union

Learn to share memory between types with union.

union

A union is like a struct but all members share the same memory. So only one member is active at a time. sizeof(union) equals the size of the largest member.

Example: union Value { int i; float f; };. Here i and f share the same 4 bytes. If you write v.i = 42; then reading v.f is undefined behavior. So you must track which member is active. A union is used for memory savings; it's common in embedded systems and protocol headers.

One active member

In a union, only one member is active at a time. Writing to one member and reading another is undefined behavior; that's why tracking the active member is required.

Example: if you write union Value v; v.i = 42;, then v.i is active. printf("%f", v.f); gives an unspecified result; the bits aren't reinterpreted as a float. The right approach is to keep which type is stored in a separate variable. This pattern is called a tagged union; you'll see it in the next lesson. A union is a powerful but careful tool for memory packing.

Exercise Store an int with a union and print

Define union Value with int i; and float f;. In main, write union Value v;; assign v.i = 42; and print with printf("%d\n", v.i);.

Show example solution

#include <stdio.h>

union Value {
    int i;
    float f;
};

int main(void) {
    union Value v;
    v.i = 42;
    printf("%d\n", v.i);
    return 0;
}

Test yourself

What does sizeof(union {int a; char b;}) give?
- sizeof(char)
- sizeof(int)
- 1
- 0
Show answer

sizeof(int) — A union's size is the size of its largest member; int is larger than char.

Complete the union definition

union Value { int i; ____ f; };

Show answer

union Value { int i; float f; };

Match the union expressions

union Value

Type definition
v.i = 42

int active
v.f = 3.14

float active
sizeof(union Value)

Largest member size

Next lesson: enum

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