Summary - Unions and enums

Combine union, enum and bit fields to write a state machine.

Summary - Unions and enums

In this summary you'll combine union, enum and bit field concepts to write a state machine. Throughout the group you learned union sharing, enum constants, the tagged union pattern, the typedef enum shortcut and bit field packing.

The state machine uses these parts: a State type with typedef enum, a struct Machine with State state and a unsigned running : 1 bit field. In a real app these parts become state transitions, event handling and flag management. C interpreters, compilers and hardware drivers start with this pattern.

Exercise Write a state machine

Define typedef enum { IDLE, ACTIVE, STOPPED } State;. Define struct Machine with State state; and unsigned running : 1;. In main, write struct Machine m;; set m.state = ACTIVE; and m.running = 1;. Print with printf("%d %d\n", m.state, m.running);.

Show example solution

#include <stdio.h>

typedef enum { IDLE, ACTIVE, STOPPED } State;

struct Machine {
    State state;
    unsigned running : 1;
};

int main(void) {
    struct Machine m;
    m.state = ACTIVE;
    m.running = 1;
    printf("%d %d\n", m.state, m.running);
    return 0;
}

Summary

In this group you learned union, enum, typedef enum and bit fields. You combined them to write a state machine.

A union shares memory, an enum gives named constants, a typedef provides short type names and bit fields pack data. These four tools are the foundation of data modeling in C: protocols, hardware registers and state machines build on them. In the next groups you'll write and read data to disk with file I/O; you'll improve safety with error handling. These foundations are the building blocks of real-world C projects.

Test yourself

Which shares memory between members?
- struct
- enum
- union
- typedef
Show answer

union — A union shares memory between members.
Match concepts to purpose

union

Shared memory
enum

Named constants
typedef

Type alias
bit field

Bit packing

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