fcntl - manipulate file descriptor
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ... /* arg */ );
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 argument
is required is determined by
cmd. The required argument type is
indicated in parentheses after each
cmd name (in most cases, the
required type is
int, and we identify the argument using the name
arg), or
void is specified if the argument is not required.
Certain of the operations below are supported only since a particular Linux
kernel version. The preferred method of checking whether the host kernel
supports a particular operation is to invoke
fcntl() with the desired
cmd value and then test whether the call failed with
EINVAL,
indicating that the kernel does not recognize this value.
- 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).
The following commands manipulate the flags associated with a file descriptor.
Currently, only one such flag is defined:
FD_CLOEXEC, the close-on-exec
flag. If the
FD_CLOEXEC bit is set, the file descriptor will
automatically be closed during a successful
execve(2). (If the
execve(2) fails, the file descriptor is left open.) If the
FD_CLOEXEC bit is not set, the file descriptor will remain open across
an
execve(2).
- F_GETFD (void)
- Return (as the function result) the file descriptor flags; arg is
ignored.
- F_SETFD (int)
- Set the file descriptor flags to the value specified by arg.
In multithreaded programs, using
fcntl()
F_SETFD to set the
close-on-exec flag at the same time as another thread performs a
fork(2) plus
execve(2) is vulnerable to a race condition that
may unintentionally leak the file descriptor to the program executed in the
child process. See the discussion of the
O_CLOEXEC flag in
open(2) for details and a remedy to the problem.
Each open file description has certain associated status flags, initialized by
open(2) and possibly modified by
fcntl(). Duplicated file
descriptors (made with
dup(2),
fcntl(F_DUPFD),
fork(2),
etc.) refer to the same open file description, and thus share the same file
status flags.
The file status flags and their semantics are described in
open(2).
- F_GETFL (void)
- Return (as the function result) the file access mode and the file status
flags; arg is ignored.
- F_SETFL (int)
- Set the file status flags to the value specified by arg. File
access mode (O_RDONLY, O_WRONLY, O_RDWR) and file
creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY,
O_TRUNC) in arg are ignored. On Linux, this 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.
Linux implements traditional ("process-associated") UNIX record locks,
as standardized by POSIX. For a Linux-specific alternative with better
semantics, see the discussion of open file description locks below.
F_SETLK,
F_SETLKW, and
F_GETLK are used to acquire,
release, and test for the existence of record locks (also known as byte-range,
file-segment, or file-region locks). The third argument,
lock, is a
pointer to a structure that has at least the following fields (in unspecified
order).
struct flock {
...
short l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(set by F_GETLK and F_OFD_GETLK) */
...
};
The
l_whence,
l_start, and
l_len fields of this structure
specify the range of bytes we wish to lock. Bytes past the end of the file may
be locked, but not bytes before the start of the file.
l_start is the starting offset for the lock, and is interpreted relative
to either: the start of the file (if
l_whence is
SEEK_SET); the
current file offset (if
l_whence is
SEEK_CUR); or the end of the
file (if
l_whence is
SEEK_END). In the final two cases,
l_start can be a negative number provided the offset does not lie
before the start of the file.
l_len specifies the number of bytes to be locked. If
l_len is
positive, then the range to be locked covers bytes
l_start up to and
including
l_start+
l_len-1. Specifying 0 for
l_len has the
special meaning: lock all bytes starting at the location specified by
l_whence and
l_start through to the end of file, no matter how
large the file grows.
POSIX.1-2001 allows (but does not require) an implementation to support a
negative
l_len value; if
l_len is negative, the interval
described by
lock covers bytes
l_start+
l_len up to and
including
l_start-1. This is supported 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 application to check for both errors.)
- F_SETLKW (struct flock *)
- As for F_SETLK, but if a conflicting lock is held on the file, then
wait for that lock to be released. If a signal is caught while waiting,
then the call is interrupted and (after the signal handler has returned)
returns immediately (with return value -1 and errno set to
EINTR; see signal(7)).
- F_GETLK (struct flock *)
- On input to this call, lock describes a lock we would like to place
on the file. If the lock could be placed, fcntl() does not actually
place it, but returns F_UNLCK in the l_type field of
lock and leaves the other fields of the structure unchanged.
- If one or more incompatible locks would prevent this lock being placed,
then fcntl() returns details about one of those locks in the
l_type, l_whence, l_start, and l_len fields of
lock. If the conflicting lock is a traditional (process-associated)
record lock, then the l_pid field is set to the PID of the process
holding that lock. If the conflicting lock is an open file description
lock, then l_pid is set to -1. Note that the returned information
may already be out of date by the time the caller inspects it.
In order to place a read lock,
fd must be open for reading. In order to
place a write lock,
fd must be open for writing. To place both types of
lock, open a file read-write.
When placing locks with
F_SETLKW, the kernel detects
deadlocks,
whereby two or more processes have their lock requests mutually blocked by
locks held by the other processes. For example, suppose process A holds a
write lock on byte 100 of a file, and process B holds a write lock on byte
200. If each process then attempts to lock the byte already locked by the
other process using
F_SETLKW, then, without deadlock detection, both
processes would remain blocked indefinitely. When the kernel detects such
deadlocks, it causes one of the blocking lock requests to immediately fail
with the error
EDEADLK; an application that encounters such an error
should release some of its locks to allow other applications to proceed before
attempting regain the locks that it requires. Circular deadlocks involving
more than two processes are also detected. Note, however, that there are
limitations to the kernel's deadlock-detection algorithm; see BUGS.
As well as being removed by an explicit
F_UNLCK, record locks are
automatically released when the process terminates.
Record locks are not inherited by a child created via
fork(2), but are
preserved across an
execve(2).
Because of the buffering performed by the
stdio(3) library, the use of
record locking with routines in that package should be avoided; use
read(2) and
write(2) instead.
The record locks described above are associated with the process (unlike the
open file description locks described below). This has some unfortunate
consequences:
- *
- If a process closes any file descriptor referring to a file, then
all of the process's locks on that file are released, regardless of the
file descriptor(s) on which the locks were obtained. This is bad: it means
that a process can lose its locks on a file such as /etc/passwd or
/etc/mtab when for some reason a library function decides to open,
read, and close the same file.
- *
- The threads in a process share locks. In other words, a multithreaded
program can't use record locking to ensure that threads don't
simultaneously access the same region of a file.
Open file description locks solve both of these problems.
Open file description locks are advisory byte-range locks whose operation is in
most respects identical to the traditional record locks described above. This
lock type is Linux-specific, and available since Linux 3.15. (There is a
proposal with the Austin Group to include this lock type in the next revision
of POSIX.1.) For an explanation of open file descriptions, see
open(2).
The principal difference between the two lock types is that whereas traditional
record locks are associated with a process, open file description locks are
associated with the open file description on which they are acquired, much
like locks acquired with
flock(2). Consequently (and unlike traditional
advisory record locks), open file description locks are inherited across
fork(2) (and
clone(2) with
CLONE_FILES), and are only
automatically released on the last close of the open file description, instead
of being released on any close of the file.
Conflicting lock combinations (i.e., a read lock and a write lock or two write
locks) where one lock is an open file description lock and the other is a
traditional record lock conflict even when they are acquired by the same
process on the same file descriptor.
Open file description locks placed via the same open file description (i.e., via
the same file descriptor, or via a duplicate of the file descriptor created by
fork(2),
dup(2),
fcntl()
F_DUPFD, and so on) are
always compatible: if a new lock is placed on an already locked region, then
the existing lock is converted to the new lock type. (Such conversions may
result in splitting, shrinking, or coalescing with an existing lock as
discussed above.)
On the other hand, open file description locks may conflict with each other when
they are acquired via different open file descriptions. Thus, the threads in a
multithreaded program can use open file description locks to synchronize
access to a file region by having each thread perform its own
open(2)
on the file and applying locks via the resulting file descriptor.
As with traditional advisory locks, the third argument to
fcntl(),
lock, is a pointer to an
flock structure. By contrast with
traditional record locks, the
l_pid field of that structure must be set
to zero when using the 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 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.)
Warning: the Linux implementation of mandatory locking is unreliable. See
BUGS below. Because of these bugs, and the fact that the feature is believed
to be little used, since Linux 4.5, mandatory locking has been made an
optional feature, governed by a configuration option
(
CONFIG_MANDATORY_FILE_LOCKING). This is an initial step toward
removing this feature completely.
By default, both traditional (process-associated) and open file description
record locks are advisory. Advisory locks are not enforced and are useful only
between cooperating processes.
Both lock types can also be mandatory. Mandatory locks are enforced for all
processes. If a process tries to perform an incompatible access (e.g.,
read(2) or
write(2)) on a file region that has an incompatible
mandatory lock, then the result depends upon whether the
O_NONBLOCK
flag is enabled for its open file description. If the
O_NONBLOCK flag
is not enabled, then the system call is blocked until the lock is removed or
converted to a mode that is compatible with the access. If the
O_NONBLOCK flag is enabled, then the system call fails with the error
EAGAIN.
To make use of mandatory locks, mandatory locking must be enabled both on the
filesystem that contains the file to be locked, and on the file itself.
Mandatory locking is enabled on a filesystem using the "-o mand"
option to
mount(8), or the
MS_MANDLOCK flag for
mount(2).
Mandatory locking is enabled on a file by disabling group execute permission
on the file and enabling the set-group-ID permission bit (see
chmod(1)
and
chmod(2)).
Mandatory locking is not specified by POSIX. Some other systems also support
mandatory locking, although the details of how to enable it vary across
systems.
When an advisory lock is obtained on a networked filesystem such as NFS it is
possible that the lock might get lost. This may happen due to administrative
action on the server, or due to a network partition (i.e., loss of network
connectivity with the server) which lasts long enough for the server to assume
that the client is no longer functioning.
When the filesystem determines that a lock has been lost, future
read(2)
or
write(2) requests may fail with the error
EIO. This error
will persist until the lock is removed or the file descriptor is closed. Since
Linux 3.12, this happens at least for NFSv4 (including all minor versions).
Some versions of UNIX send a signal (
SIGLOST) in this circumstance. Linux
does not define this signal, and does not provide any asynchronous
notification of lost locks.
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 currently
receiving SIGIO and SIGURG signals for events on file
descriptor fd. Process IDs are returned as positive values; process
group IDs are returned as negative values (but see BUGS below). arg
is ignored.
- F_SETOWN (int)
- Set the process ID or process group ID that will receive SIGIO and
SIGURG signals for events on the file descriptor fd. The
target process or process group ID is specified in arg. A process
ID is specified as a positive value; a process group ID is specified as a
negative value. Most commonly, the calling process specifies itself as the
owner (that is, arg is specified as getpid(2)).
- As well as setting the file descriptor owner, one must also enable
generation of signals on the file descriptor. This is done by using the
fcntl() F_SETFL command to set the O_ASYNC file
status flag on the file descriptor. Subsequently, a SIGIO signal is
sent whenever input or output becomes possible on the file descriptor. The
fcntl() F_SETSIG command can be used to obtain delivery of a
signal other than SIGIO.
- Sending a signal to the owner process (group) specified by F_SETOWN
is subject to the same permissions checks as are described for
kill(2), where the sending process is the one that employs
F_SETOWN (but see BUGS below). If this permission check fails, then
the signal is silently discarded. Note: The F_SETOWN
operation records the caller's credentials at the time of the
fcntl() call, and it is these saved credentials that are used for
the permission checks.
- If the file descriptor fd refers to a socket, F_SETOWN also
selects the recipient of SIGURG signals that are delivered when
out-of-band data arrives on that socket. (SIGURG is sent in any
situation where select(2) would report the socket as having an
"exceptional condition".)
- The following was true in 2.6.x kernels up to and including kernel
2.6.11:
- If a nonzero value is given to F_SETSIG in a multithreaded process
running with a threading library that supports thread groups (e.g., NPTL),
then a positive value given to F_SETOWN has a different meaning:
instead of being a process ID identifying a whole process, it is a thread
ID identifying a specific thread within a process. Consequently, it may be
necessary to pass F_SETOWN the result of gettid(2) instead
of getpid(2) to get sensible results when F_SETSIG is used.
(In current Linux threading implementations, a main thread's thread ID is
the same as its process ID. This means that a single-threaded program can
equally use gettid(2) or getpid(2) in this scenario.) Note,
however, that the statements in this paragraph do not apply to the
SIGURG signal generated for out-of-band data on a socket: this
signal is always sent to either a process or a process group, depending on
the value given to F_SETOWN.
- The above behavior was accidentally dropped in Linux 2.6.12, and won't be
restored. From Linux 2.6.32 onward, use F_SETOWN_EX to target
SIGIO and SIGURG signals at a particular thread.
- F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
- Return the current file descriptor owner settings as defined by a previous
F_SETOWN_EX operation. The information is returned in the structure
pointed to by arg, which has the following form:
-
struct f_owner_ex {
int type;
pid_t pid;
};
- The type field will have one of the values F_OWNER_TID,
F_OWNER_PID, or F_OWNER_PGRP. The pid field is a
positive integer representing a thread ID, process ID, or process group
ID. See F_SETOWN_EX for more details.
- F_SETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
- This operation performs a similar task to F_SETOWN. It allows the
caller to direct I/O availability signals to a specific thread, process,
or process group. The caller specifies the target of signals via
arg, which is a pointer to a f_owner_ex structure. The
type field has one of the following values, which define how
pid is interpreted:
- F_OWNER_TID
- Send the signal to the thread whose thread ID (the value returned by a
call to clone(2) or gettid(2)) is specified in
pid.
- F_OWNER_PID
- Send the signal to the process whose ID is specified in pid.
- F_OWNER_PGRP
- Send the signal to the process group whose ID is specified in pid.
(Note that, unlike with F_SETOWN, a process group ID is specified
as a positive value here.)
- F_GETSIG (void)
- Return (as the function result) the signal sent when input or output
becomes possible. A value of zero means SIGIO is sent. Any other
value (including SIGIO) is the signal sent instead, and in this
case additional info is available to the signal handler if installed with
SA_SIGINFO. arg is ignored.
- F_SETSIG (int)
- Set the signal sent when input or output becomes possible to the value
given in arg. A value of zero means to send the default
SIGIO signal. Any other value (including SIGIO) is the
signal to send instead, and in this case additional info is available to
the signal handler if installed with SA_SIGINFO.
- By using F_SETSIG with a nonzero value, and setting
SA_SIGINFO for the signal handler (see sigaction(2)), extra
information about I/O events is passed to the handler in a
siginfo_t structure. If the si_code field indicates the
source is SI_SIGIO, the si_fd field gives the file
descriptor associated with the event. Otherwise, there is no indication
which file descriptors are pending, and you should use the usual
mechanisms (select(2), poll(2), read(2) with
O_NONBLOCK set etc.) to determine which file descriptors are
available for I/O.
- Note that the file descriptor provided in si_fd is the one that was
specified during the F_SETSIG operation. This can lead to an
unusual corner case. If the file descriptor is duplicated (dup(2)
or similar), and the original file descriptor is closed, then I/O events
will continue to be generated, but the si_fd field will contain the
number of the now closed file descriptor.
- By selecting a real time signal (value >= SIGRTMIN), multiple
I/O events may be queued using the same signal numbers. (Queuing is
dependent on available memory.) Extra information is available if
SA_SIGINFO is set for the signal handler, as above.
- Note that Linux imposes a limit on the number of real-time signals that
may be queued to a process (see getrlimit(2) and signal(7))
and if this limit is reached, then the kernel reverts to delivering
SIGIO, and this signal is delivered to the entire process rather
than to a specific thread.
Using these mechanisms, a program can implement fully asynchronous I/O without
using
select(2) or
poll(2) most of the time.
The use of
O_ASYNC is specific to BSD and Linux. The only use of
F_GETOWN and
F_SETOWN specified in POSIX.1 is in conjunction
with the use of the
SIGURG signal on sockets. (POSIX does not specify
the
SIGIO signal.)
F_GETOWN_EX,
F_SETOWN_EX,
F_GETSIG, and
F_SETSIG are Linux-specific. POSIX has
asynchronous I/O and the
aio_sigevent structure to achieve similar
things; these are also available in Linux as part of the GNU C Library
(Glibc).
F_SETLEASE and
F_GETLEASE (Linux 2.4 onward) are used to establish
a new lease, and retrieve the current lease, on the open file description
referred to by the file descriptor
fd. A file lease provides a
mechanism whereby the process holding the lease (the "lease holder")
is notified (via delivery of a signal) when a process (the "lease
breaker") tries to
open(2) or
truncate(2) the file referred
to by that file descriptor.
- F_SETLEASE (int)
- Set or remove a file lease according to which of the following values is
specified in the integer arg:
- F_RDLCK
- Take out a read lease. This will cause the calling process to be notified
when the file is opened for writing or is truncated. A read lease can be
placed only on a file descriptor that is opened read-only.
- F_WRLCK
- Take out a write lease. This will cause the caller to be notified when the
file is opened for reading or writing or is truncated. A write lease may
be placed on a file only if there are no other open file descriptors for
the file.
- F_UNLCK
- Remove our lease from the file.
Leases are associated with an open file description (see
open(2)). This
means that duplicate file descriptors (created by, for example,
fork(2)
or
dup(2)) refer to the same lease, and this lease may be modified or
released using any of these descriptors. Furthermore, the lease is released by
either an explicit
F_UNLCK operation on any of these duplicate file
descriptors, or when all such file descriptors have been closed.
Leases may be taken out only on regular files. An unprivileged process may take
out a lease only on a file whose UID (owner) matches the filesystem UID of the
process. A process with the
CAP_LEASE capability may take out leases on
arbitrary files.
- F_GETLEASE (void)
- Indicates what type of lease is associated with the file descriptor
fd by returning either F_RDLCK, F_WRLCK, or
F_UNLCK, indicating, respectively, a read lease , a write lease, or
no lease. arg is ignored.
When a process (the "lease breaker") performs an
open(2) or
truncate(2) that conflicts with a lease established via
F_SETLEASE, the system call is blocked by the kernel and the kernel
notifies the lease holder by sending it a signal (
SIGIO by default).
The lease holder should respond to receipt of this signal by doing whatever
cleanup is required in preparation for the file to be accessed by another
process (e.g., flushing cached buffers) and then either remove or downgrade
its lease. A lease is removed by performing an
F_SETLEASE 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 compatible with the lease breaker) until the lease holder voluntarily
downgrades or removes the lease or the kernel forcibly does so after the lease
break timer expires.
Once the lease has been voluntarily or forcibly removed or downgraded, and
assuming the lease breaker has not unblocked its system call, the kernel
permits the lease breaker's system call to proceed.
If the lease breaker's blocked
open(2) or
truncate(2) is
interrupted by a signal handler, then the system call fails with the error
EINTR, but the other steps still occur as described above. If the lease
breaker is killed by a signal while blocked in
open(2) or
truncate(2), then the other steps still occur as described above. If
the lease breaker specifies the
O_NONBLOCK flag when calling
open(2), then the call immediately fails with the error
EWOULDBLOCK, but the other steps still occur as described above.
The default signal used to notify the lease holder is
SIGIO, but this can
be changed using the
F_SETSIG command to
fcntl(). If a
F_SETSIG command is performed (even one specifying
SIGIO), and
the signal handler is established using
SA_SIGINFO, then the handler
will receive a
siginfo_t structure as its second argument, and the
si_fd field of this argument will hold the file descriptor of the
leased file that has been accessed by another process. (This is useful if the
caller holds leases against multiple files.)
- F_NOTIFY (int)
- (Linux 2.4 onward) Provide notification when the directory referred to by
fd or any of the files that it contains is changed. The events to
be notified are specified in arg, which is a bit mask specified by
ORing together zero or more of the following bits:
- DN_ACCESS
- A file was accessed (read(2), pread(2), readv(2), and
similar)
- DN_MODIFY
- A file was modified (write(2), pwrite(2), writev(2),
truncate(2), ftruncate(2), and similar).
- DN_CREATE
- A file was created (open(2), creat(2), mknod(2),
mkdir(2), link(2), symlink(2), rename(2) into
this directory).
- DN_DELETE
- A file was unlinked (unlink(2), rename(2) to another
directory, rmdir(2)).
- DN_RENAME
- A file was renamed within this directory (rename(2)).
- DN_ATTRIB
- The attributes of a file were changed (chown(2), chmod(2),
utime(2), utimensat(2), and similar).
- (In order to obtain these definitions, the _GNU_SOURCE feature test
macro must be defined before including any header files.)
- Directory notifications are normally "one-shot", and the
application must reregister to receive further notifications.
Alternatively, if DN_MULTISHOT is included in arg, then
notification will remain in effect until explicitly removed.
- A series of F_NOTIFY requests is cumulative, with the events in
arg being added to the set already monitored. To disable
notification of all events, make an F_NOTIFY call specifying
arg as 0.
- Notification occurs via delivery of a signal. The default signal is
SIGIO, but this can be changed using the F_SETSIG 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
(available since kernel 2.6.13), which provides a much superior interface
for obtaining notifications of filesystem events. See
inotify(7).
- F_SETPIPE_SZ (int; since Linux 2.6.35)
- Change the capacity of the pipe referred to by fd to be at least
arg bytes. An unprivileged process can adjust the pipe capacity to
any value between the system page size and the limit defined in
/proc/sys/fs/pipe-max-size (see proc(5)). Attempts to set
the pipe capacity below the page size are silently rounded up to the page
size. Attempts by an unprivileged process to set the pipe capacity above
the limit in /proc/sys/fs/pipe-max-size yield the error
EPERM; a privileged process (CAP_SYS_RESOURCE) can override
the limit.
- When allocating the buffer for the pipe, the kernel may use a capacity
larger than arg, if that is convenient for the implementation. (In
the current implementation, the allocation is the next higher power-of-two
page-size multiple of the requested size.) The actual capacity (in bytes)
that is set is returned as the function result.
- Attempting to set the pipe capacity smaller than the amount of buffer
space currently used to store data produces the error EBUSY.
- Note that because of the way the pages of the pipe buffer are employed
when data is written to the pipe, the number of bytes that can be written
may be less than the nominal size, depending on the size of the
writes.
- F_GETPIPE_SZ (void; since Linux 2.6.35)
- Return (as the function result) the capacity of the pipe referred to by
fd.
File seals limit the set of allowed operations on a given file. For each seal
that is set on a file, a specific set of operations will fail with
EPERM on this file from now on. The file is said to be sealed. The
default set of seals depends on the type of the underlying file and
filesystem. For an overview of file sealing, a discussion of its purpose, and
some code examples, see
memfd_create(2).
Currently, file seals can be applied only to a file descriptor returned by
memfd_create(2) (if the
MFD_ALLOW_SEALING was employed). On
other filesystems, all
fcntl() operations that operate on seals will
return
EINVAL.
Seals are a property of an inode. Thus, all open file descriptors referring to
the same inode share the same set of seals. Furthermore, seals can never be
removed, only added.
- F_ADD_SEALS (int; since Linux 3.17)
- Add the seals given in the bit-mask argument arg to the set of
seals of the inode referred to by the file descriptor fd. Seals
cannot be removed again. Once this call succeeds, the seals are enforced
by the kernel immediately. If the current set of seals includes
F_SEAL_SEAL (see below), then this call will be rejected with
EPERM. Adding a seal that is already set is a no-op, in case
F_SEAL_SEAL is not set already. In order to place a seal, the file
descriptor fd must be writable.
- F_GET_SEALS (void; since Linux 3.17)
- Return (as the function result) the current set of seals of the inode
referred to by fd. If no seals are set, 0 is returned. If the file
does not support sealing, -1 is returned and errno is set to
EINVAL.
The following seals are available:
- F_SEAL_SEAL
- If this seal is set, any further call to fcntl() with
F_ADD_SEALS fails with the error EPERM. Therefore, this seal
prevents any modifications to the set of seals itself. If the initial set
of seals of a file includes F_SEAL_SEAL, then this effectively
causes the set of seals to be constant and locked.
- F_SEAL_SHRINK
- If this seal is set, the file in question cannot be reduced in size. This
affects open(2) with the O_TRUNC flag as well as
truncate(2) and ftruncate(2). Those calls fail with
EPERM if you try to shrink the file in question. Increasing the
file size is still possible.
- F_SEAL_GROW
- If this seal is set, the size of the file in question cannot be increased.
This affects write(2) beyond the end of the file,
truncate(2), ftruncate(2), and fallocate(2). These
calls fail with EPERM if you use them to increase the file size. If
you keep the size or shrink it, those calls still work as expected.
- F_SEAL_WRITE
- If this seal is set, you cannot modify the contents of the file. Note that
shrinking or growing the size of the file is still possible and allowed.
Thus, this seal is normally used in combination with one of the other
seals. This seal affects write(2) and fallocate(2) (only in
combination with the FALLOC_FL_PUNCH_HOLE flag). Those calls fail
with EPERM if this seal is set. Furthermore, trying to create new
shared, writable memory-mappings via mmap(2) will also fail with
EPERM.
- Using the F_ADD_SEALS operation to set the F_SEAL_WRITE seal
fails with EBUSY if any writable, shared mapping exists. Such
mappings must be unmapped before you can add this seal. Furthermore, if
there are any asynchronous I/O operations (io_submit(2)) pending on
the file, all outstanding writes will be discarded.
- F_SEAL_FUTURE_WRITE (since Linux 5.1)
- The effect of this seal is similar to F_SEAL_WRITE, but the
contents of the file can still be modified via shared writable mappings
that were created prior to the seal being set. Any attempt to create a new
writable mapping on the file via mmap(2) will fail with
EPERM. Likewise, an attempt to write to the file via
write(2) will fail with EPERM.
- Using this seal, one process can create a memory buffer that it can
continue to modify while sharing that buffer on a "read-only"
basis with other processes.
Write lifetime hints can be used to inform the kernel about the relative
expected lifetime of writes on a given inode or via a particular open file
description. (See
open(2) for an explanation of open file
descriptions.) In this context, the term "write lifetime" means the
expected time the data will live on media, before being overwritten or erased.
An application may use the different hint values specified below to separate
writes into different write classes, so that multiple users or applications
running on a single storage back-end can aggregate their I/O patterns in a
consistent manner. However, there are no functional semantics implied by these
flags, and different I/O classes can use the write lifetime hints in arbitrary
ways, so long as the hints are used consistently.
The following operations can be applied to the file descriptor,
fd:
- F_GET_RW_HINT (uint64_t *; since Linux 4.13)
- Returns the value of the read/write hint associated with the underlying
inode referred to by fd.
- F_SET_RW_HINT (uint64_t *; since Linux 4.13)
- Sets the read/write hint value associated with the underlying inode
referred to by fd. This hint persists until either it is explicitly
modified or the underlying filesystem is unmounted.
- F_GET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
- Returns the value of the read/write hint associated with the open file
description referred to by fd.
- F_SET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
- Sets the read/write hint value associated with the open file description
referred to by fd.
If an open file description has not been assigned a read/write hint, then it
shall use the value assigned to the inode, if any.
The following read/write hints are valid since Linux 4.13:
- RWH_WRITE_LIFE_NOT_SET
- No specific hint has been set. This is the default value.
- RWH_WRITE_LIFE_NONE
- No specific write lifetime is associated with this file or inode.
- RWH_WRITE_LIFE_SHORT
- Data written to this inode or via this open file description is expected
to have a short lifetime.
- RWH_WRITE_LIFE_MEDIUM
- Data written to this inode or via this open file description is expected
to have a lifetime longer than data written with
RWH_WRITE_LIFE_SHORT.
- RWH_WRITE_LIFE_LONG
- Data written to this inode or via this open file description is expected
to have a lifetime longer than data written with
RWH_WRITE_LIFE_MEDIUM.
- RWH_WRITE_LIFE_EXTREME
- Data written to this inode or via this open file description is expected
to have a lifetime longer than data written with
RWH_WRITE_LIFE_LONG.
All the write-specific hints are relative to each other, and no individual
absolute meaning should be attributed to them.
For a successful call, the return value depends on the operation:
- F_DUPFD
- The new file descriptor.
- F_GETFD
- Value of file descriptor flags.
- F_GETFL
- Value of file status flags.
- F_GETLEASE
- Type of lease held on file descriptor.
- F_GETOWN
- Value of file descriptor owner.
- F_GETSIG
- Value of signal sent when read or write becomes possible, or zero for
traditional SIGIO behavior.
- F_GETPIPE_SZ, F_SETPIPE_SZ
- The pipe capacity.
- F_GET_SEALS
- A bit mask identifying the seals that have been set for the inode referred
to by fd.
- All other commands
- Zero.
On error, -1 is returned, and
errno is set appropriately.
- 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
interrupted 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 maximum 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 directory.
- 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
append-only attribute set.
- EPERM
- cmd was F_ADD_SEALS, but fd was not open for writing
or the current set of seals on the file already includes
F_SEAL_SEAL.
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_SETSIG,
F_NOTIFY,
F_GETLEASE, and
F_SETLEASE are Linux-specific. (Define the
_GNU_SOURCE macro to obtain these definitions.)
F_OFD_SETLK,
F_OFD_SETLKW, and
F_OFD_GETLK are
Linux-specific (and one must define
_GNU_SOURCE to obtain their
definitions), but work is being done to have them included in the next version
of POSIX.1.
F_ADD_SEALS and
F_GET_SEALS are Linux-specific.
The errors returned by
dup2(2) are different from those returned by
F_DUPFD.
The original Linux
fcntl() system call was not designed to handle large
file offsets (in the
flock structure). Consequently, an
fcntl64() system call was added in Linux 2.4. The newer system call
employs a different structure for file locking,
flock64, and
corresponding 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 available.
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() system call was added in Linux 2.4. The newer system call
employs a different 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 available.
Before Linux 3.12, if an NFSv4 client loses contact with the server for a period
of time (defined as more than 90 seconds with no communication), it might lose
and regain a lock without ever being aware of the fact. (The period of time
after which contact is assumed lost is known as the NFSv4 leasetime. On a
Linux NFS server, this can be determined by looking at
/proc/fs/nfsd/nfsv4leasetime, which expresses the period in seconds.
The default value for this file is 90.) This scenario potentially risks data
corruption, since another process might acquire a lock in the intervening
period and perform file I/O.
Since Linux 3.12, if an NFSv4 client loses contact with the server, any I/O to
the file by a process which "thinks" it holds a lock will fail until
that process closes and reopens the file. A kernel parameter,
nfs.recover_lost_locks, can be set to 1 to obtain the pre-3.12
behavior, whereby the client will attempt to recover lost locks when contact
is reestablished with the server. Because of the attendant risk of data
corruption, this parameter defaults to 0 (disabled).
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.
A limitation of the Linux system call conventions on some architectures (notably
i386) means that if a (negative) process group ID to be returned by
F_GETOWN falls in the range -1 to -4095, then the return value is
wrongly interpreted by glibc as an error in the system call; that is, the
return value of
fcntl() will be -1, and
errno will contain the
(positive) process group ID. The Linux-specific
F_GETOWN_EX operation
avoids this problem. Since glibc version 2.11, glibc makes the kernel
F_GETOWN problem invisible by implementing
F_GETOWN using
F_GETOWN_EX.
In Linux 2.4 and earlier, there is bug that can occur when an unprivileged
process uses
F_SETOWN to specify the owner of a socket file descriptor
as a process (group) other than the caller. In this case,
fcntl() can
return -1 with
errno set to
EPERM, even when the owner process
(group) is one that the caller has permission to send signals to. Despite this
error return, the file descriptor owner is set, and signals will be sent to
the owner.
The deadlock-detection algorithm employed by the kernel when dealing with
F_SETLKW requests can yield both false negatives (failures to detect
deadlocks, leaving a set of deadlocked processes blocked indefinitely) and
false positives (
EDEADLK errors when there is no deadlock). For
example, the kernel limits the lock depth of its dependency search to 10
steps, meaning that circular deadlock chains that exceed that size will not be
detected. In addition, the kernel may falsely indicate a deadlock when two or
more processes created using the
clone(2)
CLONE_FILES flag place
locks that appear (to the kernel) to conflict.
The Linux implementation of mandatory locking is subject to race conditions
which render it unreliable: a
write(2) call that overlaps with a lock
may modify data after the mandatory lock is acquired; a
read(2) call
that overlaps with a lock may detect changes to data that were made only after
a write lock was acquired. Similar races exist between mandatory locks and
mmap(2). It is therefore inadvisable to rely on mandatory locking.
dup2(2),
flock(2),
open(2),
socket(2),
lockf(3),
capabilities(7),
feature_test_macros(7),
lslocks(8)
locks.txt,
mandatory-locking.txt, and
dnotify.txt in the
Linux kernel source directory
Documentation/filesystems/ (on older
kernels, these files are directly under the
Documentation/ directory,
and
mandatory-locking.txt is called
mandatory.txt)