Simple Union Example in C

About this program

This is an example program in c structure and union programs. Read the concept first: C Structures, then study the code and output below.

Definition

• A union is like a struct, but all members share the same memory location.
• Only one member can hold a valid value at a time.
• Unions are useful for saving memory or representing data that can be one of several types.

Syntax

union tag_name {
    type member1;
    type member2;
};

Simple Union Example Program

#include <stdio.h>

union Data {
    int i;
    float f;
    char str[20];
};

int main(void) {
    union Data data;

    data.i = 10;
    printf("data.i = %d\n", data.i);

    data.f = 3.14f;
    printf("data.f = %.2f\n", data.f);

    /* After writing f, i is no longer valid */
    return 0;
}

Sample Output

data.i = 10
data.f = 3.14

Union vs Structure

Feature	Structure	Union
Memory	Each member has its own space	All members share one space
Size	Sum of members (with padding)	Size of largest member
Usage	Store all fields together	Store one field at a time

Practical Example — Integer or Float

#include <stdio.h>

union Value {
    int asInt;
    float asFloat;
};

int main(void) {
    union Value v;
    v.asInt = 42;
    printf("As integer: %d\n", v.asInt);

    v.asFloat = 3.5f;
    printf("As float: %.1f\n", v.asFloat);
    return 0;
}

Sample Output

As integer: 42
As float: 3.5

Detailed About Union Concepts

See the full tutorial: C Unions.

Related Pages

Learn the concept first, then study the code:

• C Programs — Browse all C Programs.
• C Structures — Concept — struct declaration and member access.
• Simple Structure Example Program — More in c structure and union programs.