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FCNTL(2)		  FreeBSD System Calls Manual		      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 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
			       descriptors share the same file status flags).
			   +o   The close-on-exec flag FD_CLOEXEC associated
			       with the	new file descriptor is cleared,	so the
			       file descriptor is to remain open across
			       execve(2) system	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)	system call executes.

     F_DUP2FD	       It is functionally equivalent to

			     dup2(fd, arg)

     F_DUP2FD_CLOEXEC  Like F_DUP2FD, but the FD_CLOEXEC flag associated with
		       the new file descriptor is set.

		       The F_DUP2FD and	F_DUP2FD_CLOEXEC constants are not
		       portable, so they should	not be used if portability is
		       needed.	Use dup2() instead of F_DUP2FD.

     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 supply-
		       ing arg as negative, 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
		  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_DIRECT	  Minimize or eliminate	the cache effects of reading and writ-
		  ing.	The system will	attempt	to avoid caching the data you
		  read or write.  If it	cannot avoid caching the data, it will
		  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	*/
	     int     l_sysid;	     /*	remote system id or zero for local */
     };
     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)).

     F_READAHEAD
		Set or clear the read ahead amount for sequential access to
		the third argument, arg, which is rounded up to	the nearest
		block size.  A zero value in arg turns off read	ahead.

     F_RDAHEAD	Equivalent to Darwin counterpart which sets read ahead amount
		of 128KB when the third	argument, arg is non-zero.  A zero
		value in arg turns off read ahead.

     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	and l_sysid
     fields are	only used with F_GETLK to return the process ID	of the process
     holding a blocking	lock and the system ID of the system that owns that
     process.  Locks created by	the local system will have a system ID of
     zero.  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 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) 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.

	   F_DUP2FD   A	file descriptor	equal to arg.

	   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
     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_DUP2FD, and arg is not a valid
			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.

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

     In	addition, if fd	refers to a descriptor open on a terminal device (as
     opposed to	a descriptor open on a socket),	a cmd of F_SETOWN can fail for
     the same reasons as in tcsetpgrp(3), and a	cmd of F_GETOWN	for the	rea-
     sons as stated in tcgetpgrp(3).

SEE ALSO
     close(2), dup2(2),	execve(2), flock(2), getdtablesize(2), open(2),
     sigaction(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3)

STANDARDS
     The F_DUP2FD constant is non portable.  It	is provided for	compatibility
     with AIX and Solaris.

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

     The F_DUP2FD constant first appeared in FreeBSD 7.1.

FreeBSD	10.1			 July 27, 2012			  FreeBSD 10.1

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

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