What's the output of this program and How?

Question

#include <stdio.h>

int main()
{
union var { int a, b; };
union var v;
v.a = 10;
v.b = 30;
printf("%d\n", v.a);
return 0;
}

Answer 1

An union is a specific data type which size is the biggest value of his member. Here, assuming you're compiling with gcc on a 32bits computer, the union 'var' will have a size of 4 bytes.

The memory inside an union is shared by all the member : modifying a member will modify the other member as well. Here, you're first assigning 10 to member 'a' : inside the union, you have
0000 0000 0000 1010

Then, you're assigning 30 to member 'b' : the memory inside the union is :
0000 0000 0001 1110

Because the memory is shared, 'a' is 30 as well. That's why the output is 30.