Topic 7 of 15 · Intermediate

Dynamic Memory: malloc & free

Heap allocation with malloc, calloc, realloc and free.

Concept

Stack memory is automatic and freed when a function returns. Heap memory is requested at run time and lives until you free it.

malloc(bytes) returns uninitialised memory; calloc(n, size) zeroes it; realloc resizes a block. All return NULL on failure.

Every malloc needs exactly one free. Forgetting causes leaks; freeing twice or using after free is undefined behaviour. Tools like Valgrind find these bugs.

Cheat sheet

Function	Does
malloc(n)	n bytes, uninitialised
calloc(k, n)	k×n bytes, zeroed
realloc(p, n)	Resize, may move
free(p)	Release memory

Code (C17)

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

int main(void) {
    int n = 5;
    int *arr = malloc(n * sizeof *arr);
    if (!arr) return 1;
    for (int i = 0; i < n; i++) arr[i] = i * i;

    int *bigger = realloc(arr, 10 * sizeof *arr);
    if (!bigger) { free(arr); return 1; }
    arr = bigger;

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

Remember

Check the return value of malloc.
Assign realloc to a temp pointer first.
free + set to NULL.

Interview questions

malloc vs calloc?

malloc takes total bytes and leaves memory uninitialised; calloc takes count and size and zero-fills.

What is a memory leak?

Heap memory that is never freed after it's no longer reachable, so the program's memory use keeps growing.

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