Provided by: freebsd-manpages_8.2-1_all bug

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
     specifying 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

     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()
     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.

     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
     drastically 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
     compatibility.  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.

     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 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
                        modified.

     [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.

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
     directory currently permits searches.  The present implementation of the openat checks the
     current permissions of directory instead.