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FCNTL(2)		    BSD Programmer's Manual		      FCNTL(2)

NAME
     fcntl - file control

SYNOPSIS
     #include <sys/types.h>
     #include <fcntl.h>

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

DESCRIPTION
     Fcntl() provides control over file descriptors.  The argument fd is a
     file descriptor to be operated on by a command cmd. Some operations use a
     third argument, which is either an integer int iarg or a pointer void
     *parg as appropriate.  The commands are:

     F_DUPFD	Return a new file descriptor as follows:

		    o	It uses the lowest numbered available descriptor
			greater than or equal to iarg.
		    o	It references the same object as the original descrip-
			tor.
		    o	It shares the same file offset if the object was a
			file.
		    o	It shares file status flags with the old descriptor,
			including the access mode (read, write or read/write).
		    o	It does not share file descriptor flags (FD_CLOEXEC,
			etc.)  with the old descriptor.

     F_GETFD	Get the file descriptor flags, as specified below.  File de-
		scriptor flags apply to the given file descriptor and they are
		not copied by F_DUPFD or any other descriptor duplication op-
		eration.  When a process calls fork(2) to create a new pro-
		cess, the new process receives copies of the parent process's
		file descriptor flags along with copies of the parent pro-
		cess's file descriptors; the processes do not share file de-
		scriptor flags.

     F_SETFD	Set the file descriptor flags to iarg.

     F_GETFL	Get the file status flags, as specified below.	File status
		flags are shared among all file descriptors that are descended
		from an original file descriptor that was created by open(2)
		or some other descriptor-creating call.	 Specifically, file
		status flags are shared between old and new file descriptors
		after an F_DUPFD operation, and they are shared with (not
		copied to) the corresponding file descriptors in a child pro-
		cess after a fork(2) call.

     F_SETFL	Set the file status flags to iarg.

     F_GETOWN	Get the process ID or process group currently receiving SIGIO
		and SIGURG signals; process groups are returned as negative
		values.

     F_SETOWN	Set the process or process group to receive SIGIO and SIGURG
		signals; process groups are specified by supplying iarg as
		negative, otherwise iarg is interpreted as a process ID.

     The file descriptor flags are:

     FD_CLOEXEC	  Automatically close this descriptor immediately before an
		  execve(2) call succeeds.

     The file status flags are:

     O_NONBLOCK	  Enable non-blocking I/O.  If no data is available to a
		  read(2) operation, or if a write(2) operation would block
		  because of flow control or other reasons, and the file de-
		  scriptor for the given operation is non-blocking, then the
		  operation returns -1 with the global variable errno set to
		  EAGAIN.

     O_APPEND	  Force each write(2) operation to append data at the end of
		  the given file.  This flag 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, for example, 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:

     F_GETLK	Get the first lock that blocks the lock description pointed to
		by parg, 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 parg, 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 -1
		with the global variable errno set to EAGAIN.

     F_SETLKW	Set or clear a file segment lock like F_SETLK except that if a
		shared or exclusive lock is blocked by other locks, the pro-
		cess 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 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.

     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 pro-
     cess.  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_GETFD    The file descriptor flags.

	   F_GETFL    The file status flags.

	   F_GETOWN   The 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 segment of a file to be locked is al-
		   ready 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.

     [EMFILE]	   The argument cmd is F_DUPFD and the maximum allowed number
		   of file descriptors are currently open.

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

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

     [EINVAL]	   The argument cmd is F_DUPFD and iarg 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 parg 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 iarg 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.

     [ENOTTY]	   The filedes is not the controlling tty for the process.

     [EPERM]	   The argument cmd is F_SETOWN and the user is not permitted
		   to signal the given process or group ID.

		   The argument cmd is F_SETOWN and filedes is a tty, and the
		   specified process group (or the process group of the speci-
		   fied process) is not in the same session as the caller.

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

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

HISTORY
     The fcntl function call appeared in 4.2BSD.

BUGS
     This interface follows the completely stupid semantics of System V and
     IEEE Std1003.1-1988 (``POSIX'') 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 getpw-
     name(3) to retrieve a record, the lock will be lost because getpwname(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 inherit-
     ed 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 applica-
     tions 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 concurrently.

     The differences between file descriptor flags, file status flags and de-
     vice parameters (from ioctl(2))  are subtle and confusing.	 Device param-
     eters apply to an object; file status flags apply to an open instance of
     an object; file descriptor flags apply to a reference to an open instance
     of an object.

4.2 Berkeley Distribution	April 10, 2000				     5
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