FCNTL(2) | System Calls Manual | FCNTL(2) |
fcntl
— file
control
#include
<fcntl.h>
int
fcntl
(int
fd, int cmd,
...);
The
fcntl
()
provides control over the properties of a file that is already open. 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 is interpreted as an
int
by some commands, a pointer to a
struct flock
by others (see below), and ignored by
the rest.
The commands are:
F_DUPFD
int
).F_DUPFD_CLOEXEC
F_DUPFD
, but the
FD_CLOEXEC
flag associated with the new file
descriptor is set, so the file descriptor is closed when
execve(2) is called.F_GETFD
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
int
) is either 0 or
FD_CLOEXEC
, as described above.F_GETFL
F_SETFL
int
).F_GETOWN
SIGIO
and SIGURG
signals;
process groups are returned as negative values (arg
is ignored).F_SETOWN
SIGIO
and SIGURG
signals; process groups are specified
by supplying arg (interpreted as an
int
) as negative, otherwise
arg is taken as a process ID.The flags for the F_GETFL
and
F_SETFL
commands are as follows:
O_NONBLOCK
EAGAIN
.O_APPEND
O_APPEND
flag of
open(2).O_ASYNC
SIGIO
signal to be sent to the process
group when I/O is possible, e.g., upon availability of data to be
read.O_SYNC
O_SYNC
flag of
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
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 which is set to
F_UNLCK
.F_SETLK
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
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 area starting at
l_start+l_len and ending at
l_start-1 is locked. 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.
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) function. The
flock(2) interface has much
more rational last close semantics and allows locks to be inherited by child
processes. 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.
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD
F_DUPFD_CLOEXEC
F_GETFD
F_GETFL
F_GETOWN
Otherwise, a value of -1 is returned and errno is set to indicate the error.
fcntl
() will fail if:
EAGAIN
]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 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
]F_SETLKW
, and a deadlock condition was
detected.EINTR
]F_SETLK
or F_SETLKW
, and the function was interrupted by a
signal.EINVAL
]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
fd refers to a file that does not support
locking.
EMFILE
]F_DUPFD
and the maximum number of open file descriptors permitted for the process
are already in use, or no file descriptors greater than or equal to
arg are available.ENOLCK
]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.EOVERFLOW
]F_GETLK
,
F_SETLK
or F_SETLKW
and
the segment length of a file to be locked is too large to be represented
by an off_t.ESRCH
]F_SETOWN
and the
process ID given in arg is not in use.close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)
The fcntl
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
The fcntl
() function call appeared in
4.2BSD.
September 6, 2019 | OpenBSD-6.9 |