C Dynamic Memory malloc, calloc, realloc,: Tutorial, Examples, FAQs & Interview Tips

Stack vs Heap

C programs use two memory regions for variables:

Feature	Stack	Heap
Allocation	Automatic (on function call)	Manual (malloc/calloc/realloc)
Deallocation	Automatic (on function return)	Manual (free)
Size	Fixed, limited (~1"8 MB)	Large (limited by RAM)
Speed	Fast	Slower (OS involvement)
Use case	Local variables, function calls	Large/variable-size data, long-lived data

Dynamic Memory Functions

All dynamic memory functions are in <stdlib.h>:

Function	Description	Initialization
`malloc(size)`	Allocates size bytes	Uninitialized (garbage values)
`calloc(n, size)`	Allocates n x size bytes	Zero-initialized
`realloc(ptr, size)`	Resizes previously allocated block	Preserves existing data
`free(ptr)`	Releases allocated memory	-

Always check for NULL: malloc, calloc, and realloc return NULL if allocation fails. Always check the return value before using the pointer.

malloc and free - Dynamic Array

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

int main() {
    int n;
    printf("Enter number of elements: ");
    scanf("%d", &n);

    // malloc - allocate n integers (uninitialized)
    int *arr = (int*)malloc(n * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }

    // Fill array
    for (int i = 0; i < n; i++) {
        arr[i] = (i + 1) * 10;
    }

    // Print array
    printf("Array: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    // ALWAYS free when done
    free(arr);
    arr = NULL;  // good practice: avoid dangling pointer

    printf("Memory freed.\n");
    return 0;
}

/*
Enter number of elements: 5
Array: 10 20 30 40 50
Memory freed.
*/

calloc and realloc

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

int main() {
    // calloc - allocates and zero-initializes
    int *arr = (int*)calloc(5, sizeof(int));
    if (!arr) { printf("calloc failed\n"); return 1; }

    printf("calloc (all zeros): ");
    for (int i = 0; i < 5; i++) printf("%d ", arr[i]);  // 0 0 0 0 0
    printf("\n");

    // Fill with values
    for (int i = 0; i < 5; i++) arr[i] = i + 1;

    // realloc - resize to 10 elements
    int *bigger = (int*)realloc(arr, 10 * sizeof(int));
    if (!bigger) {
        printf("realloc failed\n");
        free(arr);
        return 1;
    }
    arr = bigger;  // arr now points to the resized block

    // Initialize new elements
    for (int i = 5; i < 10; i++) arr[i] = (i + 1) * 10;

    printf("After realloc (10 elements): ");
    for (int i = 0; i < 10; i++) printf("%d ", arr[i]);
    printf("\n");

    free(arr);
    return 0;
}

/*
calloc (all zeros): 0 0 0 0 0
After realloc (10 elements): 1 2 3 4 5 60 70 80 90 100
*/

Dynamic Array of Structs

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

typedef struct {
    char name[50];
    int  score;
} Student;

int main() {
    int n;
    printf("How many students? ");
    scanf("%d", &n);

    // Allocate array of structs dynamically
    Student *students = (Student*)malloc(n * sizeof(Student));
    if (!students) { printf("Allocation failed\n"); return 1; }

    // Input data
    for (int i = 0; i < n; i++) {
        printf("Enter name and score for student %d: ", i + 1);
        scanf("%s %d", students[i].name, &students[i].score);
    }

    // Find highest scorer
    int maxIdx = 0;
    for (int i = 1; i < n; i++) {
        if (students[i].score > students[maxIdx].score) maxIdx = i;
    }

    printf("\nAll students:\n");
    for (int i = 0; i < n; i++) {
        printf("  %-15s %d\n", students[i].name, students[i].score);
    }
    printf("Top scorer: %s (%d)\n", students[maxIdx].name, students[maxIdx].score);

    free(students);
    students = NULL;
    return 0;
}

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C Dynamic Memory malloc, calloc, realloc,: Tutorial, Examples, FAQs & Interview Tips

Stack vs Heap

Dynamic Memory Functions

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