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OPEN(2) 			     BSD 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 specify-
     ing 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.  In this case the file to be opened is determined relative to the
     directory associated with the file descriptor fd instead of the current working directory.
     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().

     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_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_SYNC	   synchronous writes
	   O_NOFOLLOW	   do not follow symlinks
	   O_NOCTTY	   don't assign controlling terminal
	   O_TTY_INIT	   restore default terminal attributes
	   O_DIRECTORY	   error if file is not a directory
	   O_CLOEXEC	   set FD_CLOEXEC upon open

     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 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() sys-
     tem call would result in the process being blocked for some reason (e.g., waiting for car-
     rier on a dialup line), open() returns immediately.  The descriptor remains in non-blocking
     mode for subsequent operations.

     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.

     O_SYNC is a synonym for O_FSYNC required by POSIX.

     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 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 dras-
     tically reduce 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 com-
     patibility.  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.

     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.

     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 simultaneously 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 immutable 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 translating the pathname.

     [EISDIR]		The named file is a directory, and the arguments specify it is to be mod-
			ified.

     [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 specified 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 directory
			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() system call requests write access.

     [EFAULT]		The path argument 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 illegal combination of
			O_RDONLY, O_WRONLY, O_RDWR and O_EXEC.

     [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 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)

HISTORY
     The open() function appeared in Version 6 AT&T UNIX.  The openat() function was introduced
     in FreeBSD 8.0.

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 searching, not whether the underlying direc-
     tory currently permits searches.  The present implementation of the openat checks the cur-
     rent permissions of directory instead.

BSD					 February 7, 2013				      BSD
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