fcntl - manipulate file descriptor
int fcntl(int fd, int cmd);
int fcntl(int fd, int cmd, long arg);
int fcntl(int fd, int cmd, struct flock *lock);
fcntl performs one of various miscellaneous operations on fd. The
operation in question is determined by cmd.
Find the lowest numbered available file descriptor greater than
or equal to arg and make it be a copy of fd. This is different
form dup2(2) which uses exactly the descriptor specified.
The old and new descriptors may be used interchangeably. They
share locks, file position pointers and flags; for example, if
the file position is modified by using lseek on one of the
descriptors, the position is also changed for the other.
The two descriptors do not share the close-on-exec flag, how-
ever. The close-on-exec flag of the copy is off, meaning that
it will not be closed on exec.
On success, the new descriptor is returned.
Read the close-on-exec flag. If the FD_CLOEXEC bit is 0, the
file will remain open across exec, otherwise it will be closed.
Set the close-on-exec flag to the value specified by the
FD_CLOEXEC bit of arg.
The file status flags
A file descriptor has certain associated flags, initialized by open(2)
and possibly modified by fcntl(2). The flags are shared between copies
(made with dup(2), fork(2), etc.) of the same file descriptor.
The flags and their semantics are described in open(2).
Read the file descriptor's flags.
Set the file status flags part of the descriptor's flags to the
value specified by arg. Remaining bits (access mode, file cre-
ation flags) in arg are ignored. On Linux this command can only
change the O_APPEND, O_NONBLOCK, O_ASYNC, and O_DIRECT flags.
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
(F_GETLK only) */
The l_whence, l_start, and l_len fields of this structure specify the
range of bytes we wish to lock. 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 is a non-negative
integer (but see the NOTES below) specifying the number of bytes to be
locked. Bytes past the end of the file may be locked, but not bytes
before the start of the file. 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
The l_type field can be used to place a read (F_RDLCK) or a write
(F_WDLCK) 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 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 pre-
cisely coincide with the range of the existing lock.)
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.
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).
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
matically released when the process terminates or if it closes any file
descriptor referring to a file on which locks are held. This is bad:
it means that a process can lose the locks on a file like /etc/passwd
or /etc/mtab when for some reason a library function decides to open,
read and close it.
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.
(Non-POSIX.) The above record locks may be either advisory or manda-
tory, and are advisory by default. To make use of mandatory locks,
mandatory locking must be enabled (using the "-o mand" option to
mount(8)) for the file system containing the file to be locked and
enabled on the file itself (by disabling group execute permission on
the file and enabling the set-GID permission bit).
Advisory locks are not enforced and are useful only between cooperating
processes. Mandatory locks are enforced for all processes.
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O avail-
Get the process ID or process group currently receiving SIGIO
and SIGURG signals for events on file descriptor fd. Process
groups are returned as negative values.
Set the process ID or process group that will receive SIGIO and
SIGURG signals for events on file descriptor fd. Process groups
are specified using negative values. (F_SETSIG can be used to
specify a different signal instead of SIGIO).
If you set the O_ASYNC status flag on a file descriptor (either
by providing this flag with the open(2) call, or by using the
F_SETFL command of fcntl), a SIGIO signal is sent whenever input
or output becomes possible on that file descriptor.
The process or process group to receive the signal can be
selected by using the F_SETOWN command to the fcntl function.
If the file descriptor is a socket, this also selects the recip-
ient 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 con-
dition".) If the file descriptor corresponds to a terminal
device, then SIGIO signals are sent to the foreground process
group of the terminal.
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 non-zero 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.
By selecting a POSIX.1b 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
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, F_GETOWN, F_SETOWN is specific to BSD and Linux.
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 onwards) are used (respectively)
to establish and retrieve the current setting of the calling process's
lease on the file referred to by fd. A file lease provides a mechanism
whereby the process holding the lease (the "lease holder") is notified
(via delivery of a signal) when another process (the "contestant")
tries to open(2) or truncate(2) that file.
Set or remove a file lease according to which of the following
values is specified in the integer arg:
Take out a read lease. This will cause us to be notified
when another process opens the file for writing or trun-
Take out a write lease. This will cause us to be noti-
fied when another process opens the file (for reading or
writing) or truncates it. A write lease may be placed on
a file only if no other process currently has the file
Remove our lease from the file.
respectively, that the calling process holds a read, a write, or
no lease on the file. (The third argument to fcntl() is
When the contestant performs an open() or truncate() that conflicts
with a lease established via F_SETLEASE, the system call is blocked by
the kernel (unless the O_NONBLOCK flag was specified to open(), in
which case it returns immediately with the error EWOULDBLOCK). The
kernel then 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
remove its lease by performing an F_SETLEASE command specifying arg as
If the lease holder fails to release the lease within the number of
seconds specified in /proc/sys/fs/lease-break-time and the contestant's
system call remains blocked (i.e., the contestant did not specify
O_NONBLOCK on its open() call, and the system call was not interrupted
by a signal handler) then the kerrnel forcibly breaks the lease
Once the lease has been voluntarily or forcibly removed, and assuming
the contestant has not unblocked its system call, the kernel permits
the contestant's system call to proceed.
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 sructure as its second argument, and the si_fd field of this
argument will hold the 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
(Linux 2.4 onwards) 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
Bit Description (event in directory)
DN_ACCESS A file was accessed (read, pread, readv)
DN_MODIFY A file was modified (write, pwrite,
writev, truncate, ftruncate)
DN_CREATE A file was created (open, creat, mknod,
mkdir, link, symlink, rename)
DN_DELETE A file was unlinked (unlink, rename to
another directory, rmdir)
DN_RENAME A file was renamed within this
DN_ATTRIB The attributes of a file were changed
(chown, chmod, utime[s])
in arg being added to the set already monitored. To disable
notification of all events, make an F_NOTIFY call specifying arg
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(). 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 struc-
ture contains the file descriptor which generated the notifica-
tion (useful when establishing notification on multiple directo-
Especially when using DN_MULTISHOT, a POSIX.1b real time signal
should be used for notication, so that multiple notifications
can be queued.
For a successful call, the return value depends on the operation:
F_DUPFD The new descriptor.
F_GETFD Value of flag.
F_GETFL Value of flags.
F_GETOWN Value of descriptor owner.
F_GETSIG Value of signal sent when read or write becomes possible, or
zero for traditional SIGIO behaviour.
All other commands
On error, -1 is returned, and errno is set appropriately.
EACCES or EAGAIN
Operation is prohibited by locks held by other processes. Or,
operation is prohibited because the file has been memory-mapped
by another process.
EBADF fd is not an open file descriptor, or the command was F_SETLK or
F_SETLKW and the file descriptor open mode doesn't match with
the type of lock requested.
It was detected that the specified F_SETLKW command would cause
EFAULT lock is outside your accessible address space.
EINTR For F_SETLKW, the command was interrupted by a signal. For
F_GETLK and F_SETLK, the command was interrupted by a signal
before the lock was checked or acquired. Most likely when lock-
ing a remote file (e.g. locking over NFS), but can sometimes
locking protocol failed (e.g. locking over NFS).
EPERM Attempted to clear the O_APPEND flag on a file that has the
append-only attribute set.
The errors returned by dup2 are different from those returned by
Since kernel 2.0, there is no interaction between the types of lock
placed by flock(2) and fcntl(2).
POSIX 1003.1-2001 allows l_len to be negative. (And if it is, the
interval described by the lock covers bytes l_start+l_len up to and
including l_start-1.) However, for current kernels the Linux system
call returns EINVAL in this situation.
Several systems have more fields in struct flock such as e.g. 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.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. 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. F_GETOWN and F_SETOWN are BSDisms not supported
in SVr4; F_GETSIG and F_SETSIG are specific to Linux. F_NOTIFY,
F_GETLEASE, and F_SETLEASE are Linux specific. (Define the _GNU_SOURCE
macro before including <fcntl.h> to obtain these definitions.) The
flags legal for F_GETFL/F_SETFL are those supported by open(2) and vary
between these systems; O_APPEND, O_NONBLOCK, O_RDONLY, and O_RDWR are
specified in POSIX.1. SVr4 supports several other options and flags
not documented here.
SVr4 documents additional EIO, ENOLINK and EOVERFLOW error conditions.
dup2(2), flock(2), lockf(3), open(2), socket(2)
See also locks.txt, mandatory.txt, and dnotify.txt in
Linux-2.5.18 2002-04-24 fcntl(2)