Dynamic Memory Allocation in C

Dynamic memory allocation allows programs to request memory at run time rather than compile time. This is managed using functions from stdlib.h.

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Why Dynamic Allocation?

• Array size not known at compile time
• Need to allocate/free memory as needed
• Efficient use of available memory

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malloc() — Memory Allocation

Allocates a block of uninitialised memory.

void *malloc(size_t size);

int *arr = (int *)malloc(5 * sizeof(int));
if (arr == NULL) {
    printf("Memory allocation failed!\n");
    exit(1);
}
for (int i = 0; i < 5; i++) arr[i] = i * 10;

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calloc() — Cleared Allocation

Allocates memory for n elements, all initialised to zero.

void *calloc(size_t n, size_t size);

int *arr = (int *)calloc(5, sizeof(int));
/* arr[0] to arr[4] are all 0 */

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realloc() — Resize Allocation

Changes the size of previously allocated memory.

void *realloc(void *ptr, size_t new_size);

int *arr = (int *)malloc(5 * sizeof(int));
arr = (int *)realloc(arr, 10 * sizeof(int));  /* expand to 10 ints */

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free() — Release Memory

Always free dynamically allocated memory to prevent memory leaks.

free(arr);
arr = NULL;   /* good practice: set to NULL after free */

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Comparison

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Complete Example: Dynamic Array

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

int main() {
    int n;
    printf("Enter size: ");
    scanf("%d", &n);
    
    int *arr = (int *)malloc(n * sizeof(int));
    if (!arr) { perror("malloc"); return 1; }
    
    for (int i = 0; i < n; i++) {
        printf("arr[%d] = ", i);
        scanf("%d", &arr[i]);
    }
    
    int sum = 0;
    for (int i = 0; i < n; i++) sum += arr[i];
    printf("Sum = %d\n", sum);
    
    free(arr);
    return 0;
}

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Common Errors