text: #ifndef FILE_H
  #define FILE_H
  
  struct file {
    enum { FD_NONE, FD_PIPE, FD_INODE, FD_DEVICE, FD_MUTEX} type;
    int ref; // reference count
    char readable;
    char writable;
    struct pipe *pipe; // FD_PIPE
    struct inode *ip;  // FD_INODE and FD_DEVICE
    uint off;          // FD_INODE and FD_DEVICE
    short major;       // FD_DEVICE
    short minor;       // FD_DEVICE
  
    // ---------------------
    struct sleeplock mutex;
    // ---------------------
  };
  
  #define major(dev)  ((dev) >> 16 & 0xFFFF)
  #define minor(dev)  ((dev) & 0xFFFF)
  #define	mkdev(m,n)  ((uint)((m)<<16| (n)))
  
  // in-memory copy of an inode
  struct inode {
    uint dev;           // Device number
    uint inum;          // Inode number
    int ref;            // Reference count
    struct sleeplock lock; // protects everything below here
    int valid;          // inode has been read from disk?
  
    short type;         // copy of disk inode
    short major;
    short minor;
    short nlink;
    uint size;
    uint addrs[NDIRECT+1];
  };
  
  // map major device number to device functions.
  struct devsw {
    int (*read)(struct file *, int, uint64, int);
    int (*write)(struct file *, int, uint64, int);
  };
  
  extern struct devsw devsw[];
  
  #define DISK 0
  #define CONSOLE 1
  #define WATCHDOG 2
  
  #endif
  