LOCKF
Section: Linux Programmer's Manual (3)
Updated: 2010-09-20
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);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
lockf():
-
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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.1-2001 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
or
F_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.
- EDEADLK
-
The command was
T_LOCK
and this lock operation would cause a deadlock.
- EINVAL
-
An invalid operation was specified in
fd.
- ENOLCK
-
Too many segment locks open, lock table is full.
CONFORMING TO
SVr4, POSIX.1-2001.
SEE ALSO
fcntl(2),
flock(2)
There are also
locks.txt
and
mandatory-locking.txt
in the 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 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:35:28 GMT, March 26, 2013