lbdb_dotlock - Lock mail spool files.

       Thomas Roessler <roessler@guug.de>

Unix                                              JANUARY 2016                                        dotlock(1)

dotlock  implements  the traditional mail spool file locking method: To lock file, a file named file.lock
       is created.

dotlock gives all diagnostics in its return values:

       0-DL_EX_OK
              The program was successful.

       1-DL_EX_ERROR
              An  unspecified  error such as bad command line parameters, lack of system memory and the like has
              occurred.

       3-DL_EX_EXIST
              The user wants to lock a file which has been locked by another process  already.   If  dotlock  is
              invoked  with  the  -f  command  line  option,  dotlock won't generate this error, but break other
              processes' locks.

       4-DL_EX_NEED_RPIVS
              This return value only occurs if dotlock has been invoked with the -t  command  line  option.   It
              means that dotlock will have to use its group mail privileges to lock file.

       5-DL_EX_IMPOSSIBLE
              This  return  value  only  occurs if dotlock has been invoked with the -t command line option.  It
              means that dotlock is unable to lock file.

file.lock
              The lock file dotlock generates.

dotlock is part of the Mutt mail user agent package.  It has been created  to  avoid  running  mutt  with
       group mail privileges.

       lbdb_dotlock - Lock mail spool files.

dotlock tries to implement an NFS-safe dotlocking method which was borrowed from lockfile(1).

       If the user can't open file for reading with his normal privileges, dotlock will return  the  DL_EX_ERROR
       exit  value  to  avoid  certain  attacks against other users' spool files. The code carefully avoids race
       conditions when checking permissions; for details of all this see the comments in dotlock.c.

       -t     Just try.  dotlock won't actually lock a file, but inform the invoking  process  if  it's  at  all
              possible to lock file.

       -f     Force  the  lock.   If  another process holds a lock on file longer than a certain amount of time,
              dotlock will break that lock by removing the lockfile.

       -u     Unlock.  dotlock will remove file.lock.

       -d     Delete.  dotlock will lock file, remove it if it has length 0, and afterwards remove file.lock.

       -r retries
              This command line option tells dotlock to try locking  retries  times  before  giving  up  or  (if
              invoked with the -f command line option) break a lock.  The default value is 5.  dotlock waits one
              second between successive locking attempts.

fcntl(2), flock(2), lockfile(1), lbdb-fetchaddr(1)

lbdb_dotlock [-t|-f|-u|-d] [-r retries] file