Interoperation of flock(2), fcntl(2), and lockf(3)
yar at comp.chem.msu.su
Sat May 15 02:21:19 PDT 2004
I've always been confused by the following sentence from the lockf(3)
The lockf(), fcntl(2) and flock(2) locks may be safely used
Does that mean that each of those calls uses a locking mechanism
of its own? Of course, in practice those calls use a mutual
mechanism, thus allowing serial access to a file from applications
using different calls. However, there's an oddity: While it's
possible for a process to obtain the same lock several times w/o
error (it's a no-op case of upgrading the lock,) intermixing flock(2)
and fcntl(2), or flock(2) and lockf(3), within the same process
results in EAGAIN upon the second locking attempt. That's while
mixing fcntl(2) and lockf(3) is all right as long as the latter
call is just a wrapper for the former one. Of course, intermixing
different lock calls within one process is a poor idea at the first
place, but I can imagine some mail application that tries to coax
all the mailbox locking schemes at once.
Considering all the above, I'd like to add the following paragraph
to the flock(2), lockf(3), and fcntl(2) man pages (replacing the
sentence quoted from lockf(3)):
The flock(2), fcntl(2), and lockf(3) locks are compatible.
Processes using different locking interfaces can cooperate
over the same file safely. However, only one of such
interfaces should be used within a process. If a file is
locked by a process through flock(2), any record within the
file will be seen as locked from the viewpoint of another
process using fcntl(2) or lockf(3), and vice versa.
Any objections or comments?
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