int
fcntl(
int fd
, int cmd
, ...
)
)
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
arg
,
which is interpreted as an int.
F_GETFD
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
fd
to
arg
,
where
arg
is either 0 or
FD_CLOEXEC
,
as described above.
F_GETFL
(arg
is ignored).
F_SETFL
arg
,
which is interpreted as an 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
as negative, otherwise
arg
is interpreted as a process ID.
The argument
arg
is interpreted as an int.
F_CLOSEM
F_MAXFD
The flags for the
F_GETFL
and
F_SETFL
flags 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.
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
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
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
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.
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 _a_n_y 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.
cmd
as follows:
F_DUPFD
F_GETFD
F_GETFL
F_GETOWN
F_MAXFD
Otherwise, a value of -1 is returned and errno is set to indicate the error.
)
will fail if:
EAGAIN
]
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
]
cmd
is
F_SETLKW
,
and a deadlock condition was detected.
EINTR
]
cmd
is
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
fildes
refers to a file that does not support locking.
EMFILE
]
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
]
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.
)
function conforms to
ISO/IEC 9945-1:1990 (``POSIX.1'') .
)
function call appeared in
4.2BSD.