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FCNTL(2)		    BSD 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
     descriptor 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
			descriptor is set to remain open across execv(2) sys‐
			tem 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
		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 supplying arg as neg‐
		ative, 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 call, or
		  if a write 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
		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
		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 EACCES.

     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
     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, 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 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
     getpwname(3) to retrieve a record, the lock will be lost because
     getpwname(3) opens, reads, and closes the password database.  The data‐
     base 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 con‐
     currently.

     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_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
     Fcntl() will fail if:

     [EACCES]		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.

     [EMFILE]		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
			interrupted 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_DUPED 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.

     [ESRCH]		Cmd is F_SETOWN and the process 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.

4.2 Berkeley Distribution      January 12, 1994	     4.2 Berkeley Distribution
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