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NetBSD 6.1.5 - man page for flock (netbsd section 2)

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

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

     Standard C Library (libc, -lc)

     #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 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 EAGAIN 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 refer-
     ences to a single lock.  If a process holding a lock on a file forks and the child explic-
     itly 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:

     [EAGAIN]		The file is locked and the LOCK_NB option was specified.

     [EBADF]		The argument fd is an invalid descriptor.

     [EINVAL]		The argument operation does not include exactly one of LOCK_EX, LOCK_SH,
			or LOCK_UN.

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

     close(2), dup(2), execve(2), fork(2), open(2), flockfile(3), lockf(3)

     The flock() function call appeared in 4.2BSD.

BSD					 October 15, 2011				      BSD

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