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FLOCK(2)		  FreeBSD 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	/* do not block	when locking */
     #define   LOCK_UN	      0x08	/* unlock file */

     flock(int fd, int operation);

     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

     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

     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
     included 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
     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.

     The flock(), fcntl(2), and	lockf(3) locks are compatible.	Processes
     using 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() system	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() system	call appeared in 4.2BSD.

FreeBSD	6.0		       December	11, 1993		   FreeBSD 6.0


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