Section: Linux Programmer's Manual (3)
lockf - apply, test or remove a POSIX lock on an open file
int lockf(int fd, int cmd, off_t len);
Feature Test Macro Requirements for glibc (see
_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
a file descriptor open for writing, the action by
and the section consists of byte positions
is positive, and
is negative, where
is the current file position, and if
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.
is just an interface on top of
Many other systems implement
in this way, but note that POSIX.1 leaves the relationship between
A portable application should probably avoid mixing calls
to these interfaces.
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 these locks.
but the call never blocks and returns an error instead if the file is
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
on some other systems),
if another process holds a lock.
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
- EACCES or EAGAIN
The file is locked and
was specified, or the operation is prohibited because the file has
been memory-mapped by another process.
is not an open file descriptor; or
is not a writable file descriptor.
The command was
and this lock operation would cause a deadlock.
While waiting to acquire a lock, the call was interrupted by
delivery of a signal caught by a handler; see
An invalid operation was specified in
Too many segment locks open, lock table is full.
For an explanation of the terms used in this section, see
POSIX.1-2001, POSIX.1-2008, SVr4.
in the Linux kernel source directory
(on older kernels, these files are directly under the
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