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LOCKF(3)	       FreeBSD Library Functions Manual		      LOCKF(3)

     lockf -- record locking on	files

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     lockf(int fd, int function, off_t size);

     The lockf() function allows sections of a file to be locked with advi-
     sory-mode locks.  Calls to	lockf()	from other processes which attempt to
     lock the locked file section will either return an	error value or block
     until the section becomes unlocked.  All the locks	for a process are re-
     moved when	the process terminates.

     The argument fd is	an open	file descriptor.  The file descriptor must
     have been opened either for write-only (O_WRONLY) or read/write (O_RDWR)

     The function argument is a	control	value which specifies the action to be
     taken.  The permissible values for	function are as	follows:
	   Function   Description
	   F_ULOCK    unlock locked sections
	   F_LOCK     lock a section for exclusive use
	   F_TLOCK    test and lock a section for exclusive use
	   F_TEST     test a section for locks by other	processes

     F_ULOCK removes locks from	a section of the file; F_LOCK and F_TLOCK both
     lock a section of a file if the section is	available; F_TEST detects if a
     lock by another process is	present	on the specified section.

     The size argument is the number of	contiguous bytes to be locked or un-
     locked.  The section to be	locked or unlocked starts at the current off-
     set in the	file and extends forward for a positive	size or	backward for a
     negative size (the	preceding bytes	up to but not including	the current
     offset).  However,	it is not permitted to lock a section that starts or
     extends before the	beginning of the file.	If size	is 0, the section from
     the current offset	through	the largest possible file offset is locked
     (that is, from the	current	offset through the present or any future end-

     The sections locked with F_LOCK or	F_TLOCK	may, in	whole or in part, con-
     tain or be	contained by a previously locked section for the same process.
     When this occurs, or if adjacent locked sections would occur, the sec-
     tions are combined	into a single locked section.  If the request would
     cause the number of locks to exceed a system-imposed limit, the request
     will fail.

     F_LOCK and	F_TLOCK	requests differ	only by	the action taken if the	sec-
     tion is not available.  F_LOCK blocks the calling process until the sec-
     tion is available.	 F_TLOCK makes the function fail if the	section	is al-
     ready locked by another process.

     File locks	are released on	first close by the locking process of any file
     descriptor	for the	file.

     F_ULOCK requests release (wholly or in part) one or more locked sections
     controlled	by the process.	 Locked	sections will be unlocked starting at
     the current file offset through size bytes	or to the end of file if size
     is	0.  When all of	a locked section is not	released (that is, when	the
     beginning or end of the area to be	unlocked falls within a	locked sec-
     tion), the	remaining portions of that section are still locked by the
     process.  Releasing the center portion of a locked	section	will cause the
     remaining locked beginning	and end	portions to become two separate	locked
     sections.	If the request would cause the number of locks in the system
     to	exceed a system-imposed	limit, the request will	fail.

     An	F_ULOCK	request	in which size is non-zero and the offset of the	last
     byte of the requested section is the maximum value	for an object of type
     off_t, when the process has an existing lock in which size	is 0 and which
     includes the last byte of the requested section, will be treated as a re-
     quest to unlock from the start of the requested section with a size equal
     to	0.  Otherwise an F_ULOCK request will attempt to unlock	only the re-
     quested section.

     A potential for deadlock occurs if	a process controlling a	locked region
     is	put to sleep by	attempting to lock the locked region of	another
     process.  This implementation detects that	sleeping until a locked	region
     is	unlocked would cause a deadlock	and fails with an EDEADLK error.

     The lockf(), fcntl(2), and	flock(2) 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(2), any record
     within the	file will be seen as locked from the viewpoint of another
     process using fcntl(2) or lockf(),	and vice versa.

     Blocking on a section is interrupted by any signal.

     The lockf() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno	is set to indicate the
     error.  In	the case of a failure, existing	locks are not changed.

     The lockf() function will fail if:

     [EAGAIN]		The argument function is F_TLOCK or F_TEST and the
			section	is already locked by another process.

     [EBADF]		The argument fd	is not a valid open file descriptor.

			The argument function is F_LOCK	or F_TLOCK, and	fd is
			not a valid file descriptor open for writing.

     [EDEADLK]		The argument function is F_LOCK	and a deadlock is de-

     [EINTR]		The argument function is F_LOCK	and lockf() was	inter-
			rupted by the delivery of a signal.

     [EINVAL]		The argument function is not one of F_ULOCK, F_LOCK,

			The argument fd	refers to a file that does not support

     [ENOLCK]		The argument function is F_ULOCK, F_LOCK or F_TLOCK,
			and satisfying the lock	or unlock request would	result
			in the number of locked	regions	in the system exceed-
			ing a system-imposed limit.

     fcntl(2), flock(2)

     The lockf() function conforms to X/Open Portability Guide Issue 4,
     Version 2 ("XPG4.2").

FreeBSD	13.0		      September	11, 2013		  FreeBSD 13.0


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