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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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lockf(3) Library Functions Manual lockf(3)
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 op, off_t size);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
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 op, and the section consists of byte positions
pos..pos+size-1 if size is positive, and pos-size..pos-1 if size
is negative, where pos is the current file position, and if size
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 op is F_LOCK or
F_TLOCK and fd is not a writable file descriptor.
EDEADLK
op 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 op.
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.1-2008.
POSIX.1-2001, 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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 lockf(3)
Pages that refer to this page: fcntl(2), fcntl_locking(2), flock(2), flockfile(3), off_t(3type), lslocks(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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