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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 <sys/file.h>
     #define   LOCK_SH	      0x01	/* shared file lock */
     #define   LOCK_EX	      0x02	/* exclusive file lock */
     #define   LOCK_NB	      0x04	/* don't block when locking */
     #define   LOCK_UN	      0x08	/* unlock file */

     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.

     The flock(), fcntl(2), and	lockf(3) locks are compatible.	Processes us-
     ing different locking interfaces can cooperate over the same file safely.
     However, only one of such interfaces should be used within	the same
     process.  If a file is locked by a	process	through	flock(), any record
     within the	file will be seen as locked from the viewpoint of another
     process using fcntl(2) or lockf(3), and vice versa.

     Processes blocked awaiting	a lock may be awakened by signals.

     The flock() function returns the value 0 if successful; 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 fd	refers to an object other than a file.

     [EOPNOTSUPP]	The argument fd	refers to an object that does not sup-
			port file locking.

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

     The flock() function call appeared	in 4.2BSD.

BSD			       December	11, 1993			   BSD


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