---
id: c-errors
title: "C Errors"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["compile-time vs runtime errors", "use-after-free", "C 에러"]
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", "errors"]
raw_sources: ["https://www.w3schools.com/c/c_errors.php"]
applied_in: []
github_commit: ""
---

# [[C Errors]]

## 🎯 한 줄 통찰 (One-line insight)
The three runtime error examples (divide-by-zero, out-of-bounds array access, use-after-free) all share a common trait the source doesn't state explicitly but demonstrates through selection: NONE of them are caught by the compiler — all three compile cleanly and only misbehave when actually EXECUTED — meaning C's compile-time checking is fundamentally limited to syntax/type issues, while logic and memory-safety issues are entirely the programmer's responsibility to prevent at runtime. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Compile-time errors** — prevent the program from compiling at all; e.g. missing semicolon, undeclared variable, mismatched types (assigning a string literal to an `int`). [S1]
- **Runtime errors** — the program compiles successfully but crashes or misbehaves when RUN; e.g. division by zero, out-of-bounds array access, using memory after `free()`. [S1]
- **Use-after-free** — reading `*ptr` after `free(ptr)` is explicitly labeled "undefined behavior," not a guaranteed crash — meaning it might appear to work sometimes, making it a particularly dangerous class of bug. [S1]
- **Good habits** — always initialize variables, use meaningful names, keep indentation clean, keep functions short/focused, verify loop/condition behavior, and read error messages carefully (they often pinpoint the exact problem). [S1]

## 📖 세부 내용 (Details)
- Missing semicolon (compile-time): `int x = 5 printf("%d", x);` → `error: expected ',' or ';' before 'printf'`. [S1]
- Type mismatch (compile-time): `int x = "Hello";` → `error: initialization makes integer from pointer without a cast`. [S1]
- Use-after-free (runtime, undefined behavior): `int* ptr = malloc(sizeof(int)); *ptr = 10; free(ptr); printf("%d\n", *ptr); // accessing freed memory`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **해제된 메모리 접근은 "미정의 동작"**: free() 이후 포인터를 읽는 것이 항상 크래시로 이어지는 것이 아니라 "undefined behavior"로 명시되어, 우연히 정상 동작하는 것처럼 보일 수도 있어 더 위험하다는 점이 확인됨. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 컴파일 타임 에러 3종과 런타임 에러 3종을 나란히 대비시킨 것이 두 에러 범주의 근본적 차이를 이해하는 실전 학습 자료로 활용된다. [S1]

## 💻 코드 패턴 (Code patterns)
Using memory after it's been freed — compiles fine, but is undefined behavior at runtime (C):
```c
int* ptr = malloc(sizeof(int));
*ptr = 10;
free(ptr);
printf("%d\n", *ptr); // Undefined behavior - accessing memory that was freed
```

## ✅ 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 Date Time]], [[C Memory Deallocate]], [[C Error Handling]]
- **참조 맥락:** 에러 섹션 첫 챕터 — 에러 처리(Error Handling) 챕터로 이어짐.

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

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