union
- C course
- Intermediate
- Lesson 36/116
- 5 min
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.
Practice 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);.
Starter code
#include <stdio.h>
// Define union Value (int i and float f)
// In main declare v, set i to 42 and print
int main(void) {
return 0;
}Sample 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;
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does sizeof(union {int a; char b;}) give?
- sizeof(char)
- sizeof(int) Correct answer
- 1
- 0
Explanation: A union's size is the size of its largest member; int is larger than char.
2 Complete the union definition
union Value { int i; ____ f; };union Value { int i; float f; };Explanation: float is the 4-byte member added to the union.
3 Match the union expressions
| Term | Meaning |
|---|---|
union Value |
Type definition |
v.i = 42 |
int active |
v.f = 3.14 |
float active |
sizeof(union Value) |
Largest member size |