---
id: c-storage-classes
title: "C Storage Classes"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["auto static register extern", "static local variable", "C 저장 클래스"]
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", "storage-classes", "static"]
raw_sources: ["https://www.w3schools.com/c/c_storage_classes.php"]
applied_in: []
github_commit: ""
---

# [[C Storage Classes]]

## 🎯 한 줄 통찰 (One-line insight)
A `static` local variable inside a function survives BETWEEN separate calls to that function — the source's `count()` example prints `1`, `2`, `3` across three separate calls specifically because `static` makes the variable retain its value instead of resetting to 0 each time, whereas an ordinary local variable would print `1`, `1`, `1` — demonstrating that `static` fundamentally changes a variable's LIFETIME, not just its scope. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Scope vs. storage class** — scope defines WHERE a variable can be used; storage class defines HOW LONG it lasts and WHERE it's stored — two independent concepts. [S1]
- **`auto`** — the default storage class for local variables inside functions; rarely written explicitly since it's already the default. [S1]
- **`static`** — a local variable keeps its value across separate function calls (rather than resetting each call); a global variable/function marked `static` becomes invisible outside its own file. [S1]
- **`register`** — suggests storing a variable in a CPU register for faster access; its address cannot be taken with `&`; considered mostly obsolete since modern compilers auto-optimize register allocation. [S1]
- **`extern`** — tells the compiler a variable/function is DEFINED in another file, used for sharing variables across multiple source files. [S1]

## 📖 세부 내용 (Details)
- `static` preserving value across calls: `void count() { static int myNum = 0; myNum++; printf("num = %d\n", myNum); } int main() { count(); count(); count(); return 0; }` — outputs `num = 1`, `num = 2`, `num = 3` (removing `static` would reset to 1 each time). [S1]
- `extern` sharing a variable across files: `main.c` declares `extern int shared;` while `data.c` defines `int shared = 50;`, compiled together via `gcc main.c data.c -o program`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **static은 값을 유지, register는 사실상 구식**: static이 함수 호출 간 값을 보존한다는 점과, register 키워드는 현대 컴파일러가 알아서 최적화하므로 명시적으로 쓸 필요가 거의 없다는 점이 함께 언급됨. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — count() 함수가 호출될 때마다 값을 누적하는 static 카운터 패턴이 함수 호출 횟수를 추적하는 실전 활용의 대표 사례다. [S1]

## 💻 코드 패턴 (Code patterns)
A static local variable retaining its value across separate function calls (C):
```c
void count() {
  static int myNum = 0; // Keeps its value between calls
  myNum++;
  printf("num = %d\n", myNum);
}
int main() {
  count(); // num = 1
  count(); // num = 2
  count(); // num = 3
  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 Constants]], [[C Scope]], [[C Fixed Width Ints]]
- **참조 맥락:** 저장 클래스와 변수 생명주기 — 고정폭 정수(Fixed Width Ints) 챕터로 이어짐.

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

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