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FCNTL(2)		  FreeBSD System Calls Manual		      FCNTL(2)

NAME
     fcntl -- file control

SYNOPSIS
     #include <fcntl.h>

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

DESCRIPTION
     The fcntl() provides control over the properties of a file	that is	al-
     ready 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	      Return a new descriptor as follows:

		      +o	  Lowest numbered available descriptor greater than or
			  equal	to arg (interpreted as an int).
		      +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)
			  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) is called.

     F_GETFD	      Get the close-on-exec flag associated with the file de-
		      scriptor 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	(interpreted as	an int)	is either 0 or
		      FD_CLOEXEC, as described above.

     F_GETFL	      Get file status flags associated with the	file descrip-
		      tor fd, as described below (arg is ignored).

     F_SETFL	      Set file status flags associated with the	file descrip-
		      tor fd to	arg (interpreted as an int).

     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 supply-
		      ing 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	  Non-blocking I/O; if no data is available to a read(2) 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_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.

     O_SYNC	  Cause	writes to be synchronous.  Data	will be	written	to the
		  physical device instead of just being	stored in the buffer
		  cache; corresponds to	the 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	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	function 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 de-
		scription pointed to by	the third argument, arg, taken as a
		pointer	to a struct flock (see above).	F_SETLK	is used	to es-
		tablish	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 de-
     scriptor 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 ex-
     tend 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 applica-
     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) 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.

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

	   F_DUPFD	    A new file descriptor.

	   F_DUPFD_CLOEXEC  A new file descriptor.

	   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 er-
     ror.

ERRORS
     fcntl() 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]		fd 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_SETLK or F_SETLKW, and the func-
			tion was interrupted by	a signal.

     [EINVAL]		The argument cmd is invalid.

			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]		The argument cmd is 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]		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.

     [EOVERFLOW]	The argument cmd is 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]		cmd is F_SETOWN	and the	process	ID given in arg	is not
			in use.

SEE ALSO
     close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)

STANDARDS
     The fcntl() function conforms to IEEE Std 1003.1-2008 ("POSIX.1").

HISTORY
     The fcntl() function call appeared	in 4.2BSD.

FreeBSD	13.0		       September 6, 2019		  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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