fcntl_locking(2) - Linux manual page (original) (raw)
fcntllocking(2) System Calls Manual fcntllocking(2)
NAME top
F_GETLK, F_SETLK, F_SETLKW, F_OFD_GETLK, F_OFD_SETLK, F_OFD_SETLKW
- lockingLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_**);**DESCRIPTION top
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 _lwhence_, _lstart_, and _llen_ 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.
_lstart_ is the starting offset for the lock, and is interpreted
relative to either: the start of the file (if _lwhence_ is
**SEEK_SET**); the current file offset (if _lwhence_ is **SEEK_CUR**); or
the end of the file (if _lwhence_ is **SEEK_END**). In the final two
cases, _lstart_ can be a negative number provided the offset does
not lie before the start of the file.
_llen_ specifies the number of bytes to be locked. If _llen_ is
positive, then the range to be locked covers bytes _lstart_ up to
and including _lstart_+_llen_-1. Specifying 0 for _llen_ has the
special meaning: lock all bytes starting at the location specified
by _lwhence_ and _lstart_ 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 _llen_ value; if _llen_ is negative, the interval
described by _lock_ covers bytes _lstart_+_llen_ up to and including
_lstart_-1. This is supported since Linux 2.4.21 and Linux 2.5.49.
The _ltype_ 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 _ltype_ is **F_RDLCK** or **F_WRLCK**) or
release a lock (when _ltype_ is **F_UNLCK**) on the bytes
specified by the _lwhence_, _lstart_, and _llen_ fields of
_lock_. If a conflicting lock is held by another process,
this call returns -1 and sets _[errno](../man3/errno.3.html)_ 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](../man3/errno.3.html)_ set to **EINTR**; see
[signal(7)](../man7/signal.7.html)).
**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
_ltype_ 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 _ltype_, _lwhence_, _lstart_, and _llen_
fields of _lock_. If the conflicting lock is a traditional
(process-associated) record lock, then the _lpid_ field is
set to the PID of the process holding that lock. If the
conflicting lock is an open file description lock, then
_lpid_ 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)](../man2/fork.2.html), but
are preserved across an [execve(2)](../man2/execve.2.html).
Because of the buffering performed by the [stdio(3)](../man3/stdio.3.html) library, the
use of record locking with routines in that package should be
avoided; use [read(2)](../man2/read.2.html) and [write(2)](../man2/write.2.html) 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)](../man2/flock.2.html). Consequently (and unlike traditional advisory
record locks), open file description locks are inherited across
[fork(2)](../man2/fork.2.html) (and [clone(2)](../man2/clone.2.html) 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)](../man2/fork.2.html), [dup(2)](../man2/dup.2.html),
[F_DUPFD(2const)](../man2/F%5FDUPFD.2const.html), 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)](../man2/open.2.html) 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 _lpid_ 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 _ltype_ is
**F_RDLCK** or **F_WRLCK**) or release an open file description
lock (when _ltype_ is **F_UNLCK**) on the bytes specified by the
_lwhence_, _lstart_, and _llen_ fields of _lock_. If a
conflicting lock is held by another process, this call
returns -1 and sets _[errno](../man3/errno.3.html)_ 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](../man3/errno.3.html)_ set to
**EINTR**; see [signal(7)](../man7/signal.7.html)).
**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 _ltype_ 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)](../man2/read.2.html) or [write(2)](../man2/write.2.html)) 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)](../man8/mount.8.html), or the **MS_MANDLOCK** flag
for [mount(2)](../man2/mount.2.html). 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)](../man1/chmod.1.html) and [chmod(2)](../man2/chmod.2.html)).
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)](../man2/read.2.html) or [write(2)](../man2/write.2.html) 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.RETURN VALUE top
Zero.
On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
See [fcntl(2)](../man2/fcntl.2.html).
**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)](../man7/signal.7.html).
**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 _lpid_
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).STANDARDS top
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.HISTORY top
SVr4, 4.3BSD, POSIX.1-2001.
Only the operations **F_GETLK**, **F_SETLK**, and **F_SETLKW** are specified
in POSIX.1-2001.NOTES top
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, _lsysid_ (to identify the machine where the lock is held).
Clearly, _lpid_ 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.recoverlostlocks_, 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).BUGS top
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.
SEE ALSO top
[fcntl(2)](../man2/fcntl.2.html), [flock(2)](../man2/flock.2.html), [lockf(3)](../man3/lockf.3.html), [lslocks(8)](../man8/lslocks.8.html)
_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_)COLOPHON top
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