fcntl
—
file control
Standard C Library (libc, -lc)
#include
<fcntl.h>
int
fcntl
(
int
fd,
int
cmd,
...);
The
fcntl
() system call provides for control
over descriptors. The argument
fd is a
descriptor to be operated on by
cmd as
described below. Depending on the value of
cmd,
fcntl
() can take an additional third
argument
int arg.
F_DUPFD
- Return a new descriptor as follows:
- Lowest numbered available descriptor greater than or equal to
arg.
- 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
FD_CLOEXEC
associated with the new file descriptor is cleared, so the file
descriptor is to remain open across
execve(2)
system calls.
F_DUPFD_CLOEXEC
- Like
F_DUPFD
, but the
FD_CLOEXEC
flag associated with the new
file descriptor is set, so the file descriptor is closed when
execve(2)
system call executes.
F_DUP2FD
- It is functionally equivalent to
F_DUP2FD_CLOEXEC
- Like
F_DUP2FD
, but the
FD_CLOEXEC
flag associated with the new
file descriptor is set.
The F_DUP2FD
and
F_DUP2FD_CLOEXEC
constants are not
portable, so they should not be used if portability is needed. Use
dup2
() instead of
F_DUP2FD
.
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.
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.
F_READAHEAD
- Set or clear the read ahead amount for sequential access to the third
argument, arg, which is rounded up to the
nearest block size. A zero value in arg
turns off read ahead, a negative value restores the system default.
F_RDAHEAD
- Equivalent to Darwin counterpart which sets read ahead amount of 128KB
when the third argument, arg is non-zero.
A zero value in arg turns off read
ahead.
The flags for the
F_GETFL
and
F_SETFL
flags are as follows:
O_NONBLOCK
- Non-blocking I/O; if no data is available to a
read(2)
system call, or if a
write(2)
operation would block, the read or write call returns -1 with the error
EAGAIN
.
O_APPEND
- Force each write to append at the end of file; corresponds to the
O_APPEND
flag of
open(2).
O_DIRECT
- Minimize or eliminate the cache effects of reading and writing. The system
will attempt to avoid caching the data you read or write. If it cannot
avoid caching the data, it will minimize the impact the data has on the
cache. Use of this flag can drastically reduce performance if not used
with care.
O_ASYNC
- Enable the
SIGIO
signal to be sent to
the process group when I/O is possible, e.g., upon availability of data to
be read.
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 */
int l_sysid; /* remote system id or zero for local */
};
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 system call except for the lock 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).
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,
l_start means end edge of the region. The
l_pid and
l_sysid fields are only used with
F_GETLK
to return the process ID of the
process holding a blocking lock and the system ID of the system that owns that
process. Locks created by the local system will have a system ID of zero.
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.
The queuing for
F_SETLKW
requests on local
files is fair; that is, while the thread is blocked, subsequent requests
conflicting with its requests will not be granted, even if these requests do
not conflict with existing locks.
This interface follows the completely stupid semantics of System V 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)
system call. The
flock(2)
interface has much more rational last close semantics and allows locks to be
inherited by child processes. The
flock(2)
system call 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.
The
fcntl
(),
flock(2),
and
lockf(3)
locks are compatible. Processes using different locking interfaces can
cooperate over the same file safely. However, only one of such interfaces
should be used within the same process. If a file is locked by a process
through
flock(2),
any record within the file will be seen as locked from the viewpoint of
another process using
fcntl
() or
lockf(3),
and vice versa. Note that
fcntl
(
F_GETLK)
returns -1 in
l_pid if the process holding a
blocking lock previously locked the file descriptor by
flock(2).
All locks associated with a file for a given process are removed when the
process terminates.
All locks obtained before a call to
execve(2)
remain in effect until the new program releases them. If the new program does
not know about the locks, they will not be released until the program exits.
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.
Upon successful completion, the value returned depends on
cmd as follows:
F_DUPFD
- A new file descriptor.
F_DUP2FD
- A file descriptor equal to arg.
F_GETFD
- Value of flag (only the low-order bit is defined).
F_GETFL
- Value of flags.
F_GETOWN
- Value of file descriptor owner.
- other
- Value other than -1.
Otherwise, a value of -1 is returned and
errno
is set to indicate the error.
The
fcntl
() system call will fail if:
- [
EAGAIN
]
- The argument cmd 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
]
- The fd argument is not a valid open file
descriptor.
The argument cmd is
F_DUP2FD
, and
arg is not a valid file descriptor.
The argument cmd is
F_SETLK
or
F_SETLKW
, the type of lock
(l_type) is a shared lock
(F_RDLCK
), and
fd 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
fd 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 system call was
interrupted by a signal.
- [
EINVAL
]
- The cmd argument is
F_DUPFD
and
arg is negative or greater than the
maximum allowable number (see
getdtablesize(2)).
The argument cmd is
F_GETLK
,
F_SETLK
or
F_SETLKW
and the data to which
arg points is not valid.
- [
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.
- [
ENOTTY
]
- The fd argument is not a valid file
descriptor for the requested operation. This may be the case if
fd is a device node, or a descriptor
returned by
kqueue(2).
- [
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.
- [
EOPNOTSUPP
]
- The argument cmd is
F_GETLK
,
F_SETLK
or
F_SETLKW
and
fd refers to a file for which locking is
not supported.
- [
EOVERFLOW
]
- The argument cmd is
F_GETLK
,
F_SETLK
or
F_SETLKW
and an
off_t calculation overflowed.
- [
EPERM
]
- The cmd argument is
F_SETOWN
and the process ID or process
group given as an argument is in a different session than the caller.
- [
ESRCH
]
- The cmd argument is
F_SETOWN
and the process ID given as
argument is not in use.
In addition, if
fd refers to a descriptor open
on a terminal device (as opposed to a descriptor open on a socket), a
cmd of
F_SETOWN
can fail for the same reasons as
in
tcsetpgrp(3),
and a
cmd of
F_GETOWN
for the reasons as stated in
tcgetpgrp(3).
close(2),
dup2(2),
execve(2),
flock(2),
getdtablesize(2),
open(2),
sigaction(2),
lockf(3),
tcgetpgrp(3),
tcsetpgrp(3)
The
F_DUP2FD
constant is non portable. It is
provided for compatibility with AIX and Solaris.
Per
Version 4 of the Single UNIX
Specification (“SUSv4”), a call with
F_SETLKW
should fail with
[
EINTR
] after any caught signal and should
continue waiting during thread suspension such as a stop signal. However, in
this implementation a call with
F_SETLKW
is
restarted after catching a signal with a
SA_RESTART
handler or a thread suspension
such as a stop signal.
The
fcntl
() system call appeared in
4.2BSD.
The
F_DUP2FD
constant first appeared in
FreeBSD 7.1.