fcntl(2) System Calls Manual fcntl(2)
NAME
fcntl - manipulate file descriptor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int fcntl(int fd, int op, ... /* arg */ );
DESCRIPTION
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 argu-
ment is required is determined by op. The required argument type is in-
dicated 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 de-
sired op value and then test whether the call failed with EINVAL, indi-
cating that the kernel does not recognize this value.
Duplicating a file descriptor
F_DUPFD (int)
Duplicate the file descriptor fd using the lowest-numbered avail-
able file descriptor greater than or equal to arg. This is dif-
ferent from dup2(2), which uses exactly the file descriptor spec-
ified.
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 use-
ful, see the description of O_CLOEXEC in open(2).
File descriptor flags
The following operations manipulate the flags associated with a file de-
scriptor. 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 ex-
ecve(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 ex-
ecve(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, initial-
ized by open(2) and possibly modified by fcntl(). Duplicated file de-
scriptors (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-seg-
ment, or file-region locks). The third argument, lock, is a pointer to
a structure that has at least the following fields (in unspecified or-
der).
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 nega-
tive 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 conver-
sions may involve splitting, shrinking, or coalescing with an existing
lock if the byte range specified by the new lock does not precisely co-
incide 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 dif-
fers across implementations, so POSIX requires a portable appli-
cation 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 hold-
ing that lock. If the conflicting lock is an open file descrip-
tion lock, then l_pid is set to -1. Note that the returned in-
formation may already be out of date by the time the caller in-
spects 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 de-
tects such deadlocks, it causes one of the blocking lock requests to im-
mediately fail with the error EDEADLK; an application that encounters
such an error should release some of its locks to allow other applica-
tions 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-de-
tection algorithm; see BUGS.
As well as being removed by an explicit F_UNLCK, record locks are auto-
matically 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 unfor-
tunate 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 multi-
threaded 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 opera-
tion is in most respects identical to the traditional record locks de-
scribed 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 tra-
ditional record locks are associated with a process, open file descrip-
tion 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 descrip-
tion, 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 de-
scriptor created by fork(2), dup(2), fcntl() F_DUPFD, and so on) are al-
ways compatible: if a new lock is placed on an already locked region,
then the existing lock is converted to the new lock type. (Such conver-
sions 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 descrip-
tion 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 analo-
gous 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 (CON-
FIG_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 descrip-
tion 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_NON-
BLOCK 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). Manda-
tory 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 ad-
ministrative 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 per-
sist until the lock is removed or the file descriptor is closed. Since
Linux 3.12, this happens at least for NFSv4 (including all minor ver-
sions).
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 en-
able 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 de-
scriptor. 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_SE-
TOWN 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 opera-
tion 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 se-
lects 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. Conse-
quently, 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 imple-
mentations, 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 inte-
ger 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 tar-
get of signals via arg, which is a pointer to a f_owner_ex struc-
ture. The type field has one of the following values, which de-
fine 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 struc-
ture. 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 sig-
nals that may be queued to a process (see getrlimit(2) and sig-
nal(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 re-
ferred 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 re-
leased 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 descrip-
tor fd by returning either F_RDLCK, F_WRLCK, or F_UNLCK, indicat-
ing, 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., flush-
ing 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 suffi-
cient 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 com-
patible with the lease breaker) until the lease holder voluntarily down-
grades 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 speci-
fies the O_NONBLOCK flag when calling open(2), then the call immediately
fails with the error EWOULDBLOCK, but the other steps still occur as de-
scribed 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 han-
dler 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 re-
ferred 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 sim-
ilar)
DN_MODIFY
A file was modified (write(2), pwrite(2), writev(2), trun-
cate(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 di-
rectory, 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 applica-
tion must reregister to receive further notifications. Alterna-
tively, 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 notifi-
cation 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 sig-
nals; switching to the use of a real-time signal means that mul-
tiple 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 descrip-
tor 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 (avail-
able since Linux 2.6.13), which provides a much superior inter-
face for obtaining notifications of filesystem events. See ino-
tify(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 ca-
pacity larger than arg, if that is convenient for the implementa-
tion. (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 em-
ployed 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 pur-
pose, 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 re-
ferring 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 pos-
sible and allowed. Thus, this seal is normally used in combina-
tion 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. Fur-
thermore, 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. Further-
more, 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 con-
tents of the file can still be modified via shared writable map-
pings 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 descrip-
tions.) 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 sep-
arate writes into different write classes, so that multiple users or ap-
plications running on a single storage back-end can aggregate their I/O
patterns in a consistent manner. However, there are no functional se-
mantics 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 un-
derlying 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 in-
ode referred to by fd. This hint persists until either it is ex-
plicitly 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 de-
scription 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 individ-
ual absolute meaning should be attributed to them.
RETURN VALUE
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.
ERRORS
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 op-
eration was interrupted by a signal before the lock was checked
or acquired. Most likely when locking a remote file (e.g., lock-
ing 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 getr-
limit(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 direc-
tory.
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 ap-
pend-only attribute set.
EPERM op was F_ADD_SEALS, but fd was not open for writing or the cur-
rent set of seals on the file already includes F_SEAL_SEAL.
STANDARDS
POSIX.1-2008.
F_GETOWN_EX, F_SETOWN_EX, F_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG, F_SET-
SIG, 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
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 def-
initions, 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
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() sys-
tem call was added in Linux 2.4. The newer system call employs a dif-
ferent structure for file locking, flock64, and corresponding opera-
tions, 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() sys-
tem call was added in Linux 2.4. The newer system call employs a dif-
ferent structure for file locking, flock64, and corresponding opera-
tions, 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 communica-
tion), 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 po-
tentially 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).
BUGS
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 re-
turned 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 (pos-
itive) 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 unprivi-
leged process uses F_SETOWN to specify the owner of a socket file de-
scriptor as a process (group) other than the caller. In this case, fc-
ntl() 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. De-
spite 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 de-
tect deadlocks, leaving a set of deadlocked processes blocked indefi-
nitely) 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 condi-
tions 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
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), fea-
ture_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 manda-
tory-locking.txt is called mandatory.txt)
Linux man-pages 6.9.1 2024-05-02 fcntl(2)
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