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

NAME
     lockf -- record locking on	files

SYNOPSIS
     #include <unistd.h>

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

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

     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.

     The F_ULOCK function 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-
     of-file).

     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.

     The F_LOCK	and F_TLOCK requests differ only by the	action taken if	the
     section is	not available.	F_LOCK blocks the calling process until	the
     section is	available.  F_TLOCK makes the function fail if the section is
     already 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) of one or more locked	sec-
     tions controlled by the process.  Locked sections will be unlocked	start-
     ing at the	current	file offset through size bytes or to the end of	the
     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 section), 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 sepa-
     rate 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.

     lockf(), fcntl(2),	and flock(2) locks may be safely used concurrently.

     Blocking on a section is interrupted by any signal.

RETURN VALUES
     If	successful, the	lockf()	function returns 0.  Otherwise,	it returns -1,
     sets the global variable errno to indicate	an error, and existing locks
     are not changed.

ERRORS
     lockf() 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 filedes is	not a valid open file descrip-
			tor.

			The argument function is F_LOCK	or F_TLOCK, and
			filedes	is not a valid file descriptor open for	writ-
			ing.

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

     [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,
			F_TLOCK, or F_TEST.

			The argument filedes refers to a file that does	not
			support	locking.

     [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.

SEE ALSO
     fcntl(2), flock(2)

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

FreeBSD	13.0			 June 5, 2013			  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS

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