key symbols (portable names for keycodes) 

typedef enum mbtk_keymod_e { /* bitfield */
  MBTK_KM_SHIFT = 1,
  MBTK_KM_ALT = 2,
  MBTK_KM_CTRL = 4,
  MBTK_KM_KEYPAD = 8    /* pressed on the keymap */
} mbtk_keymod_t;

typedef enum mbtk_keysym_e {
  MBTK_KS_INVALID = 0,  /* failed to decode */

  /* values between 32 and 127 are ASCII, not defined here */

  /* values below 32 are invalid, except for these: */
  MBTK_KS_BACKSPACE = 8,
  MBTK_KS_TAB = 9,
  MBTK_KS_ENTER = 10, /* for both \r and \n */
  MBTK_KS_ESC = 27,

  /* F1..Fx: use MBTK_KS_FN + x where x