Union - How do you know which union parameter is in use?

Question

I have the following piece of code:

struct sched_param {
    union {
        int sched_priority;
        struct rcost_sched_param rcost_params;
    };
};

I want to know which of the two parameters is "active". Is there a way for me to do that other than adding a int to the struct sched_param?

Answer 1

struct sched_param {
  int type;
  union {
       int sched_priority;
        struct rcost_sched_param rcost_params;
  };
}

you can add member named type,save the data which parameters is "active"