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

NAME
     fcntl -- file descriptor control

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <fcntl.h>

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

DESCRIPTION
     fcntl() provides for control over descriptors.  The argument fd is	a de-
     scriptor to be operated on	by cmd as described below.  The	third parame-
     ter is called arg and is technically a pointer to void, but it is inter-
     preted as an int by some commands and ignored by others.

     Commands are:

     F_DUPFD	      Return a new descriptor as follows:

			  +o   Lowest numbered available	descriptor greater
			      than or equal to arg, which is interpreted as an
			      int.
			  +o   Same object references as	the original descrip-
			      tor.
			  +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 descrip-
			      tors share the same file status flags).
			  +o   The close-on-exec	flag associated	with the new
			      file descriptor is cleared to remain open	across
			      execve(2)	system calls.

     F_DUPFD_CLOEXEC  Same as F_DUPFD, but sets	the close-on-exec property on
		      the file descriptor created.

     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	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, which	is 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 as negative, otherwise arg is interpreted	as a
		      process ID.  The argument	arg is interpreted as an int.

     F_CLOSEM	      Close all	file descriptors greater than or equal to fd.

     F_MAXFD	      Return the maximum file descriptor number	currently open
		      by the process.

     F_GETNOSIGPIPE   Return if	the O_NOSIGPIPE	flag is	set in the file	de-
		      scriptor.

     F_SETNOSIGPIPE   Set or clear the O_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_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 l_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).	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	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	speci-
		fied 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 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 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.

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

	   F_DUPFD    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.

	   F_MAXFD    Value of the highest file	descriptor open	by the
		      process.

	   other      Value other than -1.

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

COMPATIBILITY
     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 as-
     sociated with a file for a	given process are removed when any file	de-
     scriptor 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 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.	Calling	flock(2) is recom-
     mended 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.

ERRORS
     fcntl() will fail if:

     [EAGAIN]		The argument 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 seg-
			ment 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 writ-
			ing.

     [EDEADLK]		The argument cmd is F_SETLKW, and a deadlock condition
			was detected.

     [EINTR]		The argument cmd is F_SETLKW, and the function was in-
			terrupted by a signal.

     [EINVAL]		The argument cmd is invalid.

			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]		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.

     [ENFILE]		cmd is F_DUPFD and system-wide the maximum allowed
			number of file descriptors are currently open.

     [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.

     [ESRCH]		cmd is F_SETOWN	and the	process	ID given as argument
			is not in use.

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

STANDARDS
     The fcntl() function conforms to ISO/IEC 9945-1:1990 ("POSIX.1").

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

FreeBSD	13.0		       January 23, 2012			  FreeBSD 13.0

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

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