LOCKF
Section: Linux Programmer's Manual (3)
Updated: 2021-03-22
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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():
_XOPEN_SOURCE >= 500
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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 to indicate the error.
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; or
cmd
is
F_LOCK
or
F_TLOCK
and
fd
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
|
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
SEE ALSO
fcntl(2),
flock(2)
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 5.11 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:45 GMT, May 09, 2021