FCNTL(2) Linux Programmer's Manual FCNTL(2)
NAME
fcntl - manipulate file descriptor
SYNOPSIS
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ... /* arg */ );
DESCRIPTION
fcntl() performs one of the operations described below on the open file
descriptor fd. The operation is determined by cmd.
fcntl() can take an optional third argument. Whether or not this argu-
ment is required is determined by cmd. The required argument type is
indicated in parentheses after each cmd name (in most cases, the re-
quired 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 cmd value and then test whether the call failed with EIN-
VAL, 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 commands 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
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 ex-
ecve(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, ini-
tialized 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 command 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 rela-
tive 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 posi-
tive, then the range to be locked covers bytes l_start up to and in-
cluding l_start+l_len-1. Specifying 0 for l_len has the special mean-
ing: 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 by Linux since kernel versions 2.4.21 and 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 ap-
plication 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 sig-
nal 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 de-
scription 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 al-
ready locked by the other process using F_SETLKW, then, without dead-
lock 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 ker-
nel's deadlock-detection 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 (un-
like 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 de-
cides 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
traditional record locks are associated with a process, open file de-
scription locks are associated with the open file description on which
they are acquired, much like locks acquired with flock(2). Conse-
quently (and unlike traditional advisory record locks), open file de-
scription 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 ac-
quired 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
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 con-
versions may result in splitting, shrinking, or coalescing with an ex-
isting 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 result-
ing 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 commands described below.
The commands 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 re-
turn 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 is an initial step toward removing
this feature completely.
By default, both traditional (process-associated) and open file de-
scription 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 ac-
cess (e.g., read(2) or write(2)) on a file region that has an incompat-
ible mandatory lock, then the result depends upon whether the O_NON-
BLOCK 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 er-
ror 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
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 function-
ing.
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 asynchro-
nous 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 val-
ues; 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 command to set the O_ASYNC file
status flag on the file descriptor. Subsequently, a SIGIO sig-
nal is sent whenever input or output becomes possible on the
file descriptor. The fcntl() F_SETSIG command 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 oper-
ation records the caller's credentials at the time of the fc-
ntl() 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 2.6.x kernels up to and including ker-
nel 2.6.11:
If a nonzero value is given to F_SETSIG in a multi-
threaded 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_SE-
TOWN 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 sin-
gle-threaded program can equally use gettid(2) or get-
pid(2) in this scenario.) Note, however, that the state-
ments 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
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 speci-
fied 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 han-
dler 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. (Queu-
ing 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
referred to by the file descriptor fd. A file lease provides a mecha-
nism whereby the process holding the lease (the "lease holder") is no-
tified (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 de-
scriptor.
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 writ-
ing 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 modi-
fied 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 de-
scriptor 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 re-
spond 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 command 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 command 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
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 spec-
ifies the O_NONBLOCK flag when calling open(2), then the call immedi-
ately 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 command to fcntl(). If a F_SETSIG
command 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
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 direc-
tory).
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 appli-
cation must reregister to receive further notifications. Alter-
natively, 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 noti-
fication of all events, make an F_NOTIFY call specifying arg as
0.
Notification occurs via delivery of a signal. The default sig-
nal is SIGIO, but this can be changed using the F_SETSIG command
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 (avail-
able since kernel 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
capacity larger than arg, if that is convenient for the imple-
mentation. (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 re-
ferred 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 re-
jected 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 com-
bination with one of the other seals. This seal affects
write(2) and fallocate(2) (only in combination with the FAL-
LOC_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. Fur-
thermore, if there are any asynchronous I/O operations (io_sub-
mit(2)) pending on the file, all outstanding writes will be dis-
carded.
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 at-
tempt 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 rela-
tive 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 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
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 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 in-
ode.
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 indi-
vidual 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 in-
ode referred to by fd.
All other commands
Zero.
On error, -1 is returned, and errno is set appropriately.
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 cmd is F_SETLK or F_SETLKW and the file descriptor open mode
doesn't match with the type of lock requested.
EBUSY cmd 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 cmd 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 command would cause
a deadlock.
EFAULT lock is outside your accessible address space.
EINTR cmd is F_SETLKW or F_OFD_SETLKW and the operation was inter-
rupted by a signal; see signal(7).
EINTR cmd 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 cmd is not recognized by this kernel.
EINVAL cmd is F_ADD_SEALS and arg includes an unrecognized sealing bit.
EINVAL cmd is F_ADD_SEALS or F_GET_SEALS and the filesystem containing
the inode referred to by fd does not support sealing.
EINVAL cmd is F_DUPFD and arg is negative or is greater than the maxi-
mum allowable value (see the discussion of RLIMIT_NOFILE in
getrlimit(2)).
EINVAL cmd is F_SETSIG and arg is not an allowable signal number.
EINVAL cmd is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid was
not specified as zero.
EMFILE cmd 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 cmd, but fd does not refer to a direc-
tory.
EPERM cmd 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 cmd 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.
CONFORMING TO
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.)
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.
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 commands,
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 avail-
able.
Record locks
Since kernel 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 commands,
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 avail-
able.
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 behav-
ior, 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 (positive) process group ID. The Linux-specific F_GETOWN_EX opera-
tion avoids this problem. Since glibc version 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. 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 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 indi-
cate 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 mandatory-
locking.txt is called mandatory.txt)
COLOPHON
This page is part of release 5.07 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-02-09 FCNTL(2)