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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() 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_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
     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.	The getdtablesize(2) system call 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.

			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
			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.

     [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, and O_RDWR.

SEE ALSO

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

HISTORY

     The open() function appeared in Version 6 AT&T UNIX.

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