FreeBSD Man Pages

fcntl(2)

NAME

     fcntl -- file control

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <fcntl.h>

     int
     fcntl(int fd, int cmd, ...);

DESCRIPTION

     The fcntl() system call provides for control over descriptors.  The argu-
     ment 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 argu-
     ment int arg.

     F_DUPFD	Return a new descriptor as follows:

		    o	Lowest numbered available descriptor greater than or
			equal to arg.
		    o	Same object references as the original descriptor.
		    o	New descriptor shares the same file offset if the
			object was a file.
		    o	Same access mode (read, write or read/write).
		    o	Same file status flags (i.e., both file descriptors
			share the same file status flags).
		    o	The close-on-exec flag associated with the new file
			descriptor is set to remain open across execve(2) sys-
			tem calls.

     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 neg-
		ative, otherwise arg is interpreted as a process ID.

     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) sys-
		  tem call, or if a write(2) operation would block, the read
		  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 */
     };
     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 field is only
     used with F_GETLK to return the process ID of the process holding a
     tion 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 call-
     ing process has previously existing locks on bytes in the region speci-
     fied by the request, the previous lock type for each byte in the speci-
     fied 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 applica-
     tions 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 data-
     base.  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.

RETURN VALUES

     Upon successful completion, the value returned depends on cmd as follows:

	   F_DUPFD    A new file descriptor.

     Otherwise, a value of -1 is returned and errno is set to indicate the
     error.

ERRORS

     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 seg-
			ment 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_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.

     [ENOLCK]		The argument cmd is F_SETLK or F_SETLKW, and satisfy-
			ing 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 sup-
			ported.

     [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.

SEE ALSO

     close(2), execve(2), flock(2), getdtablesize(2), open(2), sigvec(2),
     lockf(3), tcgetpgrp(3), tcsetpgrp(3)

HISTORY

     The fcntl() system call appeared in 4.2BSD.

FreeBSD 5.4		       January 12, 1994 		   FreeBSD 5.4

SPONSORED LINKS