---
id: c-strings-functions
title: "C String Functions"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["strlen strcat strcpy strcmp", "string.h", "C 문자열 함수"]
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", "strings", "string-functions"]
raw_sources: ["https://www.w3schools.com/c/c_strings_functions.php"]
applied_in: []
github_commit: ""
---

# [[C String Functions]]

## 🎯 한 줄 통찰 (One-line insight)
`strlen()` and `sizeof()` measure fundamentally different things and diverge sharply once a buffer is oversized — for a 26-letter alphabet stored in a 50-byte array, `strlen()` still correctly reports 26 (it counts up to the `\0`), while `sizeof()` reports 50 (the array's total allocated memory), meaning `sizeof` is NEVER a reliable way to get "string length" once the array is larger than the string it holds. [S1]

## 🧠 핵심 개념 (Core concepts)
- **`<string.h>`** — the header required to use C's built-in string functions. [S1]
- **`strlen(str)`** — returns the string's character count UP TO (not including) the `\0` terminator. [S1]
- **`sizeof(arr)`** — returns the array's total allocated memory in bytes, INCLUDING the `\0` and any unused trailing space — not the string's actual content length. [S1]
- **`strcat(dest, src)`** — appends `src` onto the end of `dest`; `dest`'s buffer must be large enough to hold both strings combined. [S1]
- **`strcpy(dest, src)`** — copies `src`'s value into `dest`; `dest`'s buffer must be large enough for the copied content. [S1]
- **`strcmp(str1, str2)`** — returns `0` if the strings are equal, a non-zero value otherwise. [S1]

## 📖 세부 내용 (Details)
- `strlen` vs `sizeof` on an exactly-sized array: `char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; strlen(alphabet) // 26, sizeof(alphabet) // 27` (27 includes `\0`). [S1]
- `strlen` vs `sizeof` on an OVERSIZED array — the critical divergence: `char alphabet[50] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; strlen(alphabet) // still 26, sizeof(alphabet) // 50` (the array's declared capacity, not content length). [S1]
- Concatenation requiring a big-enough destination buffer: `char str1[20] = "Hello "; char str2[] = "World!"; strcat(str1, str2); printf("%s", str1);`. [S1]
- Comparison returning 0 for equal strings, non-zero otherwise: `strcmp(str1, str2); // 0 if equal` / `strcmp(str1, str3); // -4 if not equal`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **sizeof는 문자열 길이 측정 도구가 아님**: 배열 크기가 실제 문자열보다 클 경우 sizeof는 할당된 전체 메모리(50)를 반환하지만 strlen은 여전히 실제 내용 길이(26)를 정확히 반환한다는 점이 명시적으로 대비됨 — sizeof를 문자열 길이 측정에 쓰면 안 된다는 강력한 경고. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — strcat으로 "Hello "와 "World!"를 합치는 예제가 문자열 연결의 표준 실전 활용이며, 대상 버퍼 크기를 미리 충분히 확보해야 한다는 주의사항이 함께 강조된다. [S1]

## 💻 코드 패턴 (Code patterns)
strlen() and sizeof() diverging once the buffer is larger than the actual string content (C):
```c
char alphabet[50] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
printf("%zu\n", strlen(alphabet));   // 26 (actual content length)
printf("%zu\n", sizeof(alphabet));   // 50 (total allocated buffer size)
```

## ✅ 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 Strings Esc]], [[C Data Types Sizeof]], [[C Math]]
- **참조 맥락:** 문자열 섹션 마지막 — 수학 함수(Math) 섹션으로 이어짐.

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

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