NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | 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(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500
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.
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.
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.
SVr4, POSIX.1-2001.
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.)
This page is part of release 3.23 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/.
GNU 2009-07-25 LOCKF(3)