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FLOCK(2)                    BSD Programmer's 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 */

     flock(int fd, int 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 pa-
     rameter 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 opera-
     tions on files, but do not guarantee consistency (i.e., processes may
     still access files without using advisory locks possibly resulting in in-

     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 exclu-
     sive, locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice versa, sim-
     ply by specifying the appropriate lock type; this results in the previous
     lock being released and the new lock applied (possibly after other pro-
     cesses 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 in-
     cluded 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 du-
     plicated 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.

     Processes blocked awaiting a lock may be awakened by signals.

     Zero is returned if the operation was successful; on an error a -1 is re-
     turned and an error code is left in the global location errno.

     The flock() 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.

     open(2),  close(2),  dup(2),  execve(2),  fork(2)

     The flock function call appeared in 4.2BSD.

4.2 Berkeley Distribution      December 11, 1993                             2