Unions

Introduction

A union shares one memory block among members — size equals largest member; only one member active at a time.

Understanding the topic

Memory sharing Writing to one member overwrites others.

Use cases Variant types, protocol fields, embedded registers.

• Memory sharing — Writing to one member overwrites others.
• Use cases — Variant types, protocol fields, embedded registers.

Step-by-step explanation

1. Memory sharing — Writing to one member overwrites others.
2. Use cases — Variant types, protocol fields, embedded registers.

Syntax reference

Syntax reference:

union Tag { type a; type b; };

Informative example

Example program:

#include <stdio.h>

union Data { int i; float f; };

int main(void) {
    union Data d;
    d.i = 10;
    printf("%d\n", d.i);
    return 0;
}

10

Execution workflow

Best practices

• Enable warnings: gcc -Wall -Wextra -std=c11 source.c -o app
• Give every variable a defined value before it is read.
• Stay inside array bounds — C will not stop you from over-running a buffer.

Common mistakes

• Reading uninitialized storage — behavior is undefined.
• Dismissing compiler warnings instead of fixing root causes.
• Ignoring NULL returns from malloc, fopen, and similar APIs.

Hands-on exercise

Practice problems:

• Store int or float in union
• Compare struct vs union size

Summary

Unions in C — Overlapping members sharing one storage region.