Quirk with printing union values

Question

The below code results in:

0.000000

10

What is being returned from 'data' in this case? I know n.data.idata and n.data.fdata would be the correct usage, I'm just curious as to why the integer value works in this case and the float value does not.

#include<stdio.h>
#include<stdlib.h>

typedef struct node
{
    union container
    {
        int idata;
        float fdata;
    } data;
    struct node *next;
} Node;

int main()
{
    Node i = {.data.idata = 10};
    Node n = {.data.fdata = 10.0};
    printf("%f\n", n.data);
    printf("%d\n", i.data);

    printf("\nExiting program...\n");
    return 0;
}

Answer 1

Let's ignore the obvious undefined behavior for a moment, which happens because an incorrect type is passed to the printf function for the specifier f. That type being an anonymous union.

Specifier f assumes a default argument promotion from float to double, which doesn't happen in this case, because a union data is passed to the function. So the function receives the union data which consists of 4 bytes and represents a float, but tries to print 8 bytes, because it expects a double. The result is a nonsense value, in your case 0.0.

(This answer assumes IEEE 754, and sizeof(int)<=4)