NAME
with-lock-ex - file locker
SYNOPSIS
with-lock-ex -w|-q|-f lockfile command args ...
DESCRIPTION
with-lock-ex will open and lock the lockfile for writing and then feed
the remainder of its arguments to exec(2); when that process terminates
the fd will be closed and the file unlocked automatically by the
kernel.
If the file does not exist it is created, with permissions rw for each
user class for which the umask has w.
OPTIONS
-w Wait for the lock to be available.
-f Fail (printing a message to stderr and exiting 255) if the lock
cannot be acquired immediately because another process has it.
-q Silently do nothing (ie, exit 0 instead of executing the
specified process) if the lock cannot be acquired immediately
because another process has it.
STALE LOCKS
The locking protocol used does not suffer from stale locks. If the
lock cannot be acquired, one or more running processes must currently
hold the lock; if the lock needs to be freed those processes should be
killed.
Under no circumstances should ‘stale lock cleaner’ cron jobs, or the
like, be instituted. In systems where a great many locks may exist,
old lockfiles may be removed from cron but only if each lock is
acquired before the lockfile is removed, for example with
with-lock-ex -q lockfile rm lockfile
DEADLOCKS
There is no deadlock detection. In a system with several locks, a lock
hierarchy should be established, such that for every pair of locks A
and B which a process might lock simultaneously, either A>B or B>A
where the relation > is transitive and noncyclic.
Then, for any two locks X and Y with X>Y it is forbidden to acquire X
while holding Y. Instead, acquire X first, or release Y before
(re)acquiring X and Y in that order.
(There are more complicated ways of avoiding deadlocks, but a lock
hierarchy is simple to understand and implement. If it does not meet
your needs, consult the literature.)
LOCKING PROTOCOL
The locking protocol used by with-lock-ex is as follows:
The lock is held by a process (or group of processes) which holds an
fcntl exclusive lock on the first byte of the plain file which has the
specified name. A holder of the lock (and only a holder of the lock)
may delete the file or change the inode to which the name refers, and
as soon as it does so it ceases to hold the lock.
Any process may create the file if it does not exist. There is no need
for the file to contain any actual data. Indeed, actually using the
file for data storage is strongly disrecommended, as this will
foreclose most strategies for reliable update. Use a separate lockfile
instead.
Ability to obtain the lock corresponds to write permission on the file
(and of course permission to create the file, if it does not already
exist). However, processes with only read permission on the file can
prevent the lock being acquired at all; therefore lockfiles should not
usually be world-readable.
When a (group of) processes wishes to acquire the lock, it should open
the file (with O_CREAT) and lock it with fcntl(2) F_RWLCK, operation
F_SETLK or F_SETLKW. If this succeeds it should fstat the file
descriptor it has, and the file by its path. If the device and inode
match then the lock has been acquired and remains acquired until that
group of processes changes which file the name refers to, deletes the
file, or releases the fcntl lock. If they do not then another process
acquired the lock and deleted the file in the meantime; you must now
close your filedescriptor and start again. with-lock-ex follows this
specification.
Note that flock(2) is a different kind of lock to fcntl(2). with-lock-
ex uses fcntl.
AUTHOR
This Manual page was written by Matthew Vernon <matthew@debian.org> and
enhanced by Ian Jackson <ian@chiark.greenend.org.uk>, in 2003, but may
be used by anyone.
COPYRIGHT
with-lock-ex was written by Ian Jackson <ian@chiark.greenend.org.uk> in
1993, 1994, 1995, 1996, 1998, 1999. He has placed it in the public
domain.
SEE ALSO
fcntl(2), flock(2), chmod(2)