typedef struct {
  endpoint_t m_source;          /* who sent the message */
  int m_type;                   /* what kind of message is it */
  union {
        mess_1 m_m1;
        mess_2 m_m2;
        mess_3 m_m3;
        mess_4 m_m4;
        mess_5 m_m5;
        mess_7 m_m7;
        mess_8 m_m8;
        mess_6 m_m6;
        mess_9 m_m9;
  } m_u;
} message;

Minix message structures are C struct's with two fixed members for the message type and the message source and a union member.

The union member is a union of 9 different struct types.