Dynamic Memory Allocation in C

Dynamic memory allocation allows a program to request and release memory at runtime from the heap (unlike static variables which use stack or data segment).

Essential when:

Size of array/structure is not known at compile time
Need to create large data structures
Want to resize memory during execution
Managing memory efficiently in data structures (linked list, trees, graphs)

Four Main Functions for Dynamic Memory

Function	Purpose	Initializes memory?	Returns on failure
malloc()	Allocate block of memory	No (garbage values)	NULL
calloc()	Allocate + initialize to zero	Yes (all bytes 0)	NULL
realloc()	Resize previously allocated block	Preserves old data	NULL
free()	Release allocated memory	—	—

1. malloc() - Allocate Memory

              
            
C

          Dynamic array using malloc
        
#include <stdio.h>
#include <stdlib.h>

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

    int *arr = (int*)malloc(n * sizeof(int));

    if(arr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }

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

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

    free(arr);  // Very important!
    arr = NULL; // Good practice

    return 0;
}

2. calloc() - Allocate & Zero-initialize

              
C

            int *matrix = (int*)calloc(5 * 5, sizeof(int));  // 5×5 matrix, all zeros

// Safer than malloc + manual loop to set zeros

3. realloc() - Resize Memory

              
            
C

          Dynamic growing array
        
int capacity = 5;
int *arr = (int*)malloc(capacity * sizeof(int));
int count = 0;

// Adding elements...
if(count == capacity) {
    capacity *= 2;
    int *temp = (int*)realloc(arr, capacity * sizeof(int));
    if(temp == NULL) {
        printf("Reallocation failed!\n");
        free(arr);
        return 1;
    }
    arr = temp;
}

// Later when done:
free(arr);
arr = NULL;

4. Common Problems & Dangers

Problem	Description	How to Avoid
Memory Leak	Forget to call free()	Always pair malloc/calloc/realloc with free
Dangling Pointer	Use pointer after free()	Set pointer = NULL after free
Double Free	free() same memory twice	Set pointer = NULL after free
Use after realloc failure	Continue using old pointer	Check if realloc returned NULL
Access beyond allocated	Out of bounds	Keep track of size/capacity

5. Best Practices (Modern C Style)

Always check if allocation succeeded
Use `sizeof(*ptr)` instead of type name
Prefer `calloc` when you need zero-initialized memory
Set pointer to NULL after free
Don't cast malloc in C (optional but common)
Use tools like Valgrind to detect leaks
Modern style (C11+):
```c int *arr = malloc(n * sizeof(*arr)); ```