From Linux Man Pages
fcntl - manipulate file descriptor
SYNOPSIS
#include <unistd.h>
#include <fcntl.h>
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 deter-
mined 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.
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, initialized by open(2) and possibly modified by
fcntl(2). Duplicated file descriptors (made with dup(), fcntl(F_DUPFD), fork(), 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 com-
mand 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 pro-
cesses 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 exist-
ing 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 signal handler has returned)
returns immediately (with return value -1 and errno set to EINTR).
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 automatically released when the process termi-
nates 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 pack-
age should be avoided; use read(2) and write(2) instead.
Mandatory locking
(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 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). 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)).
Managing signals
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O availability 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 specified as a nega-
tive value. Most commonly, the calling process specifies itself as the owner (that is, arg is specified
as getpid()).
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. 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() instead of
getpid() 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() or getpid() 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 addi-
tional 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 sig-
nal 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).
Leases
F_SETLEASE and F_GETLEASE (Linux 2.4 onwards) are used (respectively) to establish and retrieve the current set-
ting 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 a process (the "lease
breaker") tries to open(2) or truncate(2) that file.
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 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 no other process
currently has the file open.
F_UNLCK
Remove our lease from the file.
A process may hold only one type of lease on a file.
Leases may only be taken out on regular files. An unprivileged process may only take out a lease on a file whose
UID 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 we hold on the file referred to by fd by returning either F_RDLCK, F_WRLCK,
or F_UNLCK, indicating, respectively, that the calling process holds a read, a write, or no lease on the
file. (The third argument to fcntl() is omitted.)
When a process (the "lease breaker") performs an open() or truncate() 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 we currently hold a write lease on the file, and the lease breaker is opening the file for read-
ing, then it is sufficient to downgrade the lease to a read lease. This is done by performing an F_SETLEASE com-
mand 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() or truncate() 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() or truncate(), then the other steps still occur as described above. If the
lease breaker specifies the O_NONBLOCK flag when calling open(), 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 estab-
lished 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 pro-
cess. (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:
l l ---- lB l. 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 cre-
ated (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 moni-
tored. 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 struc-
ture 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 consider using the inotify interface (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_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
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 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. For F_GETLK and F_SETLK, the command was inter-
rupted 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.
NOTES
The errors returned by dup2() 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(2).
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 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.
BUGS
A limitation of the Linux system call conventions on some architectures (notably x86) 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.
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 specific. (Define the _GNU_SOURCE macro to
obtain these definitions.)
RELATED
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_test_macros(7)
See also locks.txt, mandatory.txt, and dnotify.txt in /usr/src/linux/Documentation.
CATEGORY