---
id: c-functions-decl
title: "C Function Declaration and Definition"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["function prototype", "forward declaration", "C 함수 선언과 정의"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.85
created_at: 2026-07-04
updated_at: 2026-07-04
review_reason: ""
merge_history: []
tags: ["c", "programming-language", "w3schools", "functions", "function-declaration"]
raw_sources: ["https://www.w3schools.com/c/c_functions_decl.php"]
applied_in: []
github_commit: ""
---

# [[C Function Declaration and Definition]]

## 🎯 한 줄 통찰 (One-line insight)
Separating a function's DECLARATION (name/return-type/parameters) from its DEFINITION (actual body) is what allows `main()` to call a function that's physically written BELOW it in the source file — without a forward declaration above `main()`, the compiler would encounter the function call before ever having seen what that function even looks like, since C reads files top to bottom. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Declaration** — the function's name, return type, and parameters ONLY, ending in a semicolon (`void myFunction();`), with no body. [S1]
- **Definition** — the function's actual body/implementation. [S1]
- **Recommended file layout** — declarations placed ABOVE `main()`, definitions placed BELOW `main()`, for organization and readability. [S1]
- **Enables forward reference** — a function can be called from `main()` even though its full definition appears later in the file, as long as its declaration appeared earlier. [S1]
- **Functions calling other functions** — as long as both are DECLARED first, one function's body can call another function that's defined elsewhere in the file. [S1]

## 📖 세부 내용 (Details)
- Declaration-above, definition-below pattern: `void myFunction(); int main() { myFunction(); return 0; } void myFunction() { printf("I just got executed!"); }`. [S1]
- Same pattern with parameters and a return value: `int myFunction(int x, int y); int main() { int result = myFunction(5, 3); ... } int myFunction(int x, int y) { return x + y; }`. [S1]
- One function calling another, both pre-declared: `void myFunction(); void myOtherFunction(); int main() { myFunction(); return 0; } void myFunction() { printf("Some text in myFunction\n"); myOtherFunction(); } void myOtherFunction() { printf("Hey! Some text in myOtherFunction\n"); }`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- 현재까지 발견된 모순 사항 없음 (No contradictions found in available sources).

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 함수 선언을 main() 위에, 정의를 아래에 두는 구조가 대부분의 C 프로그램에서 표준적으로 쓰이는 코드 조직 방식이라고 원문에 명시됨. [S1]

## 💻 코드 패턴 (Code patterns)
Declaring functions above main() and defining them below, allowing one to call the other (C):
```c
// Function declarations
void myFunction();
void myOtherFunction();

int main() {
  myFunction();
  return 0;
}

// Function definitions
void myFunction() {
  printf("Some text in myFunction\n");
  myOtherFunction(); // calling another function
}
void myOtherFunction() {
  printf("Hey! Some text in myOtherFunction\n");
}
```

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.85
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[C Tutorial]]
- **관련 개념:** [[C Functions]], [[C Functions Parameters]]
- **참조 맥락:** 함수 선언과 정의 — 함수 매개변수(Functions Parameters) 챕터로 이어짐.

## 📚 출처 (Sources)
- [S1] W3Schools — C Function Declaration and Definition — https://www.w3schools.com/c/c_functions_decl.php

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