2nd/10_Wiki/Topic_Programming/Topic_C/C_Files_Read.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
c-files-read	C Read Files	Programming_Language	draft	conceptual		fgets file reading read multiple lines C 파일 읽기		B	0.86	2026-07-04	2026-07-04			c programming-language w3schools files fgets	https://www.w3schools.com/c/c_files_read.php

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):

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).