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

NAME
     open, openat -- open or create a file for reading,	writing	or executing

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <fcntl.h>

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

     int
     openat(int	fd, const char *path, int flags, ...);

DESCRIPTION
     The file name specified by	path is	opened for either execution or 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 created if it does not exist	(by specifying the O_CREAT
     flag).  In	this case open() and openat() require an additional 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 openat() function is equivalent to the	open() function	except in the
     case where	the path specifies a relative path.  For openat() and relative
     path, the file to be opened is determined relative	to the directory asso-
     ciated with the file descriptor fd	instead	of the current working direc-
     tory.  The	flag parameter and the optional	fourth parameter correspond
     exactly to	the parameters of open().  If openat() is passed the special
     value AT_FDCWD in the fd parameter, the current working directory is used
     and the behavior is identical to a	call to	open().

     When openat() is called with an absolute path, it ignores the fd argu-
     ment.

     In	capsicum(4) capability mode, open() is not permitted.  The path	argu-
     ment to openat() must be strictly relative	to a file descriptor fd.  path
     must not be an absolute path and must not contain ".." components which
     cause the path resolution to escape the directory hierarchy starting at
     fd.  Additionally,	no symbolic link in path may target absolute path or
     contain escaping ".." components.	fd must	not be AT_FDCWD.

     If	the vfs.lookup_cap_dotdot sysctl(3) MIB	is set to zero,	".." compo-
     nents in the paths, used in capability mode, are completely disabled.  If
     the vfs.lookup_cap_dotdot_nonlocal	MIB is set to zero, ".." is not	al-
     lowed if found on non-local filesystem.

     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_EXEC	   open	for execute only
	   O_SEARCH	   open	for search only, an alias for O_EXEC
	   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 (historical synonym for O_SYNC)
	   O_SYNC	   synchronous writes
	   O_DSYNC	   synchronous data writes
	   O_NOFOLLOW	   do not follow symlinks
	   O_NOCTTY	   ignored
	   O_TTY_INIT	   ignored
	   O_DIRECTORY	   error if file is not	a directory
	   O_CLOEXEC	   set FD_CLOEXEC upon open
	   O_VERIFY	   verify the contents of the file
	   O_RESOLVE_BENEATH	   path	resolution must	not cross the fd directory
	   O_PATH	   record only the target path in the opened descriptor
	   O_EMPTY_PATH	   openat, open	file referenced	by fd if path is empty

     Opening a file with O_APPEND set causes each write	on the file to be ap-
     pended 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 implement 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() system	call would result in the process being
     blocked for some reason (e.g., waiting for	carrier	on a dialup line),
     open() returns immediately.  The descriptor remains in non-blocking mode
     for subsequent operations.

     If	O_SYNC is used in the mask, all	writes will immediately	and syn-
     chronously	be written to disk.  O_FSYNC is	an historical synonym for
     O_SYNC.

     If	O_DSYNC	is used	in the mask, all data and metadata required to read
     the data will be synchronously written to disk, but changes to metadata
     such as file access and modification timestamps may be written later.

     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 file	system 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 re-
     duce performance if not used with care.

     O_NOCTTY may be used to ensure the	OS does	not assign this	file as	the
     controlling terminal when it opens	a tty device.  This is the default on
     FreeBSD, but is present for POSIX compatibility.  The open() system call
     will not assign controlling terminals on FreeBSD.

     O_TTY_INIT	may be used to ensure the OS restores the terminal attributes
     when initially opening a TTY.  This is the	default	on FreeBSD, but	is
     present for POSIX compatibility.  The initial call	to open() on a TTY
     will always restore default terminal attributes on	FreeBSD.

     O_DIRECTORY may be	used to	ensure the resulting file descriptor refers to
     a directory.  This	flag can be used to prevent applications with elevated
     privileges	from opening files which are even unsafe to open with
     O_RDONLY, such as device nodes.

     O_CLOEXEC may be used to set FD_CLOEXEC flag for the newly	returned file
     descriptor.

     O_VERIFY may be used to indicate to the kernel that the contents of the
     file should be verified before allowing the open to proceed.  The details
     of	what "verified"	means is implementation	specific.  The run-time	linker
     (rtld) uses this flag to ensure shared objects have been verified before
     operating on them.

     O_RESOLVE_BENEATH returns ENOTCAPABLE if any intermediate component of
     the specified relative path does not reside in the	directory hierarchy
     beneath the starting directory.  Absolute paths or	even the temporal es-
     cape from beneath of the starting directory is not	allowed.

     When fd is	opened with O_SEARCH, execute permissions are checked at open
     time.  The	fd may not be used for any read	operations like
     getdirentries(2).	The primary use	for this descriptor will be as the
     lookup descriptor for the *at() family of functions.

     O_PATH returns a file descriptor that can be used as a directory file de-
     scriptor for openat(2) and	other system calls taking a file descriptor
     argument, like fstatat(2) and others.  The	other functionality of the re-
     turned file descriptor is limited to the descriptor-level operations.  It
     can be used for
	   fcntl(2)	  but advisory locking is not allowed
	   dup(2)
	   close(2)
	   fstat(2)
	   fexecve(2)
	   SCM_RIGHTS	  can be passed	over a unix(4) socket using a
			  SCM_RIGHTS message
	   kqueue(2)	  using	for EVFILT_VNODE
	   readlinkat(2)
     But operations like read(2), ftruncate(2),	and any	other that operate on
     file and not on file descriptor (except fstat(2) ), are not allowed.
     File opened with the O_PATH flag does not prevent non-forced unmount of
     the volume	it belongs to.

     A file descriptor created with the	O_PATH flag can	be opened into normal
     (operable)	file descriptor	by specifying it as the	fd argument to
     openat() with empty path and flag O_EMPTY_PATH.  Such an open behaves as
     if	the current path of the	file referenced	by fd is passed, except	that
     the path walk permissions are not checked.	 See also the description of
     AT_EMPTY_PATH flag	for fstatat(2) and related syscalls.

     If	successful, open() returns a non-negative integer, termed a file de-
     scriptor.	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.

     If	a sleeping open	of a device node from devfs(5) is interrupted by a
     signal, the call always fails with	EINTR, even if the SA_RESTART flag is
     set for the signal.  A sleeping open of a fifo (see mkfifo(2)) is
     restarted as normal.

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

     Unless O_CLOEXEC flag was specified, the new descriptor is	set to remain
     open across execve(2) system calls; see close(2), fcntl(2)	and O_CLOEXEC
     description.

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

RETURN VALUES
     If	successful, open() and openat()	return a non-negative integer, termed
     a file descriptor.	 They return -1	on failure, and	set 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_TRUNC	is specified and write permission is denied.

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

     [EPERM]		O_CREAT	is specified, the file does not	exist, and the
			directory in which it is to be created has its im-
			mutable	flag set, see the chflags(2) manual page for
			more information.

     [EPERM]		The named file has its immutable flag set and the file
			is to be modified.

     [EPERM]		The named file has its append-only flag	set, the file
			is to be modified, and O_TRUNC is specified or
			O_APPEND is not	specified.

     [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	modified.

     [EISDIR]		The named file is a directory, and the flags specified
			O_CREAT	without	O_DIRECTORY.

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

     [EROFS]		O_CREAT	is specified and the named file	would reside
			on a read-only file system.

     [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]		O_NONBLOCK is set, the named file is a fifo, O_WRONLY
			is set,	and no process has the file open for reading.

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

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

     [EOPNOTSUPP]	The named file is a special file mounted through a
			file system that does not support access to it (e.g.
			NFS).

     [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.

     [EINTEGRITY]	Corrupted data was detected while reading from the
			file system.

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

     [EFAULT]		The path argument points outside the process's allo-
			cated 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, or O_RDWR, and
			O_EXEC or O_SEARCH.

     [EINVAL]		The O_RESOLVE_BENEATH flag is specified	and path is
			absolute.

     [EBADF]		The path argument does not specify an absolute path
			and the	fd argument is neither AT_FDCWD	nor a valid
			file descriptor	open for searching.

     [ENOTDIR]		The path argument is not an absolute path and fd is
			neither	AT_FDCWD nor a file descriptor associated with
			a directory.

     [ENOTDIR]		O_DIRECTORY is specified and the file is not a direc-
			tory.

     [ECAPMODE]		AT_FDCWD is specified and the process is in capability
			mode.

     [ECAPMODE]		open() was called and the process is in	capability
			mode.

     [ENOTCAPABLE]	path is	an absolute path, or contained a ".." compo-
			nent leading to	a directory outside of the directory
			hierarchy specified by fd, and the process is in capa-
			bility mode.

     [ENOTCAPABLE]	The O_RESOLVE_BENEATH flag was provided, and the rela-
			tive path escapes the fd directory.

SEE ALSO
     chmod(2), close(2), dup(2), fexecve(2), fhopen(2),	getdtablesize(2),
     getfh(2), lgetfh(2), lseek(2), read(2), umask(2), write(2), fopen(3),
     capsicum(4)

STANDARDS
     These functions are specified by IEEE Std 1003.1-2008 ("POSIX.1").
     FreeBSD sets errno	to EMLINK instead of ELOOP as specified	by POSIX when
     O_NOFOLLOW	is set in flags	and the	final component	of pathname is a sym-
     bolic link	to distinguish it from the case	of too many symbolic link tra-
     versals in	one of its non-final components.

HISTORY
     The open()	function appeared in Version 1 AT&T UNIX.  The openat()	func-
     tion was introduced in FreeBSD 8.0.  O_DSYNC appeared in 13.0.

BUGS
     The Open Group Extended API Set 2 specification requires that the test
     for whether fd is searchable is based on whether fd is open for search-
     ing, not whether the underlying directory currently permits searches.
     The present implementation	of the openat checks the current permissions
     of	directory instead.

     The mode argument is variadic and may result in different calling conven-
     tions than	might otherwise	be expected.

FreeBSD	13.0			October	9, 2021			  FreeBSD 13.0

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

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