C Union Demo

union.c

#include <stdio.h>

typedef union U1 {
    int a[2];
    char c;
    short s;
} U1;

int main() {
    long long tmp;
    char* mem = (char*)&tmp;
    mem[0] = 0xFF;
    mem[1] = 0xEF;
    mem[2] = 0xDF;
    mem[3] = 0xCF;
    mem[4] = 0xBF;
    mem[5] = 0xAF;
    mem[6] = 0x9F;
    mem[7] = 0x8F;

    void* ptr = (void*)mem;

    int* a = (int*)ptr;
    int* b = (int*)(ptr + 4);
    char* c = (char*)ptr;
    short* d = (short*)ptr;

    printf("0x%X, 0x%X, 0x%hhX, 0x%hX\n", *a, *b, *c, *d);

    U1* u = (U1*)ptr;

    printf("0x%X, 0x%X, 0x%hhX, 0x%hX\n", u->a[0], u->a[1], u->c, u->s);

    return 0;
}

Both printf statements should output the same string.

This example relies on some undefined behavior; pay no mind to that.