malloc and free — Heap Memory in C

C in 100 Seconds, Episode 20

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So far, all our variables have lived on the stack — allocated automatically and freed when the function returns. malloc lets you allocate memory on the heap, where it persists until you explicitly free it.

Allocating Memory

#include <stdlib.h>

int *nums = malloc(5 * sizeof(int));

malloc takes a size in bytes and returns a pointer to the allocated block. 5 * sizeof(int) requests enough space for 5 integers (typically 20 bytes).

Using the Memory

Once allocated, you use it like any array:

for (int i = 0; i < 5; i++) {
  nums[i] = (i + 1) * 10;
}

for (int i = 0; i < 5; i++) {
  printf("nums[%d] = %d\n", i, nums[i]);
}

Output:

nums[0] = 10
nums[1] = 20
nums[2] = 30
nums[3] = 40
nums[4] = 50

The pointer nums behaves exactly like an array name. Index access works the same way.

Freeing Memory

When you're done, release the memory back to the system:

free(nums);

After free, the pointer is dangling — it still holds the old address, but that memory is no longer yours. Using it after free is undefined behavior.

Why Does This Matter?

Stack memory is fast but limited and automatic. Heap memory is flexible:

• Dynamic sizing — allocate exactly as much as you need at runtime
• Longer lifetime — survives past the function that created it
• Your responsibility — forget to free and you leak memory

malloc and free are the core of manual memory management in C. Every data structure that grows at runtime — dynamic arrays, linked lists, hash tables — is built on them.

Full Code

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

int main() {
  int *nums = malloc(5 * sizeof(int));

  for (int i = 0; i < 5; i++) {
    nums[i] = (i + 1) * 10;
  }

  for (int i = 0; i < 5; i++) {
    printf("nums[%d] = %d\n", i, nums[i]);
  }

  free(nums);
  printf("Memory freed.\n");

  return 0;
}

Compile and Run

gcc malloc.c -o malloc
./malloc

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Next episode: calloc and realloc — zero-initialized allocation and resizing memory.

Student code: github.com/GoCelesteAI/c-in-100-seconds/episode20