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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 operations on files, but do not guar-antee guarantee
     antee consistency (i.e., processes may still access files without using advisory locks possibly result-ing resulting
     ing in inconsistencies).

     The locking mechanism allows two types of locks: shared locks and exclusive locks.  At any time multi-ple multiple
     ple shared locks may be applied to a file, but at no time are multiple exclusive, or both shared and
     exclusive, locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropri-ate appropriate
     ate lock type; this results in the previous lock being 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 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 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

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