Initialization of a union in C

Question

I came across this objective question on the C programming language. The output for the following code is supposed to be 0 2, but I don't understand why.

Please explain the initialization process. Here's the code:

#include <stdio.h>  int main() {   union a   {     int x;     char y[2];   };   union a z = {512};   printf("\n%d %d", z.y[0], z.y[1]);   return 0; } 

Answer 1

I am going to assume that you use a little endian system where sizeof int is 4 bytes (32 bits) and sizeof a char is 1 byte (8 bits), and one in which integers are represented in two's complement form. A union only has the size of its largest member, and all the members point to this exact piece of memory.

Now, you are writing to this memory an integer value of 512.

512 in binary is 1000000000.

or in 32 bit two's complement form:

00000000 00000000 00000010 00000000.

Now convert this to its little endian representation and you'll get:

00000000 00000010 00000000 00000000 |______| |______|    |         |   y[0]      y[1] 

Now see the above what happens when you access it using indices of a char array.

Thus, y[0] is 00000000 which is 0,

and y[1] is 00000010 which is 2.

Answer 2

The memory allocated for the union is the size of the largest type in the union, which is intin this case. Let's say the size of int on your system is 2 bytes then

512 will be 0x200.

Represenataion looks like:

0000 0010 0000 0000 |        |         | -------------------  Byte 1     Byte 0 

So the first byte is 0 and the second one is 2.(On Little endian systems)

char is one byte on all systems.

So the access z.y[0] and z.y[1] is per byte access.

z.y[0] = 0000 0000 = 0 z.y[1] = 0000 0010 = 2 

I am just giving you how memory is allocated and the value is stored.You need to consider the below points since the output depends on them.

Points to be noted:

1. The output is completely system dependent.
2. The endianess and the sizeof(int) matters, which will vary across the systems.

PS: The memory occupied by both the members is the same in union.