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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.