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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 <&lt;fcntl.h>&gt;

     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 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 which case
     the file is created with a mode specified by an additional argument of
     type mode_t as described in chmod(2) and modified by the process' umask
     value (see umask(2)).

     The flags specified are a bitwise OR of the following values.  Exactly
     one of the first three values (file access modes) must be specified:

           O_RDONLY     Open for reading only.
           O_WRONLY     Open for writing only.
           O_RDWR       Open for reading and writing.

     Any combination of the following flags may additionally be used:

           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.  An additional argu-
                        ment of type mode_t must be supplied to the call.
           O_TRUNC      Truncate size to 0.
           O_EXCL       Error if O_CREAT is set and file exists.
           O_SYNC       Perform synchronous I/O operations.
           O_SHLOCK     Atomically obtain a shared lock.
           O_EXLOCK     Atomically obtain an exclusive lock.
           O_NOFOLLOW   If last path element is a symlink, don't follow it.
           O_CLOEXEC    Set FD_CLOEXEC (the close-on-exec flag) on the new
                        file descriptor.
           O_DIRECTORY  Error if path does not name a directory.

     Opening a file with O_APPEND set causes each write on the file to be
     appended to the end.  If O_TRUNC and a writing mode are 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
     either of O_EXCL or O_NOFOLLOW are 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, 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 making all subsequent I/O on the open file
     non-blocking.  If the O_SYNC flag is set, all I/O operations on the file
     will be done synchronously.

     A FIFO should either be opened with O_RDONLY or with O_WRONLY.  The
     behavior for opening a FIFO with O_RDWR is undefined.

     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 open() is successful, the file pointer used to mark the current posi-
     tion 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.  getdtablesize(3) returns the current system
     limit.

     The openat() function is equivalent to open() except that where path
     specifies a relative path, the file to be opened is determined relative
     to the directory associated with file descriptor fd instead of the cur-
     rent working directory.

     If openat() is passed the special value AT_FDCWD (defined in <fcntl.h>)
     in the fd parameter, the current working directory is used and the behav-
     ior is identical to a call to open().

RETURN VALUES
     If successful, open() returns a non-negative integer, termed a file
     descriptor.  Otherwise, a value of -1 is returned and errno is set to
     indicate the error.

ERRORS
     The open() and openat() functions will fail if:

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ENOTDIR]          O_DIRECTORY is specified and path does not name a
                        directory.

     [ENAMETOOLONG]     A component of a pathname exceeded NAME_MAX charac-
                        ters, or an entire pathname (including the terminating
                        NUL) exceeded PATH_MAX bytes.

     [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, or the O_NOFOLLOW flag was specified
                        and the target is a symbolic link.

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

     [EINVAL]           The flags specified for opening the file are not
                        valid.

     [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.

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

     [ENXIO]            The named file is a FIFO, the O_NONBLOCK and O_WRONLY
                        flags are 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
                        filesystem does not support locking.

     [EWOULDBLOCK]      O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci-
                        fied and the file is already 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() 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.

     [EPERM]            The file named by path is flagged append-only but
                        O_APPEND was not specified in flags.

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

     [EBUSY]            An attempt was made to open a terminal device that
                        requires exclusive access and the specified device has
                        already be opened.

     Additionally, the openat() function will fail if:

     [EBADF]            The path argument specifies a relative path and the fd
                        argument is neither AT_FDCWD nor a valid file descrip-
                        tor.

     [ENOTDIR]          The path argument specifies a relative path and the fd
                        argument is a valid file descriptor but it does not
                        reference a directory.

     [EACCES]           The path argument specifies a relative path but search
                        permission is denied for the directory which the fd
                        file descriptor references.

SEE ALSO
     chflags(2), chmod(2), close(2), dup(2), flock(2), lseek(2), read(2),
     umask(2), write(2), getdtablesize(3)

STANDARDS
     The open() and openat() functions conform to IEEE Std 1003.1-2008
     (``POSIX.1'').

     POSIX specifies three different flavors for synchronous I/O: O_SYNC,
     O_DSYNC, and O_RSYNC.  In OpenBSD, these are all equivalent.

     The O_SHLOCK and O_EXLOCK flags are non-standard extensions and should
     not be used if portability is of concern.

HISTORY
     An open() system call first appeared in Version 1 AT&T UNIX.  The flags
     argument has been supported since 4.2BSD.  Before that, a dedicated
     creat() system call had to be used to create new files; it appeared in
     Version 1 AT&T UNIX, was deprecated in 4.3BSD-Reno, and removed in
     OpenBSD 5.0.

     The openat() system call has been available since OpenBSD 5.0.

CAVEATS
     The O_TRUNC flag requires that one of O_RDWR or O_WRONLY also be speci-
     fied, else EINVAL is returned.

BSD                              May 28, 2017                              BSD