Section (3) lockf
Name
lockf — apply, test or remove a POSIX lock on an open file
Synopsis
#include <unistd.h>
int
lockf( |
int fd, |
int cmd, | |
off_t len) ; |
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Note | ||||
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DESCRIPTION
Apply, test or remove a POSIX lock on a section of an open
file. The file is specified by fd
, a file descriptor open for
writing, the action by cmd
, and the section consists
of byte positions pos
..pos
+len
−1 if len
is positive, and pos
−len
..pos
−1 if len
is negative, where
pos
is the current
file position, and if len
is zero, the section
extends from the current file position to infinity,
encompassing the present and future end-of-file positions. In
all cases, the section may extend past current
end-of-file.
On Linux, lockf
() is just an
interface on top of fcntl(2) locking. Many
other systems implement lockf
()
in this way, but note that POSIX.1 leaves the relationship
between lockf
() and fcntl(2) locks unspecified.
A portable application should probably avoid mixing calls to
these interfaces.
Valid operations are given below:
F_LOCK
-
Set an exclusive lock on the specified section of the file. If (part of) this section is already locked, the call blocks until the previous lock is released. If this section overlaps an earlier locked section, both are merged. File locks are released as soon as the process holding the locks closes some file descriptor for the file. A child process does not inherit these locks.
F_TLOCK
-
Same as
F_LOCK
but the call never blocks and returns an error instead if the file is already locked. F_ULOCK
-
Unlock the indicated section of the file. This may cause a locked section to be split into two locked sections.
F_TEST
-
Test the lock: return 0 if the specified section is unlocked or locked by this process; return −1, set
errno
to EAGAIN (EACCES on some other systems), if another process holds a lock.
RETURN VALUE
On success, zero is returned. On error, −1 is
returned, and errno
is set
appropriately.
ERRORS
- EACCES or EAGAIN
-
The file is locked and
F_TLOCK
orF_TEST
was specified, or the operation is prohibited because the file has been memory-mapped by another process. - EBADF
-
fd
is not an open file descriptor; orcmd
isF_LOCK
orF_TLOCK
andfd
is not a writable file descriptor. - EDEADLK
-
The command was
F_LOCK
and this lock operation would cause a deadlock. - EINTR
-
While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).
- EINVAL
-
An invalid operation was specified in
cmd
. - ENOLCK
-
Too many segment locks open, lock table is full.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
lockf () |
Thread safety | MT-Safe |
SEE ALSO
locks.txt
and
mandatory-locking.txt
in the
Linux kernel source directory Documentation/filesystems
(on older
kernels, these files are directly under the Documentation
directory, and mandatory-locking.txt
is
called mandatory.txt
)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright 1997 Nicolás Lichtmaier <nickdebian.org> Created Thu Aug 7 00:44:00 ART 1997 %%%LICENSE_START(GPLv2+_DOC_FULL) This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License_zsingle_quotesz_s references to object code and executables are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, see <http://www.gnu.org/licenses/>. %%%LICENSE_END Added section stuff, aeb, 2002-04-22. Corrected include file, drepper, 2003-06-15. |