---
id: c-unions
title: "C Unions"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["union keyword", "shared memory members", "C 유니온"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.86
created_at: 2026-07-04
updated_at: 2026-07-04
review_reason: ""
merge_history: []
tags: ["c", "programming-language", "w3schools", "unions"]
raw_sources: ["https://www.w3schools.com/c/c_unions.php"]
applied_in: []
github_commit: ""
---

# [[C Unions]]

## 🎯 한 줄 통찰 (One-line insight)
Writing to ANY member of a union silently invalidates every other member's previously-stored value — the source's example shows `u1.myNum = 1000;` followed by `u1.myLetter = 'A';` leaves `myNum` printing "no longer reliable" garbage, because unlike a struct where each member owns separate memory, a union's members OVERLAP the exact same bytes, so the last write always wins and there's no compiler warning when you accidentally read a stale member. [S1]

## 🧠 핵심 개념 (Core concepts)
- **`union`** — syntactically similar to `struct` (same declaration/dot-access style), but semantically opposite: all members share ONE memory location instead of each having its own. [S1]
- **Only one valid member at a time** — writing to any member overwrites the shared memory, silently invalidating whatever the previous member held. [S1]
- **Size = largest member** — `sizeof(union)` equals the size of its biggest member, NOT the sum of all members (unlike a struct, whose size is roughly the sum of members plus padding). [S1]
- **Use case** — when you only need to store ONE of several possible types at a time and memory savings matter; structs remain the default choice for storing/accessing multiple values simultaneously. [S1]

## 📖 세부 내용 (Details)
- The stale-value trap: `union myUnion { int myNum; char myLetter; char myString[30]; }; u1.myNum = 1000; u1.myLetter = 'A'; printf("myNum: %d\n", u1.myNum); // no longer reliable printf("myLetter: %c\n", u1.myLetter); // 'A'`. [S1]
- Size determined by the largest member: `union myUnion { int myNum; char myString[36]; }; sizeof(u1); // 36 (the size of myString, the largest member)`. [S1]
- Contrast with an equivalent struct's size: the same two members (`int` + `char[36]`) as a STRUCT would total ~40 bytes (4 + 36), versus the union's 36 bytes. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **유니온의 마지막 대입만 유효**: 서로 다른 멤버에 순서대로 값을 쓰면 이전에 쓴 멤버의 값은 조용히 무효화된다는 점이 명시적으로 경고됨 — 컴파일러가 이를 알려주지 않으므로 프로그래머가 직접 관리해야 함. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 여러 타입 중 한 번에 하나만 저장하면서 메모리를 절약해야 하는 상황(예: 프로토콜 파싱)이 유니온의 대표적 활용 사례로 원문에 조건으로만 언급됨. [S1]

## 💻 코드 패턴 (Code patterns)
Writing to one union member silently invalidates the previously-stored member (C):
```c
union myUnion {
  int myNum;
  char myLetter;
  char myString[30];
};
int main() {
  union myUnion u1;
  u1.myNum = 1000;
  u1.myLetter = 'A'; // myNum is now invalid — same memory was overwritten
  printf("myLetter: %c\n", u1.myLetter); // Prints 'A'
  return 0;
}
```

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.86
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[C Tutorial]]
- **관련 개념:** [[C Enums]], [[C Structs Padding]], [[C Typedef]]
- **참조 맥락:** 유니온 — typedef 챕터로 이어짐.

## 📚 출처 (Sources)
- [S1] W3Schools — C Unions — https://www.w3schools.com/c/c_unions.php

## 📝 변경 이력 (Change history)
- 2026-07-04: Initial draft synthesized from the W3Schools "C Unions" page (Astra wiki-curation, P-Reinforce v3.1 format).