CIS241

System-Level Programming and Utilities

C Dynamic Memory Allocation

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Memory allocation!

malloc(size)

• Allocates size bytes
• On the heap
• Contiguous

void* malloc(size);

• Returns a void pointer
• Think of it like a generic pointer
• Needs to be cast to the type you need
• Pointer points to start of allocated block

Example:
int* p = (int*)malloc(4 * 100);

#include

Note, malloc (and those like it), require a new include:

• #include <stdlib.h>

What if we don't know the size?

What if we don’t know the size of our type?

• Use sizeof!

long* p = (long*)malloc(sizeof(long) * 10);

Alternatively...

calloc(count, size);

• Allocates count * size bytes
• Initializes values to zero (malloc does not)

Example:
float* p = calloc(64, sizeof(float));

What if we want more space?

Imagine we have an array of 16 ints.

• Now we need to double it to 32.
• But we don’t want to copy them by hand…
• int* p = (int*)malloc(sizeof(int) * 16);
• p = realloc(p, sizeof(int) * 32);

Allocates new memory!

• Copies the old over
• "Frees" old memory
• Not guaranteed to initialize new mem to zero

Old memory / memory leaks?

We need to free it when we’re done!

• free(pointer);

If we forget, we can have memory leaks!

• Memory leak bad!

Example:

https://www.geeksforgeeks.org/c/dynamic-memory-allocation-in-c-using-malloc-calloc-free-and-realloc/