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FCNTL(2)		       System Calls Manual			FCNTL(2)

NAME
     fcntl -- file control

SYNOPSIS
     #include <fcntl.h>

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

DESCRIPTION
     Fcntl()  provides	for  control over descriptors.	The argument fd is a de-
     scriptor to be operated on by cmd as follows:

     F_DUPFD	Return a new descriptor as follows:

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

     F_GETFD	Get the close-on-exec flag associated with the	file  descriptor
		fd.   If  the low-order bit of the returned value 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 the low order
		bit of arg (0 or 1 as above).

     F_GETFL	Get descriptor status flags, as  described  below  (arg  is  ig-
		nored).

     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 val-
		ues (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 nega-
		tive, 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)  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.

     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 informa-
		tion 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	descrip-
		tion  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 imme-
		diately 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 un-
		til 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  pre-
     vents  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 descrip-
     tor 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 nega-
     tive, 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 effi-
     cient.

     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 descrip-
     tions of shared locks and exclusive locks, an F_SETLK or  an  F_SETLKW  re-
     quest  fails or blocks respectively when another process has existing locks
     on bytes in the specified region and the type of any of  those  locks  con-
     flicts 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 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 database.	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(2) locks may be safely used concur-
     rently.

     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.

IMPLEMENTATION NOTES
     In the non-threaded library fcntl() is implemented as the fcntl syscall.

     In  the threaded library, the fcntl syscall is assembled to _thread_sys_fc-
     ntl() and fcntl() is  implemented	as  a  function  which	disables  thread
     rescheduling,  locks fd for read and write, then calls _thread_sys_fcntl().
     Before returning, fcntl() unlocks fd and enables thread rescheduling.

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.

	   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 exclu-
			sive-locked by another process; or the type is an exclu-
			sive  lock  and some portion of the segment 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 writing.

     [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]		Cmd 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,  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.

     [ENOLCK]		The argument cmd is 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]		Cmd 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 op-
     posed 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 reasons as
     stated in tcgetpgrp(3).

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

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

4.2 Berkeley Distribution	January 12, 1994			FCNTL(2)
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