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OPEN(2)			  FreeBSD System Calls Manual		       OPEN(2)

NAME
     open -- open or create a file for reading or writing

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <fcntl.h>

     int
     open(const	char *path, int	flags, ...);

DESCRIPTION
     The file name specified by	path is	opened for reading and/or writing as
     specified by the argument flags and the file descriptor returned to the
     calling process.  The flags argument may indicate the file	is to be cre-
     ated if it	does not exist (by specifying the O_CREAT flag).  In this case
     open requires a third argument mode_t mode, and the file is created with
     mode mode as described in chmod(2)	and modified by	the process' umask
     value (see	umask(2)).

     The flags specified are formed by or'ing the following values

	   O_RDONLY	   open	for reading only
	   O_WRONLY	   open	for writing only
	   O_RDWR	   open	for reading and	writing
	   O_NONBLOCK	   do not block	on open
	   O_APPEND	   append on each write
	   O_CREAT	   create file if it does not exist
	   O_TRUNC	   truncate size to 0
	   O_EXCL	   error if create and file exists
	   O_SHLOCK	   atomically obtain a shared lock
	   O_EXLOCK	   atomically obtain an	exclusive lock
	   O_DIRECT	   eliminate or	reduce cache effects
	   O_FSYNC	   synchronous writes
	   O_NOFOLLOW	   do not follow symlinks

     Opening a file with O_APPEND set causes each write	on the file to be
     appended to the end.  If O_TRUNC is specified and the file	exists,	the
     file is truncated to zero length.	If O_EXCL is set with O_CREAT and the
     file already exists, open() returns an error.  This may be	used to	imple-
     ment a simple exclusive access locking mechanism.	If O_EXCL is set and
     the last component	of the pathname	is a symbolic link, open() will	fail
     even if the symbolic link points to a non-existent	name.  If the
     O_NONBLOCK	flag is	specified and the open() call would result in the
     process being blocked for some reason (e.g., waiting for carrier on a
     dialup line), open() returns immediately.	The first time the process
     attempts to perform I/O on	the open file it will block (not currently
     implemented).

     If	O_FSYNC	is used	in the mask, all writes	will immediately be written to
     disk, the kernel will not cache written data and all writes on the
     descriptor	will not return	until the data to be written completes.

     If	O_NOFOLLOW is used in the mask and the target file passed to open() is
     a symbolic	link then the open() will fail.

     When opening a file, a lock with flock(2) semantics can be	obtained by
     setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock.
     If	creating a file	with O_CREAT, the request for the lock will never fail
     (provided that the	underlying filesystem supports locking).

     O_DIRECT may be used to minimize or eliminate the cache effects of	read-
     ing 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.

     If	successful, open() returns a non-negative integer, termed a file
     descriptor.  It returns -1	on failure.  The file pointer used to mark the
     current position within the file is set to	the beginning of the file.

     When a new	file is	created	it is given the	group of the directory which
     contains it.

     The new descriptor	is set to remain open across execve(2) system calls;
     see close(2) and fcntl(2).

     The system	imposes	a limit	on the number of file descriptors open simul-
     taneously by one process.	Getdtablesize(2) returns the current system
     limit.

RETURN VALUES
     If	successful, open() returns a non-negative integer, termed a file
     descriptor.  It returns -1	on failure, and	sets errno to indicate the
     error.

ERRORS
     The named file is opened unless:

     [ENOTDIR]		A component of the path	prefix is not a	directory.

     [ENAMETOOLONG]	A component of a pathname exceeded 255 characters, or
			an entire path name exceeded 1023 characters.

     [ENOENT]		O_CREAT	is not set and the named file does not exist.

     [ENOENT]		A component of the path	name that must exist does not
			exist.

     [EACCES]		Search permission is denied for	a component of the
			path prefix.

     [EACCES]		The required permissions (for reading and/or writing)
			are denied for the given flags.

     [EACCES]		O_CREAT	is specified, the file does not	exist, and the
			directory in which it is to be created does not	permit
			writing.

     [ELOOP]		Too many symbolic links	were encountered in translat-
			ing the	pathname.

     [EISDIR]		The named file is a directory, and the arguments spec-
			ify it is to be	opened for writing.

     [EROFS]		The named file resides on a read-only file system, and
			the file is to be modified.

     [EMFILE]		The process has	already	reached	its limit for open
			file descriptors.

     [ENFILE]		The system file	table is full.

     [EMLINK]		O_NOFOLLOW was specified and the target	is a symbolic
			link.

     [ENXIO]		The named file is a character special or block special
			file, and the device associated	with this special file
			does not exist.

     [ENXIO]		The named file is a fifo, no process has it open for
			reading, and the arguments specify it is to be opened
			for writing.

     [EINTR]		The open() operation was interrupted by	a signal.

     [EOPNOTSUPP]	O_SHLOCK or O_EXLOCK is	specified but the underlying
			filesystem does	not support locking.

     [EWOULDBLOCK]	O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci-
			fied and the file is locked.

     [ENOSPC]		O_CREAT	is specified, the file does not	exist, and the
			directory in which the entry for the new file is being
			placed cannot be extended because there	is no space
			left on	the file system	containing the directory.

     [ENOSPC]		O_CREAT	is specified, the file does not	exist, and
			there are no free inodes on the	file system on which
			the file is being created.

     [EDQUOT]		O_CREAT	is specified, the file does not	exist, and the
			directory in which the entry for the new file is being
			placed cannot be extended because the user's quota of
			disk blocks on the file	system containing the direc-
			tory has been exhausted.

     [EDQUOT]		O_CREAT	is specified, the file does not	exist, and the
			user's quota of	inodes on the file system on which the
			file is	being created has been exhausted.

     [EIO]		An I/O error occurred while making the directory entry
			or allocating the inode	for O_CREAT.

     [ETXTBSY]		The file is a pure procedure (shared text) file	that
			is being executed and the open() call requests write
			access.

     [EFAULT]		Path points outside the	process's allocated address
			space.

     [EEXIST]		O_CREAT	and O_EXCL were	specified and the file exists.

     [EOPNOTSUPP]	An attempt was made to open a socket (not currently
			implemented).

     [EINVAL]		An attempt was made to open a descriptor with an ille-
			gal combination	of O_RDONLY, O_WRONLY, and O_RDWR.

SEE ALSO
     chmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2),
     umask(2), write(2)

HISTORY
     An	open() function	call appeared in Version 6 AT&T	UNIX.

FreeBSD	10.1		       November	16, 1993		  FreeBSD 10.1

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

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