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RocketLink!--> Man page versions: OpenBSD FreeBSD NetBSD RedHat Others

FLOCK(2)                  OpenBSD Programmer's Manual                 FLOCK(2)

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

     #include <fcntl.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 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:

                   The file is locked and the LOCK_NB option was specified.

     [EBADF]       The argument fd is an invalid descriptor.

     [EINVAL]      The argument operation have 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() function call appeared in 4.2BSD.

OpenBSD 2.6                    December 11, 1993                             2

Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.

(Corrections, notes, and links courtesy of RocketAware.com)

[Detailed Topics]

[Overview Topics]

Up to: File Access Limits - Limiting access to files (permissions, locking, et al)

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