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 long, and
we identify the argument using the name arg), or void is specified if the
argument is not required.
F_DUPFD (long)
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 (long; 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 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 0, the file descriptor will remain open across an
execve(2), otherwise it will be closed.
F_GETFD (void)
Read the file descriptor flags; arg is ignored.
F_SETFD (long)
Set the file descriptor flags to the value specified by arg.
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)
Read the file status flags; arg is ignored.
F_SETFL (long)
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.
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. 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.
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 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
automatically 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 mandatory, 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 description. 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 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
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). 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)).
The Linux implementation of mandatory locking is unreliable. See BUGS below.
F_GETOWN, F_SETOWN, 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 (long)
Set the process ID or process group ID that will receive SIGIO and
SIGURG signals for events on file descriptor fd to the ID given 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)).
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 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. 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 (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 (long)
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.
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 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.
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.
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 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 (long)
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 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 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 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 (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 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 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 (long)
(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, 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 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 application
must re-register 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().
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 file system events. See inotify(7).
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.
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 doesn't 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 signal(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.
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_DUPFD_CLOEXEC is specified in POSIX.1-2008.
F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific.
(Define the _GNU_SOURCE macro to obtain these definitions.)
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().
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.
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 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 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 therefore inadvisable to rely on mandatory
locking.
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),
feature_test_macros(7)
See also locks.txt, mandatory-locking.txt, and dnotify.txt in the 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 3.23 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 2009-07-25 FCNTL(2)