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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
FCNTL(2) Linux Programmer's Manual FCNTL(2)
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.
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
descriptor. Currently, only one such flag is defined: FD_CLOEXEC,
the close-on-exec flag. If the FD_CLOEXEC bit is set, the file
descriptor will automatically be closed during a successful
execve(2). (If the execve(2) fails, the file descriptor is left
open.) If the FD_CLOEXEC bit is not set, the file descriptor will
remain open across an execve(2).
F_GETFD (void)
Return (as the function result) the file descriptor flags; arg
is ignored.
F_SETFD (int)
Set the file descriptor flags to the value specified by arg.
In multithreaded programs, using fcntl() F_SETFD to set the close-on-
exec flag at the same time as another thread performs a fork(2) plus
execve(2) is vulnerable to a race condition that may unintentionally
leak the file descriptor to the program executed in the child
process. See the discussion of the O_CLOEXEC flag in open(2) for
details and a remedy to the problem.
File status flags
Each open file description has certain associated status flags,
initialized by open(2) and possibly modified by fcntl(). Duplicated
file descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.)
refer to the same open file description, and thus share the same file
status flags.
The file status flags and their semantics are described in open(2).
F_GETFL (void)
Return (as the function result) the file access mode and the
file status flags; arg is ignored.
F_SETFL (int)
Set the file status flags to the value specified by arg. File
access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
ignored. On Linux, this 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-
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 (non-POSIX)
Open file description locks are advisory byte-range locks whose
operation is in most respects identical to the traditional record
locks described above. This lock type is Linux-specific, and
available since Linux 3.15. (There is a proposal with the Austin
Group to include this lock type in the next revision of POSIX.1.)
For an explanation of open file descriptions, see open(2).
The principal difference between the two lock types is that whereas
traditional record locks are associated with a process, open file
description locks are associated with the open file description on
which they are acquired, much like locks acquired with flock(2).
Consequently (and unlike traditional advisory record locks), open
file description locks are inherited across fork(2) (and clone(2)
with CLONE_FILES), and are only automatically released on the last
close of the open file description, instead of being released on any
close of the file.
Conflicting lock combinations (i.e., a read lock and a write lock or
two write locks) where one lock is an open file description lock and
the other is a traditional record lock conflict even when they are
acquired by the same process on the same file descriptor.
Open file description locks placed via the same open file description
(i.e., via the same file descriptor, or via a duplicate of the file
descriptor created by fork(2), dup(2), fcntl() F_DUPFD, and so on)
are always compatible: if a new lock is placed on an already locked
region, then the existing lock is converted to the new lock type.
(Such conversions may result in splitting, shrinking, or coalescing
with an existing lock as discussed above.)
On the other hand, open file description locks may conflict with each
other when they are acquired via different open file descriptions.
Thus, the threads in a multithreaded program can use open file
description locks to synchronize access to a file region by having
each thread perform its own open(2) on the file and applying locks
via the resulting file descriptor.
As with traditional advisory locks, the third argument to fcntl(),
lock, is a pointer to an flock structure. By contrast with
traditional record locks, the l_pid field of that structure must be
set to zero when using the 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.)
Mandatory locking
Warning: the Linux implementation of mandatory locking is unreliable.
See BUGS below. Because of these bugs, and the fact that the feature
is believed to be little used, since Linux 4.5, mandatory locking has
been made an optional feature, governed by a configuration option
(CONFIG_MANDATORY_FILE_LOCKING). This 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.
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 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).
Leases
F_SETLEASE and F_GETLEASE (Linux 2.4 onward) are used (respectively)
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.)
File and directory change notification (dnotify)
F_NOTIFY (int)
(Linux 2.4 onward) Provide notification when the directory
referred to by fd or any of the files that it contains is
changed. The events to be notified are specified in arg,
which is a bit mask specified by ORing together zero or more
of the following bits:
DN_ACCESS A file was accessed (read(2), pread(2), readv(2),
and similar)
DN_MODIFY A file was modified (write(2), pwrite(2),
writev(2), truncate(2), ftruncate(2), and
similar).
DN_CREATE A file was created (open(2), creat(2), mknod(2),
mkdir(2), link(2), symlink(2), rename(2) into this
directory).
DN_DELETE A file was unlinked (unlink(2), rename(2) to
another directory, rmdir(2)).
DN_RENAME A file was renamed within this directory
(rename(2)).
DN_ATTRIB The attributes of a file were changed (chown(2),
chmod(2), utime(2), utimensat(2), and similar).
(In order to obtain these definitions, the _GNU_SOURCE feature
test macro must be defined before including any header files.)
Directory notifications are normally "one-shot", and the
application must reregister to receive further notifications.
Alternatively, if DN_MULTISHOT is included in arg, then
notification will remain in effect until explicitly removed.
A series of F_NOTIFY requests is cumulative, with the events
in arg being added to the set already monitored. To disable
notification of all events, make an F_NOTIFY call specifying
arg as 0.
Notification occurs via delivery of a signal. The default
signal is SIGIO, but this can be changed using the F_SETSIG
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).
Changing the capacity of a pipe
F_SETPIPE_SZ (int; since Linux 2.6.35)
Change the capacity of the pipe referred to by fd to be at
least arg bytes. An unprivileged process can adjust the pipe
capacity to any value between the system page size and the
limit defined in /proc/sys/fs/pipe-max-size (see proc(5)).
Attempts to set the pipe capacity below the page size are
silently rounded up to the page size. Attempts by an
unprivileged process to set the pipe capacity above the limit
in /proc/sys/fs/pipe-max-size yield the error EPERM; a
privileged process (CAP_SYS_RESOURCE) can override the limit.
When allocating the buffer for the pipe, the kernel may use a
capacity larger than arg, if that is convenient for the
implementation. (In the current implementation, the
allocation is the next higher power-of-two page-size multiple
of the requested size.) The actual capacity (in bytes) that
is set is returned as the function result.
Attempting to set the pipe capacity smaller than the amount of
buffer space currently used to store data produces the error
EBUSY.
F_GETPIPE_SZ (void; since Linux 2.6.35)
Return (as the function result) the capacity of the pipe
referred to by fd.
File Sealing
File seals limit the set of allowed operations on a given file. For
each seal that is set on a file, a specific set of operations will
fail with EPERM on this file from now on. The file is said to be
sealed. The default set of seals depends on the type of the
underlying file and filesystem. For an overview of file sealing, a
discussion of its purpose, and some code examples, see
memfd_create(2).
Currently, file seals can be applied only to a file descriptor
returned by memfd_create(2) (if the MFD_ALLOW_SEALING was employed).
On other filesystems, all fcntl() operations that operate on seals
will return EINVAL.
Seals are a property of an inode. Thus, all open file descriptors
referring to the same inode share the same set of seals.
Furthermore, seals can never be removed, only added.
F_ADD_SEALS (int; since Linux 3.17)
Add the seals given in the bit-mask argument arg to the set of
seals of the inode referred to by the file descriptor fd.
Seals cannot be removed again. Once this call succeeds, the
seals are enforced by the kernel immediately. If the current
set of seals includes F_SEAL_SEAL (see below), then this call
will be rejected with EPERM. Adding a seal that is already
set is a no-op, in case F_SEAL_SEAL is not set already. In
order to place a seal, the file descriptor fd must be
writable.
F_GET_SEALS (void; since Linux 3.17)
Return (as the function result) the current set of seals of
the inode referred to by fd. If no seals are set, 0 is
returned. If the file does not support sealing, -1 is
returned and errno is set to EINVAL.
The following seals are available:
F_SEAL_SEAL
If this seal is set, any further call to fcntl() with
F_ADD_SEALS will fail with 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 will 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 will 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 will 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
will fail 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.
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.
File locking
The original Linux fcntl() system call was not designed to handle
large file offsets (in the flock structure). Consequently, an
fcntl64() system call was added in Linux 2.4. The newer system call
employs a different structure for file locking, flock64, and
corresponding 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.
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. Clearly, l_pid alone is not going to be very
useful if the process holding the lock may live on a different
machine.
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.
Record locking and NFS
Before Linux 3.12, if an NFSv4 client loses contact with the server
for a period of time (defined as more than 90 seconds with no
communication), it might lose and regain a lock without ever being
aware of the fact. (The period of time after which contact is
assumed lost is known as the NFSv4 leasetime. On a Linux NFS server,
this can be determined by looking at /proc/fs/nfsd/nfsv4leasetime,
which expresses the period in seconds. The default value for this
file is 90.) This scenario potentially risks data corruption, since
another process might acquire a lock in the intervening period and
perform file I/O.
Since Linux 3.12, if an NFSv4 client loses contact with the server,
any I/O to the file by a process which "thinks" it holds a lock will
fail until that process closes and reopens the file. A kernel
parameter, nfs.recover_lost_locks, can be set to 1 to obtain the
pre-3.12 behavior, whereby the client will attempt to recover lost
locks when contact is reestablished with the server. Because of the
attendant risk of data corruption, this parameter defaults to 0
(disabled).
F_SETFL
It is not possible to use F_SETFL to change the state of the O_DSYNC
and O_SYNC flags. Attempts to change the state of these flags are
silently ignored.
F_GETOWN
A limitation of the Linux system call conventions on some
architectures (notably i386) means that if a (negative) process group
ID to be returned by F_GETOWN falls in the range -1 to -4095, then
the return value is wrongly interpreted by glibc as an error in the
system call; that is, the return value of fcntl() will be -1, and
errno will contain the (positive) process group ID. The Linux-
specific F_GETOWN_EX operation avoids this problem. Since glibc
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
unprivileged process uses F_SETOWN to specify the owner of a socket
file descriptor as a process (group) other than the caller. In this
case, fcntl() can return -1 with errno set to EPERM, even when the
owner process (group) is one that the caller has permission to send
signals to. Despite this error return, the file descriptor owner is
set, and signals will be sent to the owner.
Deadlock detection
The deadlock-detection algorithm employed by the kernel when dealing
with F_SETLKW requests can yield both false negatives (failures to
detect deadlocks, leaving a set of deadlocked processes blocked
indefinitely) and false positives (EDEADLK errors when there is no
deadlock). For example, the kernel limits the lock depth of its
dependency search to 10 steps, meaning that circular deadlock chains
that exceed that size will not be detected. In addition, the kernel
may falsely indicate a deadlock when two or more processes created
using the clone(2) CLONE_FILES flag place locks that appear (to the
kernel) to conflict.
Mandatory locking
The Linux implementation of mandatory locking is subject to race
conditions which render it unreliable: a write(2) call that overlaps
with a lock may modify data after the mandatory lock is acquired; a
read(2) call that overlaps with a lock may detect changes to data
that were made only after a write lock was acquired. Similar races
exist between mandatory locks and mmap(2). It is therefore
inadvisable to rely on mandatory locking.
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),
feature_test_macros(7), lslocks(8)
locks.txt, mandatory-locking.txt, and dnotify.txt in the Linux kernel
source directory Documentation/filesystems/ (on older kernels, these
files are directly under the Documentation/ directory, and mandatory-
locking.txt is called mandatory.txt)
This page is part of release 4.12 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 2017-05-03 FCNTL(2)
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