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

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

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/file.h>
     #define   LOCK_SH	      0x01	/* shared file lock */
     #define   LOCK_EX	      0x02	/* exclusive file lock */
     #define   LOCK_NB	      0x04	/* do not block	when locking */
     #define   LOCK_UN	      0x08	/* unlock file */

     int
     flock(int fd, int operation);

DESCRIPTION
     The flock() system	call applies or	removes	an advisory lock on the	file
     associated	with the file descriptor fd.  A	lock is	applied	by specifying
     an	operation argument 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-
     consistencies).

     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.

NOTES
     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.

RETURN VALUES
     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
     error.

ERRORS
     The flock() system	call fails if:

     [EWOULDBLOCK]	The file is locked and the LOCK_NB option was speci-
			fied.

     [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.

     [ENOLCK]		A lock was requested, but no locks are available.

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

HISTORY
     The flock() system	call appeared in 4.2BSD.

BSD			       November	9, 2011				   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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