a/sys/vax/dkio.h

/*      dkio.h  4.2     82/08/01        */
/*
 * Structures and definitions for disk io control commands
 *
 * THIS WHOLE AREA NEEDS MORE THOUGHT.  FOR NOW JUST IMPLEMENT
 * ENOUGH TO READ AND WRITE HEADERS ON MASSBUS DISKS.  EVENTUALLY
 * SHOULD BE ABLE TO DETERMINE DRIVE TYPE AND DO OTHER GOOD STUFF.
 */

/* disk io control commands */
#define DKIOCHDR        _IO(d, 1)       /* next I/O will read/write header */