[C Language] Pointers (void* & Double Pointers)

📌 Date: 2025.08.19

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1. void* Pointer

c복사
#include <stdio.h>
int main() {
    int a = 100;
    double b = 3.14;
    void *vp;

    vp = &a;
    printf("a = %d\n", *(int*)vp);

    vp = &b;
    printf("b = %.2f\n", *(double*)vp);
}

👉 void* can be thought of as a “container for addresses.”
👉 To access the actual value, you must cast it back to the proper type.

2. Double Pointer

A double pointer: a pointer to another pointer
Commonly used in: dynamic allocation of 2D arrays

c복사
#include <stdio.h>
#include <stdlib.h>

int main() {
    int rows = 3, cols = 4;
    int **matrix = malloc(rows * sizeof(int*)); // array of row pointers

    for (int i = 0; i < rows; i++) {
        matrix[i] = malloc(cols * sizeof(int)); // allocate each row
    }

    // assign values
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            matrix[i][j] = i * cols + j;
            printf("matrix[%d][%d] = %d (address: %p)\n",
                   i, j, matrix[i][j], (void*)&matrix[i][j]);
        }
    }

    // free memory
    for (int i = 0; i < rows; i++) free(matrix[i]);
    free(matrix);
}

✅ Summary

matrix itself is an array of int*

Each matrix[i] points to another array of ints

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3. Pointer and Address/Value Relationship

c복사
int a = 10;
int *p = &a;   // p stores the address of a
*p;            // value at that address (10)

👉 Meaning of *:

next to type → pointer declaration

in front of variable → dereference (get value from address)

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📌 Key Takeaways

void* = generic pointer (needs casting)

Double pointer = essential for dynamic 2D arrays

'*' = declaration vs dereference, depending on context