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open(2)                     BSD System Calls Manual                    open(2)


NAME

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


SYNOPSIS

     #include <fcntl.h>

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

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


DESCRIPTION

     The file name specified by path is opened for reading and/or writing, as
     specified by the argument oflag; the file descriptor is returned to the
     calling process.

     The oflag argument may indicate that 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; 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 oflag
     argument and the optional fourth argument correspond exactly to the argu-
     ments for open().  If openat() is passed the special value AT_FDCWD in
     the fd argument, the current working directory is used and the behavior
     is identical to a call to open().

     The flags specified for the oflag argument are formed by or'ing the fol-
     lowing 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 or for data to become available
           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 O_CREAT and the file exists
           O_SHLOCK        atomically obtain a shared lock
           O_EXLOCK        atomically obtain an exclusive lock
           O_NOFOLLOW      do not follow symlinks
           O_SYMLINK       allow open of symlinks
           O_EVTONLY       descriptor requested for event notifications only
           O_CLOEXEC       mark as close-on-exec

     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 with
     O_CREAT 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 mak-
     ing 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 locking).

     If O_NOFOLLOW is used in the mask and the target file passed to open() is
     a symbolic link then the open() will fail.

     If O_SYMLINK is used in the mask and the target file passed to open() is
     a symbolic link then the open() will be for the symbolic link itself, not
     what it links to.

     The O_EVTONLY flag is only intended for monitoring a file for changes
     (e.g. kqueue). Note: when this flag is used, the opened file will not
     prevent an unmount of the volume that contains the file.

     The O_CLOEXEC flag causes the file descriptor to be marked as close-on-
     exec, setting the FD_CLOEXEC flag.  The state of the file descriptor
     flags can be inspected using the F_GETFD fcntl.  See fcntl(2).

     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 system calls; see
     close(2) and fcntl(2).

     The system imposes a limit on the number of file descriptors that can be
     held open simultaneously by one process.  Getdtablesize(2) 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:

     [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.

     [EACCES]           O_TRUNC is specified and write permission is denied.

     [EAGAIN]           path specifies the slave side of a locked pseudo-ter-
                        minal device.

     [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.

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

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

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

     [EINVAL]           The value of oflag is not valid.

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

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

     [ELOOP]            Too many symbolic links are encountered in translating
                        the pathname.  This is taken to be indicative of a
                        looping symbolic link.

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

     [ENAMETOOLONG]     A component of a pathname exceeds {NAME_MAX} charac-
                        ters, or an entire path name exceeded {PATH_MAX} char-
                        acters.

     [ENFILE]           The system file table is full.

     [ELOOP]            O_NOFOLLOW was specified and the target is a symbolic
                        link.

     [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.

     [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.

     [ENOTDIR]          A component of the path prefix 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.

     [ENXIO]            O_NONBLOCK and O_WRONLY are set, the file is a FIFO,
                        and no process has it open for reading.

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

     [EOPNOTSUPP]       An attempt is made to open a socket (not currently
                        implemented).

     [EOVERFLOW]        The named file is a regular file and its size does not
                        fit in an object of type off_t.

     [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.

     [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.


COMPATIBILITY

     open() on a terminal device (i.e., /dev/console) will now make that
     device a controlling terminal for the process.  Use the O_NOCTTY flag to
     open a terminal device without changing your controlling terminal.


SEE ALSO

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


HISTORY

     An open() function call appeared in Version 6 AT&T UNIX.  The openat()
     function was introduced in OS X 10.10

4th Berkeley Distribution      November 10, 2010     4th Berkeley Distribution

OS X 10.10 - Generated Thu Oct 30 20:35:17 CDT 2014