A union may only be initialized with a value of the type of its first member,K&R

Question

I am learning K&R C. In chapter 6.8 it states:

A union may only be initialized with a value of the type of its first member; thus union u described above can only be initialized with an integer value.

... and defines a union u as follows:

union u_tag {
  int ival;
  float fval;
  char *sval;
} u;

I do not know how to understand this. What does it mean, the initialization of u must be as:

u.ival = 323;

clearly, that is not the case , because I can initilize u as:

u.fval = 1.0;

or

u.sval = "hi";

What does Ritchie mean?

Answer 1

The posted code is performing assignment, not initialization. The author meant that the following is valid initialization as 323 is an int and the first member of u_tag is an int:

union u_tag u = { 323 }; /* Initializes u.ival. */

The following would be invalid as it would be attempting to initialize an int with a char[]:

union u_tag u = { "hi" };

However, since C99 this is no longer the case. In C99, designators were introduced:

union u_tag u = { .sval = "hi" };

In C89, if you wanted to initialize a union with a value intended for a member other than its first you could copy an existing union. This would particularly useful if you wanted to declare the union as const. For example:

union u_tag make_u_tag_with_fval(const float a_fval)
{
    union u_tag u;
    u.fval = a_fval;
    return u;
}

const union u_tag u_fval = make_u_tag_with_fval(1.0);