NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

     #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 */

     int
     flock(int fd, int operation);

DESCRIPTION    [Toc]    [Back]

     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 guarantee consistency (i.e., processes may
     still access files without using advisory locks possibly resulting in inconsistencies).


     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 exclusive,
 locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice
versa, simply
 by specifying the appropriate 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.

NOTES    [Toc]    [Back]

     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.

RETURN VALUES    [Toc]    [Back]

     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.

ERRORS    [Toc]    [Back]

     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 operation has an invalid value.

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

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

     The flock() function call appeared in 4.2BSD.

OpenBSD      3.6                        December     11,     1993