Re: On file locking

Curt Sampson <cjs(at)cynic(dot)net> wrote:

> At any rate, it seems to me highly unlikely that, since the child has
> the *same* descriptor as the parent had, that the lock would
> disappear.

It depends on the lock function. After fork():

o with flock() the lock continues to be held, but will be unlocked
if any child process explicitly unlocks it

o with fcntl() the lock is not inherited in the child

o with lockf() the standards and manual pages don't say

Boring reference material follows.

flock
=====

From the NetBSD manual page:

NOTES
Locks are on files, not file descriptors. That is, file descriptors du-
plicated through dup(2) or fork(2) do not result in multiple instances of
a lock, but rather multiple references to a single lock. If a process
holding a lock on a file forks and the child explicitly unlocks the file,
the parent will lose its lock.

The Red Hat Linux 8.0 manual page has similar wording. (No standards
to check here -- flock() is not standardised in POSIX, X/Open, Single
Unix Standard, ...)

fcntl
=====

The NetBSD manual page notes that these locks are not inherited by
child processes:

Another minor semantic problem with this interface is that locks
are not inherited by a child process created using the fork(2)
function.

Ditto the Single Unix Standard versions 2 and 3.

lockf()
=======

The standards and manual pages that I've checked don't discuss
fork() in relation to lockf(), which seems a peculiar ommission
and makes me suspect that behaviour has varied historically.

In practice I would expect lockf() semantics to be the same as
fcntl().

Regards,

Giles