C Programming - Structures, Unions, Enums - Discussion

Here the concept is union memory management. union memory size is largest data member.

Here
data members are : int i ; char a[2];
largest data member is int ie 4bytes.
note := 1byte=8bits => 1byte= 00000000.

4byte := 00000000 00000000 00000000 00000000.

for char u.a[0]=3; => uses first byte in memory
ie , 00000000 00000000 00000000 " 00000011 "
for char u.a[1]=1 ; => uses second byte(system allocates or selects like this).
ie , 00000000 00000000 "00000001" 00000011
whenever we are printing u.a[1] points to its 1 byte and gives 3
Similarly u.a[2] points to the second byte give 1.

when printing u.i, it is not initialized so it took the first 4 bytes ie entire memory

ie "00000000 00000000 00000001 00000011" => its value in binary is 515
2^8 + 2^1 + 2^0 =512+2+1 =515
So,u[i] prints 515.

Please explain me, why use 8 binary bits 00000011?

Anyone explain about it.

Here can we take union u instead of union a u?

Thanks @Dash.

Agree @Raju Kumar.

If I make the data types in the union ( int i; & char ch[2];) match (both int or both char, etc.) then the number of bytes match and I get a predictable and expected value for u.i. That is, it matches the value of what is in the ch[0] memory location.

How to get ui? Please explain.

#include<stdio.h>

int main()
{
union a
{
short int i;
char ch[2];
};
union a u;
u.ch[0]='a';
u.ch[1]='b';

printf("%d, %d, %d\n", u.ch[0], u.ch[1], u.i);
return 0;
}

Its output is:97, 98, 25185.

if I change 'short int' to 'char'，then the output is; 97, 98, 97.

But union takes the memory of maximum size variable. So, in this case, int i has 4 bytes which is greater than value of ch[2] (2 elements * sizeof(char)) = 2 bytes.

#include<stdio.h>
int main()
{
union a
{
int i;
char ch[2];
};
union a u;
u.ch[0]=3;
u.ch[1]=2;
printf("%d, %d, %d, %d\n", u.ch[0], u.ch[1], u.ch[2], u.i);
return 0;
}

Getting ans as 3, 2, 110, -143785469.

How that 110 came? Please help

Union uses Shared memory.

So, char[] and int i occupy the same memory location.