds of locks in
most circumstances. (In the broader universe of POSIX it's even less
specified, but in the narrower universe of things Debian supports we can
primarily focus on fcntl and flock.)
I intended to specify in the full text more precisely that there are
multiple kinds of locks, and it's okay to use one or the other as long
as all cooperating software agrees.
Generally speaking, I think very few programs actually want to use the
byte-range locking that fcntl can do, and that in practice the options
usually boil down to either flock or whole-file fcntl, which are roughly
equivalent but unfortunately distinct.
The main advantage of flock, apart from simplicity, is that flock always
follows the file descriptor, while fcntl *normally* follows the process
(unless you use the non-portable "open file description locks", which
act like flock).
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