|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | SEE ALSO | COLOPHON |
LOCKF(3) Linux Programmer's Manual LOCKF(3)
lockf - apply, test or remove a POSIX lock on an open file
#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 versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
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.
EINVAL An invalid operation was specified in cmd.
ENOLCK Too many segment locks open, lock table is full.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────┼───────────────┼─────────┤
│lockf() │ Thread safety │ MT-Safe │
└──────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008, SVr4.
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)
This page is part of release 4.12 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/.
GNU 2016-03-15 LOCKF(3)
Pages that refer to this page: fcntl(2), flock(2), flockfile(3), lslocks(8)