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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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fcntl_locking(2) System Calls Manual fcntl_locking(2)
F_GETLK, F_SETLK, F_SETLKW, F_OFD_GETLK, F_OFD_SETLK, F_OFD_SETLKW
- locking
Standard C library (libc, -lc)
#include <fcntl.h>
int fcntl(int fd, F_GETLK, struct flock *lock);
int fcntl(int fd, F_SETLK, const struct flock *lock);
int fcntl(int fd, F_SETLKW, const struct flock *lock);
int fcntl(int fd, F_OFD_GETLK, struct flock *lock);
int fcntl(int fd, F_OFD_SETLK, const struct flock *lock);
int fcntl(int fd, F_OFD_SETLKW, const struct flock *lock);
Advisory record locking
Linux implements traditional ("process-associated") UNIX record
locks, as standardized by POSIX. For a Linux-specific alternative
with better semantics, see the discussion of open file description
locks below.
F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release, and
test for the existence of record locks (also known as byte-range,
file-segment, or file-region locks). The third argument, lock, is
a pointer to a structure that has at least the following fields
(in unspecified order).
struct flock {
...
short l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(set by F_GETLK and F_OFD_GETLK) */
...
};
The l_whence, l_start, and l_len fields of this structure specify
the range of bytes we wish to lock. Bytes past the end of the
file may be locked, but not bytes before the start of the file.
l_start is the starting offset for the lock, and is interpreted
relative to either: the start of the file (if l_whence is
SEEK_SET); the current file offset (if l_whence is SEEK_CUR); or
the end of the file (if l_whence is SEEK_END). In the final two
cases, l_start can be a negative number provided the offset does
not lie before the start of the file.
l_len specifies the number of bytes to be locked. If l_len is
positive, then the range to be locked covers bytes l_start up to
and including l_start+l_len-1. Specifying 0 for l_len has the
special meaning: lock all bytes starting at the location specified
by l_whence and l_start through to the end of file, no matter how
large the file grows.
POSIX.1-2001 allows (but does not require) an implementation to
support a negative l_len value; if l_len is negative, the interval
described by lock covers bytes l_start+l_len up to and including
l_start-1. This is supported since Linux 2.4.21 and Linux 2.5.49.
The l_type field can be used to place a read (F_RDLCK) or a write
(F_WRLCK) lock on a file. Any number of processes may hold a read
lock (shared lock) on a file region, but only one process may hold
a write lock (exclusive lock). An exclusive lock excludes all
other locks, both shared and exclusive. A single process can hold
only one type of lock on a file region; if a new lock is applied
to an already-locked region, then the existing lock is converted
to the new lock type. (Such conversions may involve splitting,
shrinking, or coalescing with an existing lock if the byte range
specified by the new lock does not precisely coincide with the
range of the existing lock.)
F_SETLK
Acquire a lock (when l_type is F_RDLCK or F_WRLCK) or
release a lock (when l_type is F_UNLCK) on the bytes
specified by the l_whence, l_start, and l_len fields of
lock. If a conflicting lock is held by another process,
this call returns -1 and sets errno to EACCES or EAGAIN.
(The error returned in this case differs across
implementations, so POSIX requires a portable application
to check for both errors.)
F_SETLKW
As for F_SETLK, but if a conflicting lock is held on the
file, then wait for that lock to be released. If a signal
is caught while waiting, then the call is interrupted and
(after the signal handler has returned) returns immediately
(with return value -1 and errno set to EINTR; see
signal(7)).
F_GETLK
On input to this call, lock describes a lock we would like
to place on the file. If the lock could be placed, fcntl()
does not actually place it, but returns F_UNLCK in the
l_type field of lock and leaves the other fields of the
structure unchanged.
If one or more incompatible locks would prevent this lock
being placed, then fcntl() returns details about one of
those locks in the l_type, l_whence, l_start, and l_len
fields of lock. If the conflicting lock is a traditional
(process-associated) record lock, then the l_pid field is
set to the PID of the process holding that lock. If the
conflicting lock is an open file description lock, then
l_pid is set to -1. Note that the returned information may
already be out of date by the time the caller inspects it.
In order to place a read lock, fd must be open for reading. In
order to place a write lock, fd must be open for writing. To
place both types of lock, open a file read-write.
When placing locks with F_SETLKW, the kernel detects deadlocks,
whereby two or more processes have their lock requests mutually
blocked by locks held by the other processes. For example,
suppose process A holds a write lock on byte 100 of a file, and
process B holds a write lock on byte 200. If each process then
attempts to lock the byte already locked by the other process
using F_SETLKW, then, without deadlock detection, both processes
would remain blocked indefinitely. When the kernel detects such
deadlocks, it causes one of the blocking lock requests to
immediately fail with the error EDEADLK; an application that
encounters such an error should release some of its locks to allow
other applications to proceed before attempting regain the locks
that it requires. Circular deadlocks involving more than two
processes are also detected. Note, however, that there are
limitations to the kernel's deadlock-detection algorithm; see
BUGS.
As well as being removed by an explicit F_UNLCK, record locks are
automatically released when the process terminates.
Record locks are not inherited by a child created via fork(2), but
are preserved across an execve(2).
Because of the buffering performed by the stdio(3) library, the
use of record locking with routines in that package should be
avoided; use read(2) and write(2) instead.
The record locks described above are associated with the process
(unlike the open file description locks described below). This
has some unfortunate consequences:
• If a process closes any file descriptor referring to a file,
then all of the process's locks on that file are released,
regardless of the file descriptor(s) on which the locks were
obtained. This is bad: it means that a process can lose its
locks on a file such as /etc/passwd or /etc/mtab when for some
reason a library function decides to open, read, and close the
same file.
• The threads in a process share locks. In other words, a
multithreaded program can't use record locking to ensure that
threads don't simultaneously access the same region of a file.
Open file description locks solve both of these problems.
Open file description locks (non-POSIX)
Open file description locks are advisory byte-range locks whose
operation is in most respects identical to the traditional record
locks described above. This lock type is Linux-specific, and
available since Linux 3.15. (There is a proposal with the Austin
Group to include this lock type in the next revision of POSIX.1.)
For an explanation of open file descriptions, see open(2).
The principal difference between the two lock types is that
whereas traditional record locks are associated with a process,
open file description locks are associated with the open file
description on which they are acquired, much like locks acquired
with flock(2). Consequently (and unlike traditional advisory
record locks), open file description locks are inherited across
fork(2) (and clone(2) with CLONE_FILES), and are only
automatically released on the last close of the open file
description, instead of being released on any close of the file.
Conflicting lock combinations (i.e., a read lock and a write lock
or two write locks) where one lock is an open file description
lock and the other is a traditional record lock conflict even when
they are acquired by the same process on the same file descriptor.
Open file description locks placed via the same open file
description (i.e., via the same file descriptor, or via a
duplicate of the file descriptor created by fork(2), dup(2),
F_DUPFD(2const), and so on) are always compatible: if a new lock
is placed on an already locked region, then the existing lock is
converted to the new lock type. (Such conversions may result in
splitting, shrinking, or coalescing with an existing lock as
discussed above.)
On the other hand, open file description locks may conflict with
each other when they are acquired via different open file
descriptions. Thus, the threads in a multithreaded program can
use open file description locks to synchronize access to a file
region by having each thread perform its own open(2) on the file
and applying locks via the resulting file descriptor.
As with traditional advisory locks, the third argument to fcntl(),
lock, is a pointer to an flock structure. By contrast with
traditional record locks, the l_pid field of that structure must
be set to zero when using the operations described below.
The operations for working with open file description locks are
analogous to those used with traditional locks:
F_OFD_SETLK
Acquire an open file description lock (when l_type is
F_RDLCK or F_WRLCK) or release an open file description
lock (when l_type is F_UNLCK) on the bytes specified by the
l_whence, l_start, and l_len fields of lock. If a
conflicting lock is held by another process, this call
returns -1 and sets errno to EAGAIN.
F_OFD_SETLKW
As for F_OFD_SETLK, but if a conflicting lock is held on
the file, then wait for that lock to be released. If a
signal is caught while waiting, then the call is
interrupted and (after the signal handler has returned)
returns immediately (with return value -1 and errno set to
EINTR; see signal(7)).
F_OFD_GETLK
On input to this call, lock describes an open file
description lock we would like to place on the file. If
the lock could be placed, fcntl() does not actually place
it, but returns F_UNLCK in the l_type field of lock and
leaves the other fields of the structure unchanged. If one
or more incompatible locks would prevent this lock being
placed, then details about one of these locks are returned
via lock, as described above for F_GETLK.
In the current implementation, no deadlock detection is performed
for open file description locks. (This contrasts with process-
associated record locks, for which the kernel does perform
deadlock detection.)
Mandatory locking
Warning: the Linux implementation of mandatory locking is
unreliable. See BUGS below. Because of these bugs, and the fact
that the feature is believed to be little used, since Linux 4.5,
mandatory locking has been made an optional feature, governed by a
configuration option (CONFIG_MANDATORY_FILE_LOCKING). This
feature is no longer supported at all in Linux 5.15 and above.
By default, both traditional (process-associated) and open file
description record locks are advisory. Advisory locks are not
enforced and are useful only between cooperating processes.
Both lock types can also be mandatory. Mandatory locks are
enforced for all processes. If a process tries to perform an
incompatible access (e.g., read(2) or write(2)) on a file region
that has an incompatible mandatory lock, then the result depends
upon whether the O_NONBLOCK flag is enabled for its open file
description. If the O_NONBLOCK flag is not enabled, then the
system call is blocked until the lock is removed or converted to a
mode that is compatible with the access. If the O_NONBLOCK flag
is enabled, then the system call fails with the error EAGAIN.
To make use of mandatory locks, mandatory locking must be enabled
both on the filesystem that contains the file to be locked, and on
the file itself. Mandatory locking is enabled on a filesystem
using the "-o mand" option to mount(8), or the MS_MANDLOCK flag
for mount(2). Mandatory locking is enabled on a file by disabling
group execute permission on the file and enabling the set-group-ID
permission bit (see chmod(1) and chmod(2)).
Mandatory locking is not specified by POSIX. Some other systems
also support mandatory locking, although the details of how to
enable it vary across systems.
Lost locks
When an advisory lock is obtained on a networked filesystem such
as NFS it is possible that the lock might get lost. This may
happen due to administrative action on the server, or due to a
network partition (i.e., loss of network connectivity with the
server) which lasts long enough for the server to assume that the
client is no longer functioning.
When the filesystem determines that a lock has been lost, future
read(2) or write(2) requests may fail with the error EIO. This
error will persist until the lock is removed or the file
descriptor is closed. Since Linux 3.12, this happens at least for
NFSv4 (including all minor versions).
Some versions of UNIX send a signal (SIGLOST) in this
circumstance. Linux does not define this signal, and does not
provide any asynchronous notification of lost locks.
Zero.
On error, -1 is returned, and errno is set to indicate the error.
See fcntl(2).
EBADF op is F_SETLK or F_SETLKW and the file descriptor open mode
doesn't match with the type of lock requested.
EDEADLK
It was detected that the specified F_SETLKW operation would
cause a deadlock.
EFAULT lock is outside your accessible address space.
EINTR op is F_SETLKW or F_OFD_SETLKW and the operation was
interrupted by a signal; see signal(7).
EINTR op is F_GETLK, F_SETLK, F_OFD_GETLK, or F_OFD_SETLK, and
the operation was interrupted by a signal before the lock
was checked or acquired. Most likely when locking a remote
file (e.g., locking over NFS), but can sometimes happen
locally.
EINVAL op is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid
was not specified as zero.
ENOLCK Too many segment locks open, lock table is full, or a
remote locking protocol failed (e.g., locking over NFS).
POSIX.1-2008.
F_OFD_SETLK, F_OFD_SETLKW, and F_OFD_GETLK are Linux-specific (and
one must define _GNU_SOURCE to obtain their definitions), but work
is being done to have them included in the next version of
POSIX.1.
SVr4, 4.3BSD, POSIX.1-2001.
Only the operations F_GETLK, F_SETLK, and F_SETLKW are specified
in POSIX.1-2001.
File locking
The original Linux fcntl() system call was not designed to handle
large file offsets (in the flock structure). Consequently, an
fcntl64() system call was added in Linux 2.4. The newer system
call employs a different structure for file locking, flock64, and
corresponding operations, F_GETLK64, F_SETLK64, and F_SETLKW64.
However, these details can be ignored by applications using glibc,
whose fcntl() wrapper function transparently employs the more
recent system call where it is available.
Record locks
Since Linux 2.0, there is no interaction between the types of lock
placed by flock(2) and fcntl().
Several systems have more fields in struct flock such as, for
example, l_sysid (to identify the machine where the lock is held).
Clearly, l_pid alone is not going to be very useful if the process
holding the lock may live on a different machine; on Linux, while
present on some architectures (such as MIPS32), this field is not
used.
The original Linux fcntl() system call was not designed to handle
large file offsets (in the flock structure). Consequently, an
fcntl64() system call was added in Linux 2.4. The newer system
call employs a different structure for file locking, flock64, and
corresponding operations, F_GETLK64, F_SETLK64, and F_SETLKW64.
However, these details can be ignored by applications using glibc,
whose fcntl() wrapper function transparently employs the more
recent system call where it is available.
Record locking and NFS
Before Linux 3.12, if an NFSv4 client loses contact with the
server for a period of time (defined as more than 90 seconds with
no communication), it might lose and regain a lock without ever
being aware of the fact. (The period of time after which contact
is assumed lost is known as the NFSv4 leasetime. On a Linux NFS
server, this can be determined by looking at
/proc/fs/nfsd/nfsv4leasetime, which expresses the period in
seconds. The default value for this file is 90.) This scenario
potentially risks data corruption, since another process might
acquire a lock in the intervening period and perform file I/O.
Since Linux 3.12, if an NFSv4 client loses contact with the
server, any I/O to the file by a process which "thinks" it holds a
lock will fail until that process closes and reopens the file. A
kernel parameter, nfs.recover_lost_locks, can be set to 1 to
obtain the pre-3.12 behavior, whereby the client will attempt to
recover lost locks when contact is reestablished with the server.
Because of the attendant risk of data corruption, this parameter
defaults to 0 (disabled).
Deadlock detection
The deadlock-detection algorithm employed by the kernel when
dealing with F_SETLKW requests can yield both false negatives
(failures to detect deadlocks, leaving a set of deadlocked
processes blocked indefinitely) and false positives (EDEADLK
errors when there is no deadlock). For example, the kernel limits
the lock depth of its dependency search to 10 steps, meaning that
circular deadlock chains that exceed that size will not be
detected. In addition, the kernel may falsely indicate a deadlock
when two or more processes created using the clone(2) CLONE_FILES
flag place locks that appear (to the kernel) to conflict.
Mandatory locking
The Linux implementation of mandatory locking is subject to race
conditions which render it unreliable: a write(2) call that
overlaps with a lock may modify data after the mandatory lock is
acquired; a read(2) call that overlaps with a lock may detect
changes to data that were made only after a write lock was
acquired. Similar races exist between mandatory locks and
mmap(2). It is therefore inadvisable to rely on mandatory
locking.
fcntl(2), flock(2), lockf(3), lslocks(8)
locks.txt, mandatory-locking.txt, and dnotify.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
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Linux man-pages 6.15 2025-07-20 fcntl_locking(2)
Pages that refer to this page: fcntl(2)
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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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