---
id: c-memory-allocate
title: "C Allocate Memory"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["malloc calloc", "static vs dynamic memory", "stack memory", "C 메모리 할당"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.87
created_at: 2026-07-04
updated_at: 2026-07-04
review_reason: ""
merge_history: []
tags: ["c", "programming-language", "w3schools", "memory-management", "malloc", "calloc"]
raw_sources: ["https://www.w3schools.com/c/c_memory_allocate.php"]
applied_in: []
github_commit: ""
---

# [[C Allocate Memory]]

## 🎯 한 줄 통찰 (One-line insight)
`sizeof(*ptr1)` and `sizeof(ptr1)` look nearly identical but measure two completely different things — the FIRST measures the size of the data the pointer points to (correct for allocation), while the SECOND measures the size of the pointer variable itself (usually 8 bytes, the address size) — and the source explicitly flags forgetting the `*` as a real, easy-to-make mistake that silently allocates the wrong amount of memory. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Static memory** — reserved automatically at COMPILE time for regular variables; fixed size, cannot be resized after creation (e.g. an array declared for 20 elements always occupies that space, even if only 12 are used). [S1]
- **Dynamic memory** — reserved at RUNTIME, giving full programmer control over how much memory is used; only accessible via pointers, never owned by a plain variable. [S1]
- **`malloc(size)`** — allocates `size` bytes; the content is UNPREDICTABLE/uninitialized until written. [S1]
- **`calloc(amount, size)`** — allocates `amount` items of `size` bytes each, AND zero-initializes all of it; slightly less efficient than `malloc()` because of the zeroing step. [S1]
- **`sizeof(*ptr)` vs `sizeof(ptr)`** — the former measures the pointed-to data's size (correct for sizing an allocation); the latter measures the pointer variable's own size (usually 8 bytes on 64-bit systems), a common mistake. [S1]
- **`sizeof` can't measure total allocated memory** — it only reports the size of ONE element's type, so total dynamic allocation size must be computed manually (`count * sizeof(type)`). [S1]
- **Stack memory** — a form of dynamic memory automatically reserved for variables declared INSIDE functions; freed automatically when the function returns; excessive recursion can exhaust it, causing a "stack overflow." [S1]

## 📖 세부 내용 (Details)
- Static memory waste example: `int students[20]; // reserves 80 bytes even if only 12 students actually attend`. [S1]
- Correctly sizing a dynamic allocation with `sizeof(*ptr)`: `int *ptr1, *ptr2; ptr1 = malloc(sizeof(*ptr1)); ptr2 = calloc(1, sizeof(*ptr2));`. [S1]
- Computing total dynamic memory manually (sizeof alone can't do it): `int *students; int numStudents = 12; students = calloc(numStudents, sizeof(*students)); printf("%d", numStudents * sizeof(*students)); // 48 bytes`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **sizeof(*ptr) vs sizeof(ptr)의 차이**: 별표를 빠뜨리면 포인터가 가리키는 데이터의 크기가 아니라 포인터 자체의 크기(보통 8바이트)를 측정하게 된다는 점이 실수하기 쉬운 부분으로 명시적으로 경고됨. [S1]
- **정적 메모리의 낭비 가능성**: 20명분 배열을 만들었는데 실제 12명만 참여하면 8명분 공간이 낭비되지만, 프로그램은 정상 작동하며 손상되지 않는다는 점이 확인됨(단지 비효율적일 뿐). [S1]

## 🛠️ 적용 사례 (Applied in summary)
학생 수(numStudents)만큼만 calloc으로 메모리를 할당해 정적 배열의 낭비 문제를 해결하는 예제가 원문에서 직접 실전 활용 사례로 제시됨. [S1]

## 💻 코드 패턴 (Code patterns)
Correctly sizing an allocation using sizeof(*ptr), not sizeof(ptr) (C):
```c
int *ptr1, *ptr2;
ptr1 = malloc(sizeof(*ptr1));       // correct: size of the pointed-to int
ptr2 = calloc(1, sizeof(*ptr2));    // correct: size of the pointed-to int
// sizeof(ptr1) would instead measure the pointer itself (usually 8 bytes)
```

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.87
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[C Tutorial]]
- **관련 개념:** [[C Memory Management]], [[C Data Types Sizeof]], [[C Memory Deallocate]]
- **참조 맥락:** 메모리 할당 — 메모리 해제(Memory Deallocate) 챕터로 이어짐.

## 📚 출처 (Sources)
- [S1] W3Schools — C Allocate Memory — https://www.w3schools.com/c/c_memory_allocate.php

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