Doesn't even need two return values. One tagged union suffices:

  Result(double) x = strtod·or·error(s);
  
  if (!x.ok) {
      handle(x.error);
      return;
  }
  
  use(x.value);

  typedef struct {
      double value;
      int    error;   /* 0 = ok */
  } maybe·double;
  
  maybe·double x = strtod·or·error(s, NULL);
  if (x.error) { /* handle */ }