typedef struct
{
  int m1i1;
  int m1i2;
  char* m1p1;
} mess_1;

typedef struct
{
  int m_type;
  long m_source;
  union {
    mess_1 m_m1;
    mess_2 m_m2;
  } m_u;
} message;

message msg;

Access the m1i2 member of msg and assign the value 5:

  msg.m_u.m_m1.m1i1 = 5;

Here is a more complete example.