Allocation patterns

Learn common memory allocation patterns.

malloc, calloc and realloc differences

C has three core allocation functions. malloc allocates uninitialized memory; calloc allocates zero-initialized memory; realloc resizes an existing block.

Examples: int *a = malloc(3 * sizeof(int)); may hold garbage. int *b = calloc(3, sizeof(int)); sets all elements to zero. b = realloc(b, 5 * sizeof(int)); grows the block. calloc takes two parameters (count and element size) whose product is the total size. When zero initialization is needed, prefer calloc.

The NULL check idiom

For large allocations malloc and calloc can fail; they return NULL. So a check after allocation is essential. Pattern: allocate, check, on error exit.

Example: int *arr = malloc(1000 * sizeof(int)); if (arr == NULL) return 1;. If the allocation fails the program exits with 1; otherwise it continues. In production code, on error you should also free previously allocated memory. This pattern is standard for allocations driven by user input; it's the foundation of reliable programs.

Exercise Zeroed array with calloc

Allocate a three-element zeroed array with int *arr = calloc(3, sizeof(int));. Assign 5 and 7 to the first two elements. Print all three with printf("%d ", arr[i]) in a for loop; the third element should be zero thanks to calloc. End with printf("\n"). Free with free(arr);.

Show example solution

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

int main(void) {
    int *arr = calloc(3, sizeof(int));
    arr[0] = 5;
    arr[1] = 7;
    for (int i = 0; i < 3; i++) printf("%d ", arr[i]);
    printf("\n");
    free(arr);
    return 0;
}

Test yourself

Match function to use case

malloc(n * sizeof(T))

Uninitialized alloc
calloc(n, sizeof(T))

Zeroed alloc
realloc(ptr, n)

Resize
free(ptr)

Release

Complete the calloc call

int *arr = ____(3, sizeof(int));

Show answer

int *arr = calloc(3, sizeof(int));

Which function zero-initializes memory?
- malloc
- calloc
- realloc
- free
Show answer

calloc — answer: calloc. malloc doesn't initialize, calloc zeroes.

Next lesson: Summary - Memory management

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