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NetBSD 6.1.5 - man page for open (netbsd section 2)

OPEN(2) 			     BSD 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 are spec-
     ified by or'ing the values listed below.  Applications must specify exactly one of the first
     three values (file access methods):

	   O_RDONLY	Open for reading only.

	   O_WRONLY	Open for writing only.

	   O_RDWR	Open for reading and writing.

     Any combination of the following may be used:

	   O_NONBLOCK	Do not block on open or for data to become available.

	   O_APPEND	Append to the file on each write.

	   O_CREAT	Create the file if it does not exist.  The third argument of type mode_t
			is used to compute the mode bits of the file as described in chmod(2) and
			modified by the process' umask value (see umask(2)).

	   O_TRUNC	Truncate size to 0.

	   O_EXCL	Error if O_CREAT and the file already exists.

	   O_SHLOCK	Atomically obtain a shared lock.

	   O_EXLOCK	Atomically obtain an exclusive lock.

	   O_NOFOLLOW	If last path element is a symlink, don't follow it.  This option is pro-
			vided for compatibility with other operating systems, but its security
			value is questionable.

	   O_CLOEXEC	Set the close(2) on exec(3) flag.

	   O_NOSIGPIPE	Return EPIPE instead of raising SIGPIPE.

	   O_DSYNC	If set, write operations will be performed according to synchronized I/O
			data integrity completion: each write will wait for the file data to be
			committed to stable storage.

	   O_SYNC	If set, write operations will be performed according to synchronized I/O
			file integrity completion: each write will wait for both the file data
			and file status to be committed to stable storage.

	   O_RSYNC	If set, read operations will complete at the same level of integrity
			which is in effect for write operations: if specified together with
			O_SYNC, each read will wait for the file status to be committed to stable
			storage.

			Combining O_RSYNC with O_DSYNC only, or specifying it without any other
			synchronized I/O integrity completion flag set, has no further effect.

	   O_ALT_IO	Alternate I/O semantics will be used for read and write operations on the
			file descriptor.  Alternate semantics are defined by the underlying lay-
			ers and will not have any alternate effect in most cases.

	   O_NOCTTY	If the file is a terminal device, the opened device is not made the con-
			trolling terminal for the session.  This flag has no effect on NetBSD,
			since the system defaults to the abovementioned behaviour.  The flag is
			present only for standards conformance.

	   O_DIRECT	If set on a regular file, data I/O operations will not buffer the data
			being transferred in the kernel's cache, but rather transfer the data
			directly between user memory and the underlying device driver if possi-
			ble.  This flag is advisory; the request may be performed in the normal
			buffered fashion if certain conditions are not met, e.g. if the request
			is not sufficiently aligned or if the file is mapped.

			To meet the alignment requirements for direct I/O, the file offset, the
			length of the I/O and the address of the buffer in memory must all be
			multiples of DEV_BSIZE (512 bytes).  If the I/O request is made using an
			interface that supports scatter/gather via struct iovec, each element of
			the request must meet the above alignment constraints.

	   O_DIRECTORY	Fail if the file is not a directory.

	   O_ASYNC	Enable the SIGIO signal to be sent to the process group when I/O is pos-
			sible, e.g., upon availability of data to be read.

     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, do not wait for the device or file to be ready or
     available.  If 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.	This flag also
     has the effect of making all subsequent I/O on the open file non-blocking.

     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 lock-
     ing).

     If open() is successful, 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 simultaneously by one
     process.  Calling getdtablesize(3) returns the current system limit.

RETURN VALUES
     If successful, open() returns a non-negative integer, termed a file descriptor.  Otherwise,
     a value of -1 is returned and errno is set to indicate the error.

ERRORS
     The named file is opened unless:

     [EACCES]		Search permission is denied for a component of the path prefix, the
			required permissions (for reading and/or writing) are denied for the
			given flags, or O_CREAT is specified, the file does not exist, and the
			directory in which it is to be created does not permit writing.

     [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; or 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.

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

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

     [EFTYPE]		O_NOFOLLOW was specified, but the last path component is a symlink.
			Note: IEEE Std 1003.1-2008 (``POSIX.1'') specifies returning [ELOOP] for
			this case.

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

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

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

     [ELOOP]		Too many symbolic links were encountered in translating the pathname.

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

     [ENAMETOOLONG]	A component of a pathname exceeded {NAME_MAX} characters, or an entire
			path name exceeded {PATH_MAX} characters.

     [ENFILE]		The system file table is full.

     [ENOENT]		O_CREAT is not set and the named file does not exist, or a component of
			the path name that must exist does not exist.

     [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; or
			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.

     [ENOTDIR]		A component of the path prefix is not a directory; or O_DIRECTORY is
			specified and the last path component is not a directory.

     [ENXIO]		The named file is a character special or block special file, and the
			device associated with this special file does not exist, or the named
			file is a FIFO, O_NONBLOCK and O_WRONLY is set and no process has the
			file open for reading.

     [EOPNOTSUPP]	O_SHLOCK or O_EXLOCK is specified but the underlying filesystem does not
			support locking; or an attempt was made to open a socket (not currently
			implemented).

     [EPERM]		The file's flags (see chflags(2)) don't allow the file to be opened.

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

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

SEE ALSO
     chmod(2), close(2), dup(2), lseek(2), read(2), umask(2), write(2), getdtablesize(3)

STANDARDS
     The open() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').  The flags values
     O_DSYNC, O_SYNC and O_RSYNC are extensions defined in IEEE Std 1003.1b-1993 (``POSIX.1'').

     The O_SHLOCK and O_EXLOCK flags are non-standard extensions and should not be used if porta-
     bility is of concern.

HISTORY
     An open() function call appeared in Version 2 AT&T UNIX.

BSD					 January 23, 2012				      BSD


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