[Main]

Open System Call

 

     #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

     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() 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 per-
     form I/O on the open file it will block (not currently implemented).

     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).

     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.

     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;


RETURN VALUES

     If successful, open() returns a non-negative integer, termed a file de-
     scriptor.	It returns -1 on failure, and sets errno to indicate the er-
     ror.


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 ex-
		   ist.

     [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 di-
		   rectory in which it is to be created does not permit writ-
		   ing.

     [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 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 de-
		   scriptors.

     [ENFILE]	   The system file table is full.

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

     [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.

		   hausted.

     [EDQUOT]	   O_CREAT is specified, the file does not exist, and the us-
		   er'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 be-
		   ing 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 imple-
		   mented).

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


HISTORY

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