2nd/10_Wiki/Topic_Programming/Topic_C/C_Unions.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

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

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):

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).