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FCNTL(2) Linux Programmer’s Manual FCNTL(2)
NAME
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 various miscellaneous operations on fd. The operation in
question is determined by cmd.
Handling close-on-exec
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 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 modi-
fied 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, however. 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.
F_GETFD
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.
F_SETFD
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).
F_GETFL
Read the file descriptor’s flags.
F_SETFL
Set the file status flags part of the descriptor’s flags to the value speci-
fied by arg. Remaining bits (access mode, file creation flags) in arg are
ignored. On Linux this command can only change the O_APPEND, O_NONBLOCK,
O_ASYNC, and O_DIRECT flags.
Advisory locking
F_GETLK, F_SETLK and F_SETLKW are used to acquire, release, and test for the exis-
tence 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 inter-
preted 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 mat-
ter how large the file grows.
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 precisely coin-
cide 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 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 lock-
ing 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 mandatory, 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 per-
mission 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.
Managing signals
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O availability sig-
nals:
F_GETOWN
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.
F_SETOWN
Set the process ID or process group that will receive SIGIO and SIGURG sig-
nals 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 recipient of SIGURG signals that are delivered
when out-of-band data arrives on that socket. (SIGURG is sent in any situa-
tion 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.
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.
By using F_SETSIG with a non-zero value, and setting SA_SIGINFO for the sig-
nal 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 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 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 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 "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 us to be notified when
another process opens the file for writing or truncates it.
F_WRLCK
Take out a write lease. This will cause us to be notified 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 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.
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 con-
flicts 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 the sys-
tem call will return with the error EWOULDBLOCK. 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 reading, then it is suf-
ficient 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.
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_SIG-
INFO, 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
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 noti-
fied are specified in arg, which is a bit mask specified by ORing together
zero or more of the following bits:
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
directory (rename)
DN_ATTRIB The attributes of a file were changed
(chown, chmod, utime[s])
(In order to obtain these definitions, the _GNU_SOURCE macro must be defined
before including <fcntl.h>.)
Directory notifications are normally "one-shot", and the application must
re-register to receive further notifications. Alternatively, if DN_MULTI-
SHOT 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 lat-
ter 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 directo-
ries).
Especially when using DN_MULTISHOT, a POSIX.1b real time signal should be
used for notication, so that multiple notifications can be queued.
RETURN VALUE
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 tra-
ditional 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. 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.
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 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 proto-
col 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 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.)
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.
CONFORMING TO
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.
SEE ALSO
dup2(2), flock(2), lockf(3), open(2), socket(2)
See also locks.txt, mandatory.txt, and dnotify.txt in /usr/src/linux/Documentation.
Linux-2.6.3 2004-03-03 FCNTL(2)
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