lockf - apply, test or remove a POSIX lock on an open file
int lockf(int fd, int cmd, off_t
Feature Test Macro Requirements for glibc (see feature_test_macros
_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
, and the section consists of byte positions
-1 if len
is positive, and
-1 if len
is negative, where
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
Many other systems implement lockf
() in this way, but note that POSIX.1
leaves the relationship between lockf
() and fcntl
unspecified. A portable application should probably avoid mixing calls to
Valid operations are given below:
- 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
- Same as F_LOCK but the call never blocks and returns an error
instead if the file is already locked.
- Unlock the indicated section of the file. This may cause a locked section
to be split into two locked sections.
- 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
- 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
- fd is not an open file descriptor; or cmd is F_LOCK
or F_TLOCK and fd is not a writable file descriptor.
- The command was F_LOCK and this lock operation would cause a
- While waiting to acquire a lock, the call was interrupted by delivery of a
signal caught by a handler; see signal(7).
- An invalid operation was specified in cmd.
- Too many segment locks open, lock table is full.
For an explanation of the terms used in this section, see attributes
POSIX.1-2001, POSIX.1-2008, SVr4.
in the Linux kernel source
(on older kernels, these files are
directly under the Documentation
is called mandatory.txt