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

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

     #include <sys/file.h>
     #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
     parameter 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 operations
     tions on files, but do not guarantee consistency (i.e., processes may
     still access files without using advisory locks possibly resulting 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, exclusive,
     sive, locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice versa, sim-ply simply
     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 processes
     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
     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(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
     returned 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 speci-fied. specified.

     [EBADF]            The argument fd is an invalid descriptor.

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

     [ENOTSUP]          The referenced descriptor is not of the correct type.

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

     The flock() function call appeared in 4.2BSD.

4.2 Berkeley Distribution      December 11, 1993     4.2 Berkeley Distribution