FCNTL(2) Linux Programmers Manual FCNTL(2)
NAME
fcntl - manipulate file descriptor
SYNOPSIS
#include
#include
int fcntl(int fd, int cmd);
int fcntl(int fd, int cmd, long arg);
int fcntl(int fd, int cmd, struct flock *lock);
DESCRIPTION
fcntl() performs one of the operations described below on the open file
descriptor fd. The operation is determined by cmd.
Duplicating a file descriptor
F_DUPFD
Find the lowest numbered available file descriptor greater than
or equal to arg and make it be a copy of fd. This is different
from dup2(2) which uses exactly the descriptor specified.
On success, the new descriptor is returned.
See dup(2) for further details.
F_DUPFD_CLOEXEC (since Linux 2.6.24)
As for F_DUPFD, but additionally set the close-on-exec flag for
the duplicate descriptor. Specifying this flag permits a pro
gram 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 0, the file descriptor
will remain open across an execve(2), otherwise it will be closed.
F_GETFD
Read the file descriptor flags.
F_SETFD
Set the file descriptor flags to the value specified by arg.
File status flags
Each open file description has certain associated status flags, ini
tialized by open(2) and possibly modified by fcntl(). Duplicated file
descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.) refer to
the same open file description, and thus share the same file status
flags.
The file status flags and their semantics are described in open(2).
F_GETFL
Read the file status flags.
F_SETFL
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 only change the O_APPEND, O_ASYNC,
O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
Advisory locking
F_GETLK, F_SETLK and F_SETLKW are used to acquire, release, and test
for the existence of record locks (also known as 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
(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
grows.
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
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.
F_SETLKW
As for F_SETLK, but if a conflicting lock is held on the file,
then wait for that lock to be released. If a signal is caught
while waiting, then the call is interrupted and (after the sig
nal handler has returned) returns immediately (with return value
-1 and errno set to EINTR; see signal(7)).
F_GETLK
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 these locks in
the l_type, l_whence, l_start, and l_len fields of lock and sets
l_pid to be the PID of the process holding that lock.
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.
As well as being removed by an explicit F_UNLCK, record locks are auto
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.
Mandatory locking
(Non-POSIX.) The above record locks may be either advisory or manda
tory, and are advisory by default.
Advisory locks are not enforced and are useful only between cooperating
processes.
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 descrip
tion. If the O_NONBLOCK flag is not enabled, then system call is
blocked until the lock is removed or converted to a mode that is com
patible with the access. If the O_NONBLOCK flag is enabled, then the
system call fails with the error EAGAIN or EWOULDBLOCK.
To make use of mandatory locks, mandatory locking must be enabled both
on the file system that contains the file to be locked, and on the file
itself. Mandatory locking is enabled on a file system using the "-o
mand" option to mount(8), or the MS_MANDLOCK flag for mount(2). Manda
tory locking is enabled on a file by disabling group execute permission
on the file and enabling the set-group-ID permission bit (see chmod(1)
and chmod(2)).
The Linux implementation of mandatory locking is unreliable. See BUGS
below.
Managing signals
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O avail
ability signals:
F_GETOWN
Get 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).
F_SETOWN
Set the process ID or process group ID that will receive SIGIO
and SIGURG signals for events on file descriptor fd. A process
ID is specified as a positive value; a process group ID is spec
ified as a negative value. Most commonly, the calling process
specifies itself as the owner (that is, arg is specified as get
pid(2)).
If you set the O_ASYNC status flag on a file descriptor by using
the F_SETFL command of fcntl(), a SIGIO signal is sent whenever
input or output becomes possible on that file descriptor.
F_SETSIG can be used to obtain delivery of a signal other than
SIGIO. If this permission check fails, then the signal is
silently discarded.
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 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".)
If a non-zero value is given to F_SETSIG in a multi-threaded
process running with a threading library that supports thread
groups (e.g., NPTL), then a positive value given to F_SETOWN has
a different meaning: instead of being a process ID identifying a
whole process, it is a thread ID identifying a specific thread
within a process. Consequently, it may be necessary to pass
F_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 threads 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. Note also 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.
F_GETSIG
Get 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.
F_SETSIG
Sets the signal sent when input or output becomes possible. 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.
Additionally, passing a non-zero value to F_SETSIG changes the
signal recipient from a whole process to a specific thread
within a process. See the description of F_SETOWN for more
details.
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 real time signal (value >= SIGRTMIN), multiple
I/O events may be queued using the same signal numbers. (Queu
ing is dependent on available memory). Extra information is
available if SA_SIGINFO is set for the signal handler, as above.
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).
Leases
F_SETLEASE and F_GETLEASE (Linux 2.4 onwards) 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
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 pro
cess to be notified when the file is opened for writing
or is truncated. A read lease can only be placed on a
file descriptor that is opened read-only.
F_WRLCK
Take out a write lease. This will cause the caller to be
notified when the file is opened for reading or writing
or is truncated. A write lease may be placed on a file
only if there are no other open file descriptors for the
file.
F_UNLCK
Remove our lease from the file.
Leases are associated with an open file description (see open(2)).
This means that duplicate file descriptors (created by, for example,
fork(2) or dup(2)) refer to the same lease, and this lease may be modi
fied or released using any of these descriptors. Furthermore, the
lease is released by either an explicit F_UNLCK operation on any of
these duplicate descriptors, or when all such descriptors have been
closed.
Leases may only be taken out on regular files. An unprivileged process
may only take out a lease on a file whose UID (owner) matches the file
system UID of the process. A process with the CAP_LEASE capability may
take out leases on arbitrary files.
F_GETLEASE
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. (The third argument to fcntl() is omitted.)
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 holders lease.
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 breakers system call to proceed.
If the lease breakers blocked open(2) or truncate(2) is interrupted by
a signal handler, then the system call fails with the error EINTR, but
the other steps still occur as described above. If the lease breaker
is killed by a signal while blocked in open(2) or truncate(2), then the
other steps still occur as described above. If the lease breaker spec
ifies the O_NONBLOCK flag when calling open(2), then the call immedi
ately fails with the error EWOULDBLOCK, but the other steps still occur
as described above.
The default signal used to notify the lease holder is SIGIO, but this
can be changed using the F_SETSIG command to fcntl(). If a F_SETSIG
command is performed (even one specifying SIGIO), and the signal han
dler is established using SA_SIGINFO, then the handler will receive a
siginfo_t structure as its second argument, and the si_fd field of this
argument will hold the 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
(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
following bits:
DN_ACCESS A file was accessed (read, pread, readv)
DN_MODIFY A file was modified (write, pwrite, writev, trun
cate, 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 directory (rename).
DN_ATTRIB The attributes of a file were changed (chown, chmod,
utime[s]).
(In order to obtain these definitions, the _GNU_SOURCE feature
test macro must be defined.)
Directory notifications are normally "one-shot", and the appli
cation must re-register to receive further notifications.
Alternatively, if DN_MULTISHOT is included in arg, then notifi
cation 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 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
ries).
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 consider using the inotify inter
face (available since kernel 2.6.13), which provides a superior
interface for obtaining notifications of file system events.
See inotify(7).
RETURN VALUE
For a successful call, the return value depends on the operation:
F_DUPFD The new descriptor.
F_GETFD Value of flags.
F_GETFL Value of flags.
F_GETLEASE
Type of lease held on file descriptor.
F_GETOWN Value of descriptor owner.
F_GETSIG Value of signal sent when read or write becomes possible, or
zero for traditional SIGIO behavior.
All other commands
Zero.
On error, -1 is returned, and errno is set appropriately.
ERRORS
EACCES or EAGAIN
Operation is prohibited by locks held by other processes.
EAGAIN The operation is prohibited because the file has been memory-
mapped by another process.
EBADF fd is not an open file descriptor, or the command was F_SETLK or
F_SETLKW and the file descriptor open mode doesnt match with
the type of lock requested.
EDEADLK
It was detected that the specified F_SETLKW command would cause
a deadlock.
EFAULT lock is outside your accessible address space.
EINTR For F_SETLKW, the command was interrupted by a signal; see sig
nal(7). For F_GETLK and F_SETLK, the command 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 For F_DUPFD, arg is negative or is greater than the maximum
allowable value. For F_SETSIG, arg is not an allowable signal
number.
EMFILE For F_DUPFD, the process already has the maximum number of file
descriptors open.
ENOLCK Too many segment locks open, lock table is full, or a remote
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.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. Only the operations F_DUPFD, F_GETFD,
F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK, F_SETLKW, F_GETOWN, and
F_SETOWN are specified in POSIX.1-2001.
F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-spe
cific. (Define the _GNU_SOURCE macro to obtain these definitions.)
NOTES
The errors returned by dup2(2) are different from those returned by
F_DUPFD.
Since kernel 2.0, there is no interaction between the types of lock
placed by flock(2) and fcntl().
POSIX.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.) This is supported by Linux since Linux 2.4.21 and 2.5.49.
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.
BUGS
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.
In Linux 2.4 and earlier, there is bug that can occur when an unprivi
leged process uses F_SETOWN to specify the owner of a socket file
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 pro
cess (group) is one that the caller has permission to send signals to.
Despite this error return, the file descriptor owner is set, and sig
nals will be sent to the owner.
The implementation of mandatory locking in all known versions of Linux
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 there
fore inadvisable to rely on mandatory locking.
SEE ALSO
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), fea
ture_test_macros(7)
See also Documentation/locks.txt, Documentation/mandatory.txt, and Doc
umentation/dnotify.txt in the kernel source.
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-12-12 FCNTL(2)
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