| FCNTL(2) | System Calls Manual | FCNTL(2) |
fcntl —
#include <fcntl.h>
int
fcntl(int
fd, int cmd,
...);
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_DUPFDF_DUPFD_CLOEXECF_DUPFD, but sets the close-on-exec
property on the file descriptor created.F_GETFDFD_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_SETFDFD_CLOEXEC, as described above.F_GETFLF_SETFLF_GETOWNSIGIO and SIGURG signals;
process groups are returned as negative values (arg
is ignored).F_SETOWNSIGIO
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_CLOSEMF_MAXFDF_GETNOSIGPIPEO_NOSIGPIPE flag is set in the file
descriptor.F_SETNOSIGPIPEO_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_GETLKfcntl 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_SETLKF_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_SETLKWF_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.
Otherwise, a value of -1 is returned and errno is set to indicate the error.
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.
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 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]F_SETLKW, and a deadlock condition was
detected.EINTR]F_SETLKW, and the function was interrupted by a
signal.EINVAL]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]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]F_DUPFD and
system-wide the maximum allowed number of file descriptors are currently
open.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.ESRCH]F_SETOWN and the
process ID given as argument is not in use.fcntl() function conforms to IEEE
Std 1003.1-1990 (“POSIX.1”).
fcntl() function call appeared in
4.2BSD.
| January 23, 2012 | NetBSD 9.2 |