A result like type in C using a struct and a union

rslt.c

/*
Output:
$ gcc rslt.c -o rslt && ./rslt
[OK] kind=0     data=15.159000  errno=8389
[!!] kind=1     data=0.000000   errno=-3
size = 16
*/

#include <stdio.h>

enum _rslt_kind_ { Ok = 0, Err = 1 };

typedef struct {
  enum _rslt_kind_ kind;
  union {
    double value;
    short errno;
  } data;
} rslt_dbl;

void rst_rslt_dbl(rslt_dbl *rslt) {
  rslt->kind = Ok;
  rslt->data.errno = 0;
  rslt->data.value = 0.0;
}

void ok_test(rslt_dbl *rslt) {
  rslt->kind = Ok;
  rslt->data.value = 15.159;
}

void err_test(rslt_dbl *rslt) {
  rslt->kind = Err;
  rslt->data.errno = -3;
}

void print_rslt_dbl(const rslt_dbl *my_rslt) {
  switch (my_rslt->kind) {
  case Ok:
    printf("[OK] kind=%d\tdata=%f\terrno=%d\n", my_rslt->kind,
           my_rslt->data.value, my_rslt->data.errno);
    break;
  default:
    printf("[!!] kind=%d\tdata=%f\terrno=%d\n", my_rslt->kind,
           my_rslt->data.value, my_rslt->data.errno);
  }
}

void main() {
  rslt_dbl my_rslt;

  ok_test(&my_rslt);
  print_rslt_dbl(&my_rslt);

  // Reset the struct otherwise the previous data is still there
  rst_rslt_dbl(&my_rslt);

  err_test(&my_rslt);
  print_rslt_dbl(&my_rslt);

  printf("size = %d\n", sizeof(my_rslt));
}