fcntl() - Unix, Linux System Call
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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);
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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
Tag | Description |
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.
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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.
Tag | Description |
F_GETFD |
Read the file descriptor flags.
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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).
Tag | Description |
F_GETFL |
Read the file status flags.
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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.
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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) */
...
};
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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.)
Tag | Description |
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.
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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.
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:
Tag | Description |
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).
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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 negative 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 threads 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.
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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.
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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 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.
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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 processs 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.
Tag | Description |
F_SETLEASE |
Set or remove a file lease according to which of the following
values is specified in the integer
arg:
Tag | Description |
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.
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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.
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F_UNLCK |
Remove our lease from the file.
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A process may hold only one type of lease on a file.
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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.
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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.)
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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 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 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() 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 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)
Tag | Description |
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:
Bit | Description (event in directory) |
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.v
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 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).
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RETURN VALUE
For a successful call, the return value depends on the operation:
Tag | Description |
F_DUPFD |
The new descriptor.
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F_GETFD |
Value of flags.
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F_GETFL |
Value of flags.
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F_GETOWN |
Value of descriptor owner.
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F_GETSIG |
Value of signal sent when read or write becomes possible, or zero
for traditional SIGIO behaviour.
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All other commands
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Zero.
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On error, -1 is returned, and
errno is set appropriately.
ERRORS
Tag | Description |
EACCES or EAGAIN |
Operation is prohibited by locks held by other processes.
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EAGAIN |
The operation is prohibited because the file has been memory-mapped by
another process.
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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.
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EDEADLK |
It was detected that the specified
F_SETLKW command would cause a deadlock.
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EFAULT |
lock is outside your accessible address space.
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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.
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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.
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EMFILE |
For
F_DUPFD, the process already has the maximum number of file descriptors open.
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ENOLCK |
Too many segment locks open, lock table is full, or a remote locking
protocol failed (e.g. locking over NFS).
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EPERM |
Attempted to clear the
O_APPEND flag on a file that has the append-only attribute set.
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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.)
SEE ALSO
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