NAME
fcntl —
file descriptor control
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int
fcntl(
int
fd,
int cmd,
...);
DESCRIPTION
fcntl() provides for control over descriptors. The argument
fd is a descriptor to be operated on by
cmd as described below. The third parameter is called
arg and is technically a pointer to void, but it is
interpreted as an int by some commands and ignored by others.
Commands are:
-
-
F_DUPFD
- Return a new descriptor as follows:
- Lowest numbered
available descriptor greater than or equal to
arg, which is interpreted as an int.
- Same object references
as the original descriptor.
- New descriptor shares
the same file offset if the object was a file.
- Same access mode (read,
write or read/write).
- Same file status flags
(i.e., both file descriptors share the same file status flags).
- The close-on-exec flag
associated with the new file descriptor is cleared to remain open
across execve(2) system
calls.
-
-
F_DUPFD_CLOEXEC
- Same as
F_DUPFD
, but sets the
close-on-exec property on the file descriptor created.
-
-
F_GETFD
- Get the close-on-exec flag associated with the file
descriptor fd as
FD_CLOEXEC
.
If the returned value ANDed with FD_CLOEXEC
is 0,
the file will remain open across exec(), otherwise the
file will be closed upon execution of exec()
(arg is ignored).
-
-
F_SETFD
- Set the close-on-exec flag associated with
fd to arg, where
arg is either 0 or
FD_CLOEXEC
, as described above.
-
-
F_GETFL
- Get descriptor status flags, as described below
(arg is ignored).
-
-
F_SETFL
- Set descriptor status flags to arg,
which is interpreted as an int.
-
-
F_GETOWN
- Get the process ID or process group currently receiving
SIGIO
and SIGURG
signals;
process groups are returned as negative values (arg
is ignored).
-
-
F_SETOWN
- Set the process or process group to receive
SIGIO
and SIGURG
signals;
process groups are specified by supplying arg as
negative, otherwise arg is interpreted as a process
ID. The argument arg is interpreted as an int.
-
-
F_CLOSEM
- Close all file descriptors greater than or equal to
fd.
-
-
F_MAXFD
- Return the maximum file descriptor number currently open by
the process.
-
-
F_GETNOSIGPIPE
- Return if the
O_NOSIGPIPE
flag is
set in the file descriptor.
-
-
F_SETNOSIGPIPE
- Set or clear the
O_NOSIGPIPE
in the
file descriptor.
The set of valid flags for the
F_GETFL
and
F_SETFL
flags are as follows:
O_APPEND
,
O_ASYNC
,
O_SYNC
,
O_NONBLOCK
,
O_DSYNC
,
O_RSYNC
,
O_ALT_IO
,
O_DIRECT
,
O_NOSIGPIPE
. These flags are described in
open(2).
Several commands are available for doing advisory file locking; they all operate
on the following structure:
struct flock {
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
};
The commands available for advisory record locking are as follows:
-
-
F_GETLK
- Get the first lock that blocks the lock description pointed
to by the third argument, arg, taken as a pointer to
a struct flock (see above). The information
retrieved overwrites the information passed to fcntl in
the flock structure. If no lock is found that would
prevent this lock from being created, the structure is left unchanged by
this function call except for the lock type l_type,
which is set to
F_UNLCK
.
-
-
F_SETLK
- Set or clear a file segment lock according to the lock
description pointed to by the third argument, arg,
taken as a pointer to a struct flock (see above). As
specified by the value of l_type,
F_SETLK
is used to establish shared (or read)
locks (F_RDLCK
) or exclusive (or write) locks,
(F_WRLCK
), as well as remove either type of lock
(F_UNLCK
). If a shared or exclusive lock cannot be
set, fcntl returns immediately with
EAGAIN
.
-
-
F_SETLKW
- This command is the same as
F_SETLK
except that if a shared or exclusive lock is blocked by other locks, the
process waits until the request can be satisfied. If a signal that is to
be caught is received while fcntl is waiting for a
region, the fcntl will be interrupted if the signal
handler has not specified the SA_RESTART
(see
sigaction(2)).
When a shared lock has been set on a segment of a file, other processes can set
shared locks on that segment or a portion of it. A shared lock prevents any
other process from setting an exclusive lock on any portion of the protected
area. A request for a shared lock fails if the file descriptor was not opened
with read access.
An exclusive lock prevents any other process from setting a shared lock or an
exclusive lock on any portion of the protected area. A request for an
exclusive lock fails if the file was not opened with write access.
The value of
l_whence is
SEEK_SET
,
SEEK_CUR
, or
SEEK_END
to
indicate that the relative offset,
l_start bytes, will
be measured from the start of the file, current position, or end of the file,
respectively. The value of
l_len is the number of
consecutive bytes to be locked. If
l_len is negative,
the result is undefined. The
l_pid field is only used
with
F_GETLK
to return the process ID of the process
holding a blocking lock. After a successful
F_GETLK
request, the value of
l_whence is
SEEK_SET
.
Locks may start and extend beyond the current end of a file, but may not start
or extend before the beginning of the file. A lock is set to extend to the
largest possible value of the file offset for that file if
l_len is set to zero. If
l_whence
and
l_start point to the beginning of the file, and
l_len is zero, the entire file is locked. If an
application wishes only to do entire file locking, the
flock(2) system call is much more
efficient.
There is at most one type of lock set for each byte in the file. Before a
successful return from an
F_SETLK
or an
F_SETLKW
request when the calling process has
previously existing locks on bytes in the region specified by the request, the
previous lock type for each byte in the specified region is replaced by the
new lock type. As specified above under the descriptions of shared locks and
exclusive locks, an
F_SETLK
or an
F_SETLKW
request fails or blocks respectively when
another process has existing locks on bytes in the specified region and the
type of any of those locks conflicts with the type specified in the request.
RETURN VALUES
Upon successful completion, the value returned depends on
cmd as follows:
-
-
F_DUPFD
- A new file descriptor.
-
-
F_GETFD
- Value of flag (only the low-order bit is defined).
-
-
F_GETFL
- Value of flags.
-
-
F_GETOWN
- Value of file descriptor owner.
-
-
F_MAXFD
- Value of the highest file descriptor open by the
process.
-
-
- other
- Value other than -1.
Otherwise, a value of -1 is returned and
errno is set to
indicate the error.
COMPATIBILITY
This interface follows the completely stupid semantics of
AT&T System V UNIX and
IEEE
Std 1003.1-1988 (“POSIX.1”) that require that all locks
associated with a file for a given process are removed when
any file
descriptor for that file is closed by that process. This semantic means that
applications must be aware of any files that a subroutine library may access.
For example if an application for updating the password file locks the
password file database while making the update, and then calls
getpwnam(3) to retrieve a
record, the lock will be lost because
getpwnam(3) opens, reads, and
closes the password database. The database close will release all locks that
the process has associated with the database, even if the library routine
never requested a lock on the database.
Another minor semantic problem with this interface is that locks are not
inherited by a child process created using the
fork(2) function. The
flock(2) interface has much more
rational last close semantics and allows locks to be inherited by child
processes. Calling
flock(2) is
recommended for applications that want to ensure the integrity of their locks
when using library routines or wish to pass locks to their children. Note that
flock(2) and
fcntl locks may be safely used concurrently.
All locks associated with a file for a given process are removed when the
process terminates.
A potential for deadlock occurs if a process controlling a locked region is put
to sleep by attempting to lock the locked region of another process. This
implementation detects that sleeping until a locked region is unlocked would
cause a deadlock and fails with an
EDEADLK
error.
ERRORS
fcntl() will fail if:
-
-
- [
EAGAIN
]
- The argument arg is
F_SETLK
, the type of lock
(l_type) is a shared lock
(F_RDLCK
) or exclusive lock
(F_WRLCK
), and the segment of a file to be locked
is already exclusive-locked by another process; or the type is an
exclusive lock and some portion of the segment of a file to be locked is
already shared-locked or exclusive-locked by another process.
-
-
- [
EBADF
]
- fildes is not a valid open file
descriptor.
The argument cmd is
F_SETLK
or
F_SETLKW
, the type of lock
(l_type) is a shared lock
(F_RDLCK
), and fildes is not
a valid file descriptor open for reading.
The argument cmd is F_SETLK
or
F_SETLKW
, the type of lock
(l_type) is an exclusive lock
(F_WRLCK
), and fildes is not
a valid file descriptor open for writing.
-
-
- [
EDEADLK
]
- The argument cmd is
F_SETLKW
, and a deadlock condition was
detected.
-
-
- [
EINTR
]
- The argument cmd is
F_SETLKW
, and the function was interrupted by a
signal.
-
-
- [
EINVAL
]
- The argument cmd is invalid.
The argument cmd is
F_DUPFD
and arg is negative or greater than the maximum
allowable number (see
getdtablesize(3)).
The argument cmd is F_GETLK
,
F_SETLK
, or F_SETLKW
and
the data to which arg points is not valid, or
fildes refers to a file that does not support
locking.
-
-
- [
EMFILE
]
- The argument cmd is
F_DUPFD
and the maximum number of file descriptors
permitted for the process are already in use, or no file descriptors
greater than or equal to arg are available.
-
-
- [
ENFILE
]
- cmd is
F_DUPFD
and system-wide the maximum allowed number
of file descriptors are currently open.
-
-
- [
ENOLCK
]
- The argument cmd is
F_SETLK
or F_SETLKW
, and
satisfying the lock or unlock request would result in the number of locked
regions in the system exceeding a system-imposed limit.
-
-
- [
ESRCH
]
- cmd is
F_SETOWN
and the process ID given as argument is
not in use.
SEE ALSO
close(2),
execve(2),
flock(2),
open(2),
sigaction(2),
getdtablesize(3)
STANDARDS
The
fcntl() function conforms to
IEEE Std
1003.1-1990 (“POSIX.1”).
HISTORY
The
fcntl() function call appeared in
4.2BSD.