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FCNTL(2) 			     BSD 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 argument 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 argument 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 descriptor.
			   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 supplying arg as negative, otherwise arg
		       is interpreted as a process ID.

     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, a negative value restores the system
		       default.

     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.

     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)  system 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 writing.  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 fol-
     lowing 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 argu-
		ment, 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 struc-
		ture 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) ).

     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 off-
     set, 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 application
     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 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
     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 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 data-
     base 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) 
     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 termi-
     nates.

     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 segment 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 maxi-
			mum 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 satisfying 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 supported.

     [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 reasons 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.

BSD					 February 8, 2013				      BSD
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