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10_Wiki/Dev/Topic_C/C_Input_Validation.md

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+---
+id: c-input-validation
+title: "C Input Validation"
+category: "Programming_Language"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["validate number range", "sscanf validation", "C 입력 검증"]
+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", "input-validation", "scanf", "sscanf"]
+raw_sources: ["https://www.w3schools.com/c/c_input_validation.php"]
+applied_in: []
+github_commit: ""
+---
+
+# [[C Input Validation]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+The integer-validation example doesn't use `scanf()` directly to detect bad input — it reads the RAW LINE with `fgets()` first, then tries to parse it with `sscanf()` and checks whether that parse SUCCEEDED (`== 1`), meaning robust validation in C separates "getting text from the user" from "trying to interpret that text as a number," rather than trusting `scanf()`'s own format-matching to gracefully reject non-numeric input. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Range validation via a do-while loop** — repeatedly prompt until the entered value falls within an allowed range; `while (getchar() != '\n');` clears any leftover characters in the input buffer after `scanf()`. [S1]
+- **Empty-text validation** — read with `fgets()`, strip the trailing newline (`name[strcspn(name, "\n")] = 0;`), then loop while the resulting string length is 0. [S1]
+- **Integer-format validation** — read the raw line as a STRING via `fgets()`, then attempt `sscanf(input, "%d", &number)`; if it returns `1` (one successful conversion), the input was a valid integer — otherwise, prompt again. [S1]
+- **Buffer-clearing necessity** — leftover characters (like an unconsumed newline) in the input stream after `scanf()` can silently corrupt the NEXT read unless explicitly cleared. [S1]
+
+## 📖 세부 내용 (Details)
+- Range-bounded input with buffer clearing: `do { printf("Choose a number between 1 and 5: "); scanf("%d", &number); while (getchar() != '\n'); } while (number < 1 || number > 5);`. [S1]
+- Rejecting empty text input: `do { fgets(name, sizeof(name), stdin); name[strcspn(name, "\n")] = 0; } while (strlen(name) == 0);`. [S1]
+- Validating that input is actually an integer via sscanf's return value: `while (fgets(input, sizeof(input), stdin)) { if (sscanf(input, "%d", &number) == 1) { break; } else { printf("Invalid input. Try again: "); } }`. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+- **문자열로 먼저 읽고 sscanf로 파싱을 검증하는 전략**: scanf()로 직접 숫자를 받는 대신 fgets()로 원문을 통째로 받은 뒤 sscanf()의 반환값(성공한 변환 개수)을 확인하는 방식이 잘못된 입력을 더 안전하게 걸러낸다는 점이 확인됨. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+1~5 범위의 숫자 선택, 빈 이름 거부, 문자가 아닌 정수만 허용하는 세 가지 검증 예제가 원문에서 직접 실전 활용 사례로 제시됨. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+Validating that user input is actually an integer using fgets() + sscanf()'s return value (C):
+```c
+int number;
+char input[100];
+printf("Enter a number: ");
+while (fgets(input, sizeof(input), stdin)) {
+  if (sscanf(input, "%d", &number) == 1) {
+    break; // valid integer
+  } else {
+    printf("Invalid input. Try again: ");
+  }
+}
+```
+
+## ✅ 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 Error Handling]], [[C User Input]], [[C Debugging]]
+- **참조 맥락:** 입력 검증 — 디버깅(Debugging) 챕터로 이어짐.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — C Input Validation — https://www.w3schools.com/c/c_input_validation.php
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "C Input Validation" page (Astra wiki-curation, P-Reinforce v3.1 format).