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

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

     #include <&lt;fcntl.h>&gt;

     flock(int fd, int operation);

     flock() applies or removes an advisory lock on the file associated with
     the file descriptor fd.  The operation argument is one of:

           LOCK_SH  Apply a shared lock.
           LOCK_EX  Apply an exclusive lock.
           LOCK_UN  Remove an existing lock.

     LOCK_SH and LOCK_EX may be combined with the optional LOCK_NB for non-
     blocking mode.

     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

     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 operation is the
     bitwise OR of LOCK_NB and LOCK_SH or LOCK_EX, 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.

     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     The flock() call fails if:

     [EWOULDBLOCK]      The file is locked and the LOCK_NB option was speci-

     [EBADF]            The argument fd is an invalid descriptor.

     [EINVAL]           The argument operation has an invalid value.

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

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

     The flock() system call first appeared in 4.1cBSD.

BSD                            December 10, 2014                           BSD