dotlock(1) [debian man page]

dotlock(1)							Mail Avenger 0.8.3							dotlock(1)

NAME

       dotlock - execute a command with a lock on a mailbox

SYNOPSIS

       dotlock [-LPW] mbox-file command [arg ...]

DESCRIPTION

       dotlock acquires a lock on the mailbox file mbox-file using both flock and a lock file, then executes command with any arguments specified.
       When command exits, dotlock releases the lock.

       dotlock attempts to clean up stale lockfiles.  If it succeeds in locking an mbox-file with flock, and roughly 30 seconds elapse without
       there being any changes to mbox-file or the lockfile, then dotlock will delete the lockfile and try again.

       While it holds a lock, lockfile will keep updating the modification time of the lockfile every 15 seconds, to prevent the lock from getting
       cleaned up in the event that command is slow.

   OPTION
       --noflock (-L)
	   Ordinarily, dotlock uses both flock and dotfile locking.  (It uses flock first, but releases that lock in the even that dotfile locking
	   fails, so as to avoid deadlocking with applications that proceed in the reverse order.)  The -L option disables flock locking, so that
	   dotlock only uses dotfile locking.

	   This is primarily useful as a wrapper around an application that already does flock locking, but to which you want to add dotfile
	   locking.  (Even if your mail delivery system doesn't use flock, flock actually improves the efficiency of dotlock, so there is no
	   reason to disable it.)

       --fcntl (-P)
	   This option enables fcntl (a.k.a. POSIX) file locking of mail spools, in addition to flock and dotfile locking.  The advantage of fcntl
	   locking is that it may do the right thing over NFS.	However, if either the NFS client or server does not properly support fcntl
	   locking, or if the file system is not mounted with the appropriate options, fcntl locking can fail in one of several ways.  It can
	   allow different processes to lock the same file concurrently--even on the same machine.  It can simply hang when trying to acquire a
	   lock, even if no other process holds a lock on the file.  Also, on some OSes it can interact badly with flock locking, because those
	   OSes actually implement flock in terms of fcntl.

       --nowait (-W)
	   With this option, dotlock simply exits non-zero and does not run command if it cannot immediately acquire the lock.

SEE ALSO

       avenger(1), deliver(1), avenger.local(8)

       The Mail Avenger home page: <http://www.mailavenger.org/>.

BUGS

       dotlock does not perform fcntl/lockf-style locking by default.  Thus, if your mail reader exclusively uses fcntl for locking, there will be
       race conditions unless you specify the --fcntl option.

       flock does not work over network file systems.  Thus, because of dotlock's mechanism for cleaning stale lock files, there is a possibility
       that a network outage could lead to a race condition where the lockfile is cleared before command finishes executing.  If lockfile detects
       that the lock has been stolen, it prints a message to standard error, but does not do anything else (like try to kill command).

AUTHOR

       David Mazieres

Mail Avenger 0.8.3						    2012-04-05								dotlock(1)

FLOCK(2)						      BSD System Calls Manual							  FLOCK(2)

NAME

     flock -- apply or remove an advisory lock on an open file

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/file.h>
     #define   LOCK_SH	      0x01	/* shared file lock */
     #define   LOCK_EX	      0x02	/* exclusive file lock */
     #define   LOCK_NB	      0x04	/* do not block when locking */
     #define   LOCK_UN	      0x08	/* unlock file */

     int
     flock(int fd, int operation);

DESCRIPTION

     The flock() system call applies or removes an advisory lock on the file associated with the file descriptor fd.  A lock is applied by speci-
     fying an operation argument that is one of LOCK_SH or LOCK_EX with the optional addition of LOCK_NB.  To unlock an existing lock operation
     should be LOCK_UN.

     Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee consistency (i.e., processes may
     still access files without using advisory locks possibly resulting in inconsistencies).

     The locking mechanism allows two types of locks: shared locks and exclusive locks.  At any time multiple shared locks may be applied to a
     file, but at no time are multiple exclusive, or both shared and exclusive, locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate lock type; this results in the pre-
     vious lock being released and the new lock applied (possibly after other processes have gained and released the lock).

     Requesting a lock on an object that is already locked normally causes the caller to be blocked until the lock may be acquired.  If LOCK_NB is
     included in operation, then this will not happen; instead the call will fail and the error EWOULDBLOCK will be returned.

NOTES

     Locks are on files, not file descriptors.	That is, file descriptors duplicated 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 flock(), 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 the same process.  If a file is locked by a process through flock(), 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.

     Processes blocked awaiting a lock may be awakened by signals.

RETURN VALUES

     The flock() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The flock() system call fails if:

     [EWOULDBLOCK]	The file is locked and the LOCK_NB option was specified.

     [EBADF]		The argument fd is an invalid descriptor.

     [EINVAL]		The argument fd refers to an object other than a file.

     [EOPNOTSUPP]	The argument fd refers to an object that does not support file locking.

     [ENOLCK]		A lock was requested, but no locks are available.

SEE ALSO

     close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), flopen(3), lockf(3)

HISTORY

     The flock() system call appeared in 4.2BSD.

BSD
								 November 9, 2011							       BSD