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FLOCK(2)                      System Calls Manual                     FLOCK(2)

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

       #include <&lt;sys/file.h>&gt;

       #define   LOCK_SH   1    /* shared lock */
       #define   LOCK_EX   2    /* exclusive lock */
       #define   LOCK_NB   4    /* don't block when locking */
       #define   LOCK_UN   8    /* unlock */

       int flock(fd, operation)
       int fd, operation;

       flock() applies or removes an advisory lock on the file associated with
       the file descriptor fd.  A lock is applied by specifying  an  operation
       parameter that is the inclusive OR of LOCK_SH or LOCK_EX and, possibly,
       LOCK_NB.  To unlock an existing lock, the operation should be LOCK_UN.

       Advisory locks allow cooperating processes to perform consistent opera-
       tions  on  files,  but do not guarantee exclusive access (that is, pro-
       cesses 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.  More than one process may hold a shared  lock  for  a
       file  at  any  given  time,  but multiple exclusive, or both shared and
       exclusive, locks may not exist simultaneously on a file.

       A shared lock may be upgraded to an exclusive  lock,  and  vice  versa,
       simply  by specifying the appropriate lock type; the previous lock will
       be 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 block 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.

       Locks are on files, not file descriptors.  That  is,  file  descriptors
       duplicated  through  dup(2V)  or  fork(2V))  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.

       Processes blocked awaiting a lock may be awakened by signals.

       flock() returns:

       0      on success.

       -1     on failure and sets errno to indicate the error.

       EBADF               The argument fd is an invalid descriptor.

       EOPNOTSUPP          The argument fd refers to an object  other  than  a

       EWOULDBLOCK         The file is locked and the LOCK_NB option was spec-

       close(2V),  dup(2V),   execve(2V),   fcntl(2V),   fork(2V),   open(2V),
       lockf(3), lockd(8C)

       Locks  obtained through the flock() mechanism are known only within the
       system on which they were placed.  Thus, multiple clients may  success-
       fully  acquire exclusive locks on the same remote file.  If this behav-
       ior is not explicitly desired, the fcntl(2V) or lockf(3)  system  calls
       should  be  used instead; these make use of the services of the network
       lock manager (see lockd(8C)).

                                21 January 1990                       FLOCK(2)