Where would you use union in C language?

Answer 1

Subudhi Pamisetty

With a union, you're only supposed to use one of the elements, because they're all stored at the same spot. This makes it useful when you want to store something that could be one of several types. A struct, on the other hand, has a separate memory location for each of its elements and they all can be used at once.

union foo {

int a; // can't use both a and b at once

char b;

} foo;

struct bar {

int a; // can use both a and b simultaneously

char b;

} bar;

union foo x;

x.a = 3; // OK

x.b = 'c'; // NO! this affects the value of x.a!

struct bar y;

y.a = 3; // OK

y.b = 'c'; // OK

edit: If you're wondering what setting x.b to 'c' changes the value of x.a to, technically speaking it's undefined. On most modern machines a char is 1 byte and an int is 4 bytes, so giving x.b the value 'c' also gives the first byte of x.a that same value: union foo x;

x.a = 3;

x.b = 'c';

printf("%i, %i\n", x.a, x.b);

prints 99, 99

Why are the two values the same? Because the last 3 bytes of the int 3 are all zero, so it's also read as 99. If we put in a larger number for x.a, you'll see that this is not always the case: union foo x;

x.a = 387439;

x.b = 'c';

printf("%i, %i\n", x.a, x.b);

prints 387427, 99

To get a closer look at the actual memory values, let's set and print out the values in hex: union foo x;

x.a = 0xDEADBEEF;

x.b = 0x22;

printf("%x, %x\n", x.a, x.b);

prints deadbe22, 22

You can clearly see where the 0x22 overwrote the 0xEF.

BUT

In C, the order of bytes in an int are not defined. This program overwrote the 0xEF with 0x22 on my Mac, but there are other platforms where it would overwrite the 0xDE instead because the order of the bytes that make up the int were reversed. Therefore, when writing a program, you should never rely on the behavior of overwriting specific data in a union because it's not portable.

Answer 2

When you have two objects and they are not used together, you can use union to save memory.

Example:

In a hotel, a room is shared by two members, but at any cost of time they won't overlap or stay together. Just assuming a person in Night shift work and another one in Day shift. We gave them one room to save rooms.

Answer 3

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