declaration of anonymous union must be a definition

Yes, a declaration of an anonymous union must also be a definition. In C language, an anonymous union is a union that does not have a tag name associated with it. It can only be declared and defined at the same time, without a separate declaration and definition. Here's an example:

#include <stdio.h>

int main() {
    union {
        int x;
        float y;
    } u; // Anonymous union declaration and definition

    u.x = 10;
    printf("Value of x: %d\n", u.x);

    u.y = 3.14;
    printf("Value of y: %.2f\n", u.y);

    return 0;
}

In the above code, the anonymous union `u` is declared and defined within the `main` function. It contains two members, `x` and `y`, which can be accessed interchangeably.