lockf - apply, test or remove a POSIX lock on an open file
Standard C library (
libc,
-lc)
#include <unistd.h>
int lockf(int fd, int cmd, off_t len);
lockf():
_XOPEN_SOURCE >= 500
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 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
to indicate the error.
-
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.
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)