---
id: c-files-read
title: "C Read Files"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["fgets file reading", "read multiple lines", "C 파일 읽기"]
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", "files", "fgets"]
raw_sources: ["https://www.w3schools.com/c/c_files_read.php"]
applied_in: []
github_commit: ""
---

# [[C Read Files]]

## 🎯 한 줄 통찰 (One-line insight)
A single `fgets()` call only reads ONE LINE, not the whole file — the source's own example proves it by showing "filename.txt" has two lines but only "Hello World!" prints from one call, and the fix (wrapping `fgets()` in a `while` loop) works because `fgets()` naturally returns falsy/NULL once it hits end-of-file, making the loop condition double as the "keep reading until there's nothing left" check with no separate end-of-file test needed. [S1]

## 🧠 핵심 개념 (Core concepts)
- **`fopen(filename, "r")`** — opens a file specifically for reading. [S1]
- **`fgets(buffer, maxSize, fptr)`** — reads UP TO `maxSize` characters (or until a newline) from `fptr` into `buffer`; reads only ONE line per call. [S1]
- **`while (fgets(...))` pattern** — repeatedly calling `fgets()` inside a `while` loop reads the ENTIRE file line by line; the loop naturally stops once `fgets()` reaches end-of-file (returns a falsy value). [S1]
- **NULL-check for missing files** — `fopen()` returns `NULL` if the file doesn't exist for reading; checking `if (fptr == NULL)` before use prevents crashes. [S1]

## 📖 세부 내용 (Details)
- Reading only the FIRST line (a common early mistake): `FILE *fptr = fopen("filename.txt", "r"); char myString[100]; fgets(myString, 100, fptr); printf("%s", myString); // only "Hello World!", missing the second line`. [S1]
- Reading the ENTIRE file with a while loop: `while(fgets(myString, 100, fptr)) { printf("%s", myString); } // prints both "Hello World!" and "Hi everybody!"`. [S1]
- Combining NULL-checking with the full-file read pattern: `if(fptr != NULL) { while(fgets(myString, 100, fptr)) { printf("%s", myString); } } else { printf("Not able to open the file."); }`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **fgets 한 번 호출로는 첫 줄만 읽힘**: 파일에 여러 줄이 있어도 fgets()를 한 번만 호출하면 첫 줄만 읽힌다는 점이 직접 비교 예제로 증명되며, 전체 파일을 읽으려면 while 루프로 감싸야 한다는 해법이 제시됨. [S1]

## 🛠️ 적용 사례 (Applied in summary)
파일이 존재하지 않을 경우 NULL을 체크해 "파일을 열 수 없습니다" 메시지를 출력하는 안전한 파일 읽기 패턴이 원문에서 직접 실전 활용 사례로 제시됨. [S1]

## 💻 코드 패턴 (Code patterns)
Safely reading an entire file line by line, with a NULL check for missing files (C):
```c
FILE *fptr;
fptr = fopen("filename.txt", "r");
char myString[100];
if (fptr != NULL) {
  while (fgets(myString, 100, fptr)) {
    printf("%s", myString);
  }
} else {
  printf("Not able to open the file.");
}
fclose(fptr);
```

## ✅ 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 Files]], [[C User Input]], [[C Files Write]]
- **참조 맥락:** 파일 읽기 — 파일 쓰기(Files Write) 챕터로 이어짐.

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

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