FCNTL(2) Linux Programmer's 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 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 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, 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 modi-
fied 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 specified 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 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_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 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 inter-
rupted 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 hold-
ing 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.
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 permission on the file and enabling
the set-GID permission bit).
Advisory locks are not enforced and are useful only between cooperating processes. Manda-
tory locks are enforced for all processes.
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 groups are returned as negative values.
F_SETOWN
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 sig-
nal 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 situation where select(2)
would report the socket as having an "exceptional condition".) 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 han-
dler if installed with SA_SIGINFO.
By using F_SETSIG with a non-zero value, and setting SA_SIGINFO for the signal han-
dler (see sigaction(2)), extra information about I/O events is passed to the han-
dler 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 han-
dler, 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_SET-
SIG 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 pro-
cess opens the file for writing or truncates it.
F_WRLCK
Take out a write lease. This will cause us to be notified when another pro-
cess 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 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 the system 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 sufficient 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_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
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 speci-
fied 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 .)
Directory notifications are normally "one-shot", and the application must re-regis-
ter 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 han-
dler was established using SA_SIGINFO) and the si_fd field of this structure con-
tains the file descriptor which generated the notification (useful when establish-
ing notification on multiple directories).
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 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. Or, operation is prohib-
ited 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 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 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 differ-
ent 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 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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