fcntl(2) - Linux manual page (original) (raw)
fcntl(2) System Calls Manual fcntl(2)
NAME top
fcntl - manipulate file descriptorLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <fcntl.h>**
**int fcntl(int** _fd_**, int** _op_**, ... /*** _arg_ ***/ );**DESCRIPTION top
**fcntl**() performs one of the operations described below on the open
file descriptor _fd_. The operation is determined by _op_.
**fcntl**() can take an optional third argument. Whether or not this
argument is required is determined by _op_. The required argument
type is indicated in parentheses after each _op_ name (in most
cases, the required type is _int_, and we identify the argument
using the name _arg_), or _void_ is specified if the argument is not
required.
Certain of the operations below are supported only since a
particular Linux kernel version. The preferred method of checking
whether the host kernel supports a particular operation is to
invoke **fcntl**() with the desired _op_ value and then test whether the
call failed with **EINVAL**, indicating that the kernel does not
recognize this value.Duplicating a file descriptor F_DUPFD (int) Duplicate the file descriptor fd using the lowest-numbered available file descriptor greater than or equal to arg. This is different from dup2(2), which uses exactly the file descriptor specified.
On success, the new file descriptor is returned.
See [dup(2)](../man2/dup.2.html) for further details.
**F_DUPFD_CLOEXEC** (_int_; since Linux 2.6.24)
As for **F_DUPFD**, but additionally set the close-on-exec flag
for the duplicate file descriptor. Specifying this flag
permits a program to avoid an additional **fcntl**() **F_SETFD**
operation to set the **FD_CLOEXEC** flag. For an explanation
of why this flag is useful, see the description of
**O_CLOEXEC** in [open(2)](../man2/open.2.html).File descriptor flags The following operations manipulate the flags associated with a file descriptor. Currently, only one such flag is defined: FD_CLOEXEC, the close-on-exec flag. If the FD_CLOEXEC bit is set, the file descriptor will automatically be closed during a successful execve(2). (If the execve(2) fails, the file descriptor is left open.) If the FD_CLOEXEC bit is not set, the file descriptor will remain open across an execve(2).
**F_GETFD** (_void_)
Return (as the function result) the file descriptor flags;
_arg_ is ignored.
**F_SETFD** (_int_)
Set the file descriptor flags to the value specified by
_arg_.
In multithreaded programs, using **fcntl**() **F_SETFD** to set the close-
on-exec flag at the same time as another thread performs a [fork(2)](../man2/fork.2.html)
plus [execve(2)](../man2/execve.2.html) is vulnerable to a race condition that may
unintentionally leak the file descriptor to the program executed
in the child process. See the discussion of the **O_CLOEXEC** flag in
[open(2)](../man2/open.2.html) for details and a remedy to the problem.File status flags Each open file description has certain associated status flags, initialized by open(2) and possibly modified by fcntl(). Duplicated file descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.) refer to the same open file description, and thus share the same file status flags.
The file status flags and their semantics are described in
[open(2)](../man2/open.2.html).
**F_GETFL** (_void_)
Return (as the function result) the file access mode and
the file status flags; _arg_ is ignored.
**F_SETFL** (_int_)
Set the file status flags to the value specified by _arg_.
File access mode (**O_RDONLY**, **O_WRONLY**, **O_RDWR**) and file
creation flags (i.e., **O_CREAT**, **O_EXCL**, **O_NOCTTY**, **O_TRUNC**)
in _arg_ are ignored. On Linux, this operation can change
only the **O_APPEND**, **O_ASYNC**, **O_DIRECT**, **O_NOATIME**, and
**O_NONBLOCK** flags. It is not possible to change the **O_DSYNC**
and **O_SYNC** flags; see BUGS, below.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** (_struct flock *_)
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** (_struct flock *_)
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** (_struct flock *_)
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),
**fcntl**() **F_DUPFD**, 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** (_struct flock *_)
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** (_struct flock *_)
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** (_struct flock *_)
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.Managing signals F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, and F_SETSIG are used to manage I/O availability signals:
**F_GETOWN** (_void_)
Return (as the function result) the process ID or process
group ID currently receiving **SIGIO** and **SIGURG** signals for
events on file descriptor _fd_. Process IDs are returned as
positive values; process group IDs are returned as negative
values (but see BUGS below). _arg_ is ignored.
**F_SETOWN** (_int_)
Set the process ID or process group ID that will receive
**SIGIO** and **SIGURG** signals for events on the file descriptor
_fd_. The target process or process group ID is specified in
_arg_. A process ID is specified as a positive value; a
process group ID is specified as a negative value. Most
commonly, the calling process specifies itself as the owner
(that is, _arg_ is specified as [getpid(2)](../man2/getpid.2.html)).
As well as setting the file descriptor owner, one must also
enable generation of signals on the file descriptor. This
is done by using the **fcntl**() **F_SETFL** operation to set the
**O_ASYNC** file status flag on the file descriptor.
Subsequently, a **SIGIO** signal is sent whenever input or
output becomes possible on the file descriptor. The
**fcntl**() **F_SETSIG** operation can be used to obtain delivery
of a signal other than **SIGIO**.
Sending a signal to the owner process (group) specified by
**F_SETOWN** is subject to the same permissions checks as are
described for [kill(2)](../man2/kill.2.html), where the sending process is the one
that employs **F_SETOWN** (but see BUGS below). If this
permission check fails, then the signal is silently
discarded. _Note_: The **F_SETOWN** operation records the
caller's credentials at the time of the **fcntl**() call, and
it is these saved credentials that are used for the
permission checks.
If the file descriptor _fd_ refers to a socket, **F_SETOWN** also
selects the recipient of **SIGURG** signals that are delivered
when out-of-band data arrives on that socket. (**SIGURG** is
sent in any situation where [select(2)](../man2/select.2.html) would report the
socket as having an "exceptional condition".)
The following was true in Linux 2.6.x up to and including
Linux 2.6.11:
If a nonzero value is given to **F_SETSIG** in a
multithreaded process running with a threading
library that supports thread groups (e.g., NPTL),
then a positive value given to **F_SETOWN** has a
different meaning: instead of being a process ID
identifying a whole process, it is a thread ID
identifying a specific thread within a process.
Consequently, it may be necessary to pass **F_SETOWN**
the result of [gettid(2)](../man2/gettid.2.html) instead of [getpid(2)](../man2/getpid.2.html) to get
sensible results when **F_SETSIG** is used. (In current
Linux threading implementations, a main thread's
thread ID is the same as its process ID. This means
that a single-threaded program can equally use
[gettid(2)](../man2/gettid.2.html) or [getpid(2)](../man2/getpid.2.html) in this scenario.) Note,
however, that the statements in this paragraph do
not apply to the **SIGURG** signal generated for out-of-
band data on a socket: this signal is always sent to
either a process or a process group, depending on
the value given to **F_SETOWN**.
The above behavior was accidentally dropped in Linux
2.6.12, and won't be restored. From Linux 2.6.32 onward,
use **F_SETOWN_EX** to target **SIGIO** and **SIGURG** signals at a
particular thread.
**F_GETOWN_EX** (_struct fownerex *_) (since Linux 2.6.32)
Return the current file descriptor owner settings as
defined by a previous **F_SETOWN_EX** operation. The
information is returned in the structure pointed to by _arg_,
which has the following form:
struct f_owner_ex {
int type;
pid_t pid;
};
The _type_ field will have one of the values **F_OWNER_TID**,
**F_OWNER_PID**, or **F_OWNER_PGRP**. The _pid_ field is a positive
integer representing a thread ID, process ID, or process
group ID. See **F_SETOWN_EX** for more details.
**F_SETOWN_EX** (_struct fownerex *_) (since Linux 2.6.32)
This operation performs a similar task to **F_SETOWN**. It
allows the caller to direct I/O availability signals to a
specific thread, process, or process group. The caller
specifies the target of signals via _arg_, which is a pointer
to a _fownerex_ structure. The _type_ field has one of the
following values, which define how _pid_ is interpreted:
**F_OWNER_TID**
Send the signal to the thread whose thread ID (the
value returned by a call to [clone(2)](../man2/clone.2.html) or [gettid(2)](../man2/gettid.2.html))
is specified in _pid_.
**F_OWNER_PID**
Send the signal to the process whose ID is specified
in _pid_.
**F_OWNER_PGRP**
Send the signal to the process group whose ID is
specified in _pid_. (Note that, unlike with **F_SETOWN**,
a process group ID is specified as a positive value
here.)
**F_GETSIG** (_void_)
Return (as the function result) the signal sent when input
or output becomes possible. A value of zero means **SIGIO** is
sent. Any other value (including **SIGIO**) is the signal sent
instead, and in this case additional info is available to
the signal handler if installed with **SA_SIGINFO**. _arg_ is
ignored.
**F_SETSIG** (_int_)
Set the signal sent when input or output becomes possible
to the value given in _arg_. A value of zero means to send
the default **SIGIO** signal. Any other value (including
**SIGIO**) is the signal to send instead, and in this case
additional info is available to the signal handler if
installed with **SA_SIGINFO**.
By using **F_SETSIG** with a nonzero value, and setting
**SA_SIGINFO** for the signal handler (see [sigaction(2)](../man2/sigaction.2.html)), extra
information about I/O events is passed to the handler in a
_siginfot_ structure. If the _sicode_ field indicates the
source is **SI_SIGIO**, the _sifd_ field gives the file
descriptor associated with the event. Otherwise, there is
no indication which file descriptors are pending, and you
should use the usual mechanisms ([select(2)](../man2/select.2.html), [poll(2)](../man2/poll.2.html),
[read(2)](../man2/read.2.html) with **O_NONBLOCK** set etc.) to determine which file
descriptors are available for I/O.
Note that the file descriptor provided in _sifd_ is the one
that was specified during the **F_SETSIG** operation. This can
lead to an unusual corner case. If the file descriptor is
duplicated ([dup(2)](../man2/dup.2.html) or similar), and the original file
descriptor is closed, then I/O events will continue to be
generated, but the _sifd_ field will contain the number of
the now closed file descriptor.
By selecting a real time signal (value >= **SIGRTMIN**),
multiple I/O events may be queued using the same signal
numbers. (Queuing is dependent on available memory.)
Extra information is available if **SA_SIGINFO** is set for the
signal handler, as above.
Note that Linux imposes a limit on the number of real-time
signals that may be queued to a process (see [getrlimit(2)](../man2/getrlimit.2.html)
and [signal(7)](../man7/signal.7.html)) and if this limit is reached, then the
kernel reverts to delivering **SIGIO**, and this signal is
delivered to the entire process rather than to a specific
thread.
Using these mechanisms, a program can implement fully asynchronous
I/O without using [select(2)](../man2/select.2.html) or [poll(2)](../man2/poll.2.html) most of the time.
The use of **O_ASYNC** is specific to BSD and Linux. The only use of
**F_GETOWN** and **F_SETOWN** specified in POSIX.1 is in conjunction with
the use of the **SIGURG** signal on sockets. (POSIX does not specify
the **SIGIO** signal.) **F_GETOWN_EX**, **F_SETOWN_EX**, **F_GETSIG**, and
**F_SETSIG** are Linux-specific. POSIX has asynchronous I/O and the
_aiosigevent_ structure to achieve similar things; these are also
available in Linux as part of the GNU C Library (glibc).Leases F_SETLEASE and F_GETLEASE (Linux 2.4 onward) are used to establish a new lease, and retrieve the current lease, on the open file description referred to by the file descriptor fd. A file lease provides a mechanism whereby the process holding the lease (the "lease holder") is notified (via delivery of a signal) when a process (the "lease breaker") tries to open(2) or truncate(2) the file referred to by that file descriptor.
**F_SETLEASE** (_int_)
Set or remove a file lease according to which of the
following values is specified in the integer _arg_:
**F_RDLCK**
Take out a read lease. This will cause the calling
process to be notified when the file is opened for
writing or is truncated. A read lease can be placed
only on a file descriptor that is opened read-only.
**F_WRLCK**
Take out a write lease. This will cause the caller
to be notified when the file is opened for reading
or writing or is truncated. A write lease may be
placed on a file only if there are no other open
file descriptors for the file.
**F_UNLCK**
Remove our lease from the file.
Leases are associated with an open file description (see [open(2)](../man2/open.2.html)).
This means that duplicate file descriptors (created by, for
example, [fork(2)](../man2/fork.2.html) or [dup(2)](../man2/dup.2.html)) refer to the same lease, and this
lease may be modified or released using any of these descriptors.
Furthermore, the lease is released by either an explicit **F_UNLCK**
operation on any of these duplicate file descriptors, or when all
such file descriptors have been closed.
Leases may be taken out only on regular files. An unprivileged
process may take out a lease only on a file whose UID (owner)
matches the filesystem UID of the process. A process with the
**CAP_LEASE** capability may take out leases on arbitrary files.
**F_GETLEASE** (_void_)
Indicates what type of lease is associated with the file
descriptor _fd_ by returning either **F_RDLCK**, **F_WRLCK**, or
**F_UNLCK**, indicating, respectively, a read lease , a write
lease, or no lease. _arg_ is ignored.
When a process (the "lease breaker") performs an [open(2)](../man2/open.2.html) or
[truncate(2)](../man2/truncate.2.html) that conflicts with a lease established via
**F_SETLEASE**, the system call is blocked by the kernel and the
kernel notifies the lease holder by sending it a signal (**SIGIO** by
default). The lease holder should respond to receipt of this
signal by doing whatever cleanup is required in preparation for
the file to be accessed by another process (e.g., flushing cached
buffers) and then either remove or downgrade its lease. A lease
is removed by performing an **F_SETLEASE** operation specifying _arg_ as
**F_UNLCK**. If the lease holder currently holds a write lease on the
file, and the lease breaker is opening the file for reading, then
it is sufficient for the lease holder to downgrade the lease to a
read lease. This is done by performing an **F_SETLEASE** operation
specifying _arg_ as **F_RDLCK**.
If the lease holder fails to downgrade or remove the lease within
the number of seconds specified in _/proc/sys/fs/lease-break-time_,
then the kernel forcibly removes or downgrades the lease holder's
lease.
Once a lease break has been initiated, **F_GETLEASE** returns the
target lease type (either **F_RDLCK** or **F_UNLCK**, depending on what
would be compatible with the lease breaker) until the lease holder
voluntarily downgrades or removes the lease or the kernel forcibly
does so after the lease break timer expires.
Once the lease has been voluntarily or forcibly removed or
downgraded, and assuming the lease breaker has not unblocked its
system call, the kernel permits the lease breaker's system call to
proceed.
If the lease breaker's blocked [open(2)](../man2/open.2.html) or [truncate(2)](../man2/truncate.2.html) is
interrupted by a signal handler, then the system call fails with
the error **EINTR**, but the other steps still occur as described
above. If the lease breaker is killed by a signal while blocked
in [open(2)](../man2/open.2.html) or [truncate(2)](../man2/truncate.2.html), then the other steps still occur as
described above. If the lease breaker specifies the **O_NONBLOCK**
flag when calling [open(2)](../man2/open.2.html), then the call immediately fails with
the error **EWOULDBLOCK**, but the other steps still occur as
described above.
The default signal used to notify the lease holder is **SIGIO**, but
this can be changed using the **F_SETSIG** operation to **fcntl**(). If a
**F_SETSIG** operation is performed (even one specifying **SIGIO**), and
the signal handler is established using **SA_SIGINFO**, then the
handler will receive a _siginfot_ structure as its second argument,
and the _sifd_ field of this argument will hold the file descriptor
of the leased file that has been accessed by another process.
(This is useful if the caller holds leases against multiple
files.)File and directory change notification (dnotify) F_NOTIFY (int) (Linux 2.4 onward) Provide notification when the directory referred to by fd or any of the files that it contains is changed. The events to be notified are specified in arg, which is a bit mask specified by ORing together zero or more of the following bits:
**DN_ACCESS**
A file was accessed ([read(2)](../man2/read.2.html), [pread(2)](../man2/pread.2.html), [readv(2)](../man2/readv.2.html),
and similar)
**DN_MODIFY**
A file was modified ([write(2)](../man2/write.2.html), [pwrite(2)](../man2/pwrite.2.html), [writev(2)](../man2/writev.2.html),
[truncate(2)](../man2/truncate.2.html), [ftruncate(2)](../man2/ftruncate.2.html), and similar).
**DN_CREATE**
A file was created ([open(2)](../man2/open.2.html), [creat(2)](../man2/creat.2.html), [mknod(2)](../man2/mknod.2.html),
[mkdir(2)](../man2/mkdir.2.html), [link(2)](../man2/link.2.html), [symlink(2)](../man2/symlink.2.html), [rename(2)](../man2/rename.2.html) into this
directory).
**DN_DELETE**
A file was unlinked ([unlink(2)](../man2/unlink.2.html), [rename(2)](../man2/rename.2.html) to another
directory, [rmdir(2)](../man2/rmdir.2.html)).
**DN_RENAME**
A file was renamed within this directory
([rename(2)](../man2/rename.2.html)).
**DN_ATTRIB**
The attributes of a file were changed ([chown(2)](../man2/chown.2.html),
[chmod(2)](../man2/chmod.2.html), [utime(2)](../man2/utime.2.html), [utimensat(2)](../man2/utimensat.2.html), and similar).
(In order to obtain these definitions, the **_GNU_SOURCE**
feature test macro must be defined before including _any_
header files.)
Directory notifications are normally "one-shot", and the
application must reregister to receive further
notifications. Alternatively, if **DN_MULTISHOT** is included
in _arg_, then notification will remain in effect until
explicitly removed.
A series of **F_NOTIFY** requests is cumulative, with the
events in _arg_ being added to the set already monitored. To
disable notification of all events, make an **F_NOTIFY** call
specifying _arg_ as 0.
Notification occurs via delivery of a signal. The default
signal is **SIGIO**, but this can be changed using the **F_SETSIG**
operation to **fcntl**(). (Note that **SIGIO** is one of the
nonqueuing standard signals; switching to the use of a
real-time signal means that multiple notifications can be
queued to the process.) In the latter case, the signal
handler receives a _siginfot_ structure as its second
argument (if the handler was established using **SA_SIGINFO**)
and the _sifd_ field of this structure contains the file
descriptor which generated the notification (useful when
establishing notification on multiple directories).
Especially when using **DN_MULTISHOT**, a real time signal
should be used for notification, so that multiple
notifications can be queued.
**NOTE:** New applications should use the _inotify_ interface
(available since Linux 2.6.13), which provides a much
superior interface for obtaining notifications of
filesystem events. See [inotify(7)](../man7/inotify.7.html).Changing the capacity of a pipe F_SETPIPE_SZ (int; since Linux 2.6.35) Change the capacity of the pipe referred to by fd to be at least arg bytes. An unprivileged process can adjust the pipe capacity to any value between the system page size and the limit defined in /proc/sys/fs/pipe-max-size (see proc(5)). Attempts to set the pipe capacity below the page size are silently rounded up to the page size. Attempts by an unprivileged process to set the pipe capacity above the limit in /proc/sys/fs/pipe-max-size yield the error EPERM; a privileged process (CAP_SYS_RESOURCE) can override the limit.
When allocating the buffer for the pipe, the kernel may use
a capacity larger than _arg_, if that is convenient for the
implementation. (In the current implementation, the
allocation is the next higher power-of-two page-size
multiple of the requested size.) The actual capacity (in
bytes) that is set is returned as the function result.
Attempting to set the pipe capacity smaller than the amount
of buffer space currently used to store data produces the
error **EBUSY**.
Note that because of the way the pages of the pipe buffer
are employed when data is written to the pipe, the number
of bytes that can be written may be less than the nominal
size, depending on the size of the writes.
**F_GETPIPE_SZ** (_void_; since Linux 2.6.35)
Return (as the function result) the capacity of the pipe
referred to by _fd_.File Sealing File seals limit the set of allowed operations on a given file. For each seal that is set on a file, a specific set of operations will fail with EPERM on this file from now on. The file is said to be sealed. The default set of seals depends on the type of the underlying file and filesystem. For an overview of file sealing, a discussion of its purpose, and some code examples, see memfd_create(2).
Currently, file seals can be applied only to a file descriptor
returned by [memfd_create(2)](../man2/memfd%5Fcreate.2.html) (if the **MFD_ALLOW_SEALING** was
employed). On other filesystems, all **fcntl**() operations that
operate on seals will return **EINVAL**.
Seals are a property of an inode. Thus, all open file descriptors
referring to the same inode share the same set of seals.
Furthermore, seals can never be removed, only added.
**F_ADD_SEALS** (_int_; since Linux 3.17)
Add the seals given in the bit-mask argument _arg_ to the set
of seals of the inode referred to by the file descriptor
_fd_. Seals cannot be removed again. Once this call
succeeds, the seals are enforced by the kernel immediately.
If the current set of seals includes **F_SEAL_SEAL** (see
below), then this call will be rejected with **EPERM**. Adding
a seal that is already set is a no-op, in case **F_SEAL_SEAL**
is not set already. In order to place a seal, the file
descriptor _fd_ must be writable.
**F_GET_SEALS** (_void_; since Linux 3.17)
Return (as the function result) the current set of seals of
the inode referred to by _fd_. If no seals are set, 0 is
returned. If the file does not support sealing, -1 is
returned and _[errno](../man3/errno.3.html)_ is set to **EINVAL**.
The following seals are available:
**F_SEAL_SEAL**
If this seal is set, any further call to **fcntl**() with
**F_ADD_SEALS** fails with the error **EPERM**. Therefore, this
seal prevents any modifications to the set of seals itself.
If the initial set of seals of a file includes **F_SEAL_SEAL**,
then this effectively causes the set of seals to be
constant and locked.
**F_SEAL_SHRINK**
If this seal is set, the file in question cannot be reduced
in size. This affects [open(2)](../man2/open.2.html) with the **O_TRUNC** flag as
well as [truncate(2)](../man2/truncate.2.html) and [ftruncate(2)](../man2/ftruncate.2.html). Those calls fail
with **EPERM** if you try to shrink the file in question.
Increasing the file size is still possible.
**F_SEAL_GROW**
If this seal is set, the size of the file in question
cannot be increased. This affects [write(2)](../man2/write.2.html) beyond the end
of the file, [truncate(2)](../man2/truncate.2.html), [ftruncate(2)](../man2/ftruncate.2.html), and [fallocate(2)](../man2/fallocate.2.html).
These calls fail with **EPERM** if you use them to increase the
file size. If you keep the size or shrink it, those calls
still work as expected.
**F_SEAL_WRITE**
If this seal is set, you cannot modify the contents of the
file. Note that shrinking or growing the size of the file
is still possible and allowed. Thus, this seal is normally
used in combination with one of the other seals. This seal
affects [write(2)](../man2/write.2.html) and [fallocate(2)](../man2/fallocate.2.html) (only in combination with
the **FALLOC_FL_PUNCH_HOLE** flag). Those calls fail with
**EPERM** if this seal is set. Furthermore, trying to create
new shared, writable memory-mappings via [mmap(2)](../man2/mmap.2.html) will also
fail with **EPERM**.
Using the **F_ADD_SEALS** operation to set the **F_SEAL_WRITE**
seal fails with **EBUSY** if any writable, shared mapping
exists. Such mappings must be unmapped before you can add
this seal. Furthermore, if there are any asynchronous I/O
operations ([io_submit(2)](../man2/io%5Fsubmit.2.html)) pending on the file, all
outstanding writes will be discarded.
**F_SEAL_FUTURE_WRITE** (since Linux 5.1)
The effect of this seal is similar to **F_SEAL_WRITE**, but the
contents of the file can still be modified via shared
writable mappings that were created prior to the seal being
set. Any attempt to create a new writable mapping on the
file via [mmap(2)](../man2/mmap.2.html) will fail with **EPERM**. Likewise, an
attempt to write to the file via [write(2)](../man2/write.2.html) will fail with
**EPERM**.
Using this seal, one process can create a memory buffer
that it can continue to modify while sharing that buffer on
a "read-only" basis with other processes.File read/write hints Write lifetime hints can be used to inform the kernel about the relative expected lifetime of writes on a given inode or via a particular open file description. (See open(2) for an explanation of open file descriptions.) In this context, the term "write lifetime" means the expected time the data will live on media, before being overwritten or erased.
An application may use the different hint values specified below
to separate writes into different write classes, so that multiple
users or applications running on a single storage back-end can
aggregate their I/O patterns in a consistent manner. However,
there are no functional semantics implied by these flags, and
different I/O classes can use the write lifetime hints in
arbitrary ways, so long as the hints are used consistently.
The following operations can be applied to the file descriptor,
_fd_:
**F_GET_RW_HINT** (_uint64t *_; since Linux 4.13)
Returns the value of the read/write hint associated with
the underlying inode referred to by _fd_.
**F_SET_RW_HINT** (_uint64t *_; since Linux 4.13)
Sets the read/write hint value associated with the
underlying inode referred to by _fd_. This hint persists
until either it is explicitly modified or the underlying
filesystem is unmounted.
**F_GET_FILE_RW_HINT** (_uint64t *_; since Linux 4.13)
Returns the value of the read/write hint associated with
the open file description referred to by _fd_.
**F_SET_FILE_RW_HINT** (_uint64t *_; since Linux 4.13)
Sets the read/write hint value associated with the open
file description referred to by _fd_.
If an open file description has not been assigned a read/write
hint, then it shall use the value assigned to the inode, if any.
The following read/write hints are valid since Linux 4.13:
**RWH_WRITE_LIFE_NOT_SET**
No specific hint has been set. This is the default value.
**RWH_WRITE_LIFE_NONE**
No specific write lifetime is associated with this file or
inode.
**RWH_WRITE_LIFE_SHORT**
Data written to this inode or via this open file
description is expected to have a short lifetime.
**RWH_WRITE_LIFE_MEDIUM**
Data written to this inode or via this open file
description is expected to have a lifetime longer than data
written with **RWH_WRITE_LIFE_SHORT**.
**RWH_WRITE_LIFE_LONG**
Data written to this inode or via this open file
description is expected to have a lifetime longer than data
written with **RWH_WRITE_LIFE_MEDIUM**.
**RWH_WRITE_LIFE_EXTREME**
Data written to this inode or via this open file
description is expected to have a lifetime longer than data
written with **RWH_WRITE_LIFE_LONG**.
All the write-specific hints are relative to each other, and no
individual absolute meaning should be attributed to them.RETURN VALUE top
For a successful call, the return value depends on the operation:
**F_DUPFD**
The new file descriptor.
**F_GETFD**
Value of file descriptor flags.
**F_GETFL**
Value of file status flags.
**F_GETLEASE**
Type of lease held on file descriptor.
**F_GETOWN**
Value of file descriptor owner.
**F_GETSIG**
Value of signal sent when read or write becomes possible,
or zero for traditional **SIGIO** behavior.
**F_GETPIPE_SZ**
**F_SETPIPE_SZ**
The pipe capacity.
**F_GET_SEALS**
A bit mask identifying the seals that have been set for the
inode referred to by _fd_.
All other operations
Zero.
On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**EACCES** or **EAGAIN**
Operation is prohibited by locks held by other processes.
**EAGAIN** The operation is prohibited because the file has been
memory-mapped by another process.
**EBADF** _fd_ is not an open file descriptor
**EBADF** _op_ is **F_SETLK** or **F_SETLKW** and the file descriptor open mode
doesn't match with the type of lock requested.
**EBUSY** _op_ is **F_SETPIPE_SZ** and the new pipe capacity specified in
_arg_ is smaller than the amount of buffer space currently
used to store data in the pipe.
**EBUSY** _op_ is **F_ADD_SEALS**, _arg_ includes **F_SEAL_WRITE**, and there
exists a writable, shared mapping on the file referred to
by _fd_.
**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** The value specified in _op_ is not recognized by this kernel.
**EINVAL** _op_ is **F_ADD_SEALS** and _arg_ includes an unrecognized sealing
bit.
**EINVAL** _op_ is **F_ADD_SEALS** or **F_GET_SEALS** and the filesystem
containing the inode referred to by _fd_ does not support
sealing.
**EINVAL** _op_ is **F_DUPFD** and _arg_ is negative or is greater than the
maximum allowable value (see the discussion of
**RLIMIT_NOFILE** in [getrlimit(2)](../man2/getrlimit.2.html)).
**EINVAL** _op_ is **F_SETSIG** and _arg_ is not an allowable signal number.
**EINVAL** _op_ is **F_OFD_SETLK**, **F_OFD_SETLKW**, or **F_OFD_GETLK**, and _lpid_
was not specified as zero.
**EMFILE** _op_ is **F_DUPFD** and the per-process limit on the number of
open file descriptors has been reached.
**ENOLCK** Too many segment locks open, lock table is full, or a
remote locking protocol failed (e.g., locking over NFS).
**ENOTDIR**
**F_NOTIFY** was specified in _op_, but _fd_ does not refer to a
directory.
**EPERM** _op_ is **F_SETPIPE_SZ** and the soft or hard user pipe limit has
been reached; see [pipe(7)](../man7/pipe.7.html).
**EPERM** Attempted to clear the **O_APPEND** flag on a file that has the
append-only attribute set.
**EPERM** _op_ was **F_ADD_SEALS**, but _fd_ was not open for writing or the
current set of seals on the file already includes
**F_SEAL_SEAL**.STANDARDS top
POSIX.1-2008.
**F_GETOWN_EX**, **F_SETOWN_EX**, **F_SETPIPE_SZ**, **F_GETPIPE_SZ**, **F_GETSIG**,
**F_SETSIG**, **F_NOTIFY**, **F_GETLEASE**, and **F_SETLEASE** are Linux-specific.
(Define the **_GNU_SOURCE** macro to obtain these definitions.)
**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.
**F_ADD_SEALS** and **F_GET_SEALS** are Linux-specific.HISTORY top
SVr4, 4.3BSD, POSIX.1-2001.
Only the operations **F_DUPFD**, **F_GETFD**, **F_SETFD**, **F_GETFL**, **F_SETFL**,
**F_GETLK**, **F_SETLK**, and **F_SETLKW** are specified in POSIX.1-2001.
**F_GETOWN** and **F_SETOWN** are specified in POSIX.1-2001. (To get
their definitions, define either **_XOPEN_SOURCE** with the value 500
or greater, or **_POSIX_C_SOURCE** with the value 200809L or greater.)
**F_DUPFD_CLOEXEC** is specified in POSIX.1-2008. (To get this
definition, define **_POSIX_C_SOURCE** with the value 200809L or
greater, or **_XOPEN_SOURCE** with the value 700 or greater.)NOTES top
The errors returned by [dup2(2)](../man2/dup2.2.html) are different from those returned
by **F_DUPFD**.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
F_SETFL It is not possible to use F_SETFL to change the state of the O_DSYNC and O_SYNC flags. Attempts to change the state of these flags are silently ignored.
F_GETOWN A limitation of the Linux system call conventions on some architectures (notably i386) means that if a (negative) process group ID to be returned by F_GETOWN falls in the range -1 to -4095, then the return value is wrongly interpreted by glibc as an error in the system call; that is, the return value of fcntl() will be -1, and errno will contain the (positive) process group ID. The Linux-specific F_GETOWN_EX operation avoids this problem. Since glibc 2.11, glibc makes the kernel F_GETOWN problem invisible by implementing F_GETOWN using F_GETOWN_EX.
F_SETOWN In Linux 2.4 and earlier, there is bug that can occur when an unprivileged process uses F_SETOWN to specify the owner of a socket file descriptor as a process (group) other than the caller. In this case, fcntl() can return -1 with errno set to EPERM, even when the owner process (group) is one that the caller has permission to send signals to. Despite this error return, the file descriptor owner is set, and signals will be sent to the owner.
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
[dup2(2)](../man2/dup2.2.html), [flock(2)](../man2/flock.2.html), [open(2)](../man2/open.2.html), [socket(2)](../man2/socket.2.html), [lockf(3)](../man3/lockf.3.html), [capabilities(7)](../man7/capabilities.7.html),
[feature_test_macros(7)](../man7/feature%5Ftest%5Fmacros.7.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
This page is part of the _man-pages_ (Linux kernel and C library
user-space interface documentation) project. Information about
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part of the original manual page), send a mail to
man-pages@man7.orgLinux man-pages 6.10 2024-07-23 fcntl(2)
Pages that refer to this page:flock(1), pipesz(1), accept(2), bpf(2), chmod(2), clone(2), close(2), dup(2), eventfd(2), execve(2), fallocate(2), flock(2), fork(2), getrlimit(2), gettid(2), inotify_init(2), ioctl(2), ioctl_console(2), memfd_create(2), memfd_secret(2), mknod(2), mmap(2), mount(2), NS_GET_USERNS(2const), open(2), perf_event_open(2), pidfd_getfd(2), pipe(2), read(2), recv(2), select_tut(2), send(2), sigaction(2), signalfd(2), socket(2), statfs(2), syscalls(2), timerfd_create(2), truncate(2), userfaultfd(2), write(2), audit_open(3), dbopen(3), fopen(3), lockf(3), sd_event_add_io(3), shm_open(3), sockatmark(3), statvfs(3), nfs(5), proc_locks(5), proc_sys_fs(5), systemd.socket(5), capabilities(7), credentials(7), epoll(7), inotify(7), landlock(7), man-pages(7), pipe(7), pthreads(7), signal(7), signal-safety(7), socket(7), tcp(7), lslocks(8), mount(8)