Originální popis anglicky:
fcntl - manipulate file descriptor
Návod, kniha: Linux Programmer's Manual
#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);
fcntl performs one of various miscellaneous operations on
fd. The
operation in question is determined by
cmd.
- 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.
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 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.
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 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 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. 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. Mandatory locks are enforced for all processes.
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 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 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 handler if installed with
SA_SIGINFO.
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 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).
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. 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, 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).
- 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_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_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 POSIX.1b real time signal
should be used for notication, so that multiple notifications can be
queued.
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.
- 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 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.
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.
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.
dup2(2),
flock(2),
open(2),
socket(2),
lockf(3),
capabilities(7)
See also locks.txt, mandatory.txt, and dnotify.txt in
/usr/src/linux/Documentation.
