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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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fcntl(2) System Calls Manual fcntl(2)
fcntl - manipulate file descriptor
Standard C library (libc, -lc)
#include <fcntl.h>
int fcntl(int fd, int op, ... /* arg */ );
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) 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).
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) plus execve(2) 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) 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).
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 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 (struct flock *)
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 (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 set to EINTR;
see signal(7)).
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 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),
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) 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 (struct flock *)
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 (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 set to
EINTR; see signal(7)).
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 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.
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)).
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), 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) 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) instead of getpid(2) 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) or getpid(2) 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 f_owner_ex *) (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 f_owner_ex *) (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 f_owner_ex 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) or gettid(2))
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)),
extra information about I/O events is passed to the
handler in a siginfo_t structure. If the si_code field
indicates the source is SI_SIGIO, the si_fd 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),
poll(2), read(2) with O_NONBLOCK set etc.) to determine
which file descriptors are available for I/O.
Note that the file descriptor provided in si_fd 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) or similar), and the
original file descriptor is closed, then I/O events will
continue to be generated, but the si_fd 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)
and signal(7)) 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) or poll(2) 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
aio_sigevent 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)). This means that duplicate file descriptors (created
by, for example, fork(2) or dup(2)) 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) or
truncate(2) 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) or truncate(2) 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) or truncate(2), then the other steps still occur as
described above. If the lease breaker specifies the O_NONBLOCK
flag when calling open(2), 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 siginfo_t structure as its second
argument, and the si_fd 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), pread(2), readv(2),
and similar)
DN_MODIFY
A file was modified (write(2), pwrite(2),
writev(2), truncate(2), ftruncate(2), and similar).
DN_CREATE
A file was created (open(2), creat(2), mknod(2),
mkdir(2), link(2), symlink(2), rename(2) into this
directory).
DN_DELETE
A file was unlinked (unlink(2), rename(2) to
another directory, rmdir(2)).
DN_RENAME
A file was renamed within this directory
(rename(2)).
DN_ATTRIB
The attributes of a file were changed (chown(2),
chmod(2), utime(2), utimensat(2), 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 siginfo_t structure as its second
argument (if the handler was established using SA_SIGINFO)
and the si_fd 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).
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) (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 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) with the O_TRUNC
flag as well as truncate(2) and ftruncate(2). 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) beyond the end
of the file, truncate(2), ftruncate(2), and fallocate(2).
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) and fallocate(2) (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) 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)) 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) will fail with EPERM. Likewise,
an attempt to write to the file via write(2) 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 (uint64_t *; 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 (uint64_t *; 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 (uint64_t *; 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 (uint64_t *; 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.
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 is set to indicate the error.
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).
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)).
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 l_pid
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).
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.
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.
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.)
The errors returned by dup2(2) 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, 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).
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.
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),
feature_test_macros(7), 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
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for this manual page, see
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Linux man-pages 6.9.1 2024-05-02 fcntl(2)
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