NAME
        OpenDevice -- gain access to a device

   SYNOPSIS
        error = OpenDevice(devName, unitNumber, iORequest, flags)
        D0                 A0       D0          A1         D1

        byte opendevice(strptr,ulong,struct iorequest *,ulong);

   FUNCTION
        This function opens the named device/unit and initializes the given
        I/O request block.  Specific documentation on opening procedures
        may come with certain devices.

        The device may exist in memory, or on disk; this is transparent to
        the OpenDevice caller.

        A full path name for the device name is legitimate.  For example
        "test:devs/fred.device".  This allows the use of custom devices
        without requiring the user to copy the device into the system's
        DEVS: directory.

   NOTES
        All calls to OpenDevice should have matching calls to CloseDevice!

        Devices on disk cannot be opened until after DOS has been
        started.

        As of V36 tasks can safely call OpenDevice, though DOS may open
        system requesters (e.g., asking the user to insert the Workbench
        disk if DEVS: is not online).  You must call this function from a
        DOS process if you want to turn off dos requesters.

   INPUTS
        devName - requested device name

        unitNumber - the unit number to open on that device.  The format of
                the unit number is device specific.  If the device does
                not have separate units, send a zero.

        iORequest - the I/O request block to be returned with
                appropriate fields initialized.

        flags - additional driver specific information.  This is sometimes
                used to request opening a device with exclusive access.

   RESULTS
        error - Returns a sign-extended copy of the io_Error field
                of the iorequest.  zero if successful, else an error code
                is returned.

   BUGS
        AmigaDOS file names are not case sensitive, but Exec lists are.  If
        the library name is specified in a different case than it exists on
        disk, unexpected results may occur.

        Prior to V36, tasks could not make OpenDevice calls requiring disk
        access (since tasks are not allowed to make dos.library calls).
        Now OpenDevice is protected from tasks.

   SEE ALSO
        closedevice(), doio(), sendio(), checkio(), abortio(), waitio()