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FLOCK(2)		  FreeBSD System Calls Manual		      FLOCK(2)

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

SYNOPSIS
     #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 */

     int
     flock(int fd, int operation);

DESCRIPTION
     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 opera-
     tions 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 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.

NOTES
     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.

IMPLEMENTATION NOTES
     In	the non-threaded library flock() is implemented	as the flock syscall.

     In	the threaded library, the flock	syscall	is assembled to
     _thread_sys_flock() and flock() is	implemented as a function which	locks
     fd	for read and write, then calls _thread_sys_flock().  Before returning,
     flock() unlocks fd.

RETURN VALUES
     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
     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 fd refers to an object	other than a file.

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

SEE ALSO
     close(2), dup(2), execve(2), fork(2), open(2)

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

4.2 Berkeley Distribution      December	11, 1993     4.2 Berkeley Distribution

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

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