Unit 4 · Lesson 532 minAcademic review pending

Ask for memory, own the cleanup

Allocate runtime-sized objects and arrays with malloc, calloc, realloc, and free while preserving ownership.

Choose explanation

After this lesson

You should be able to

Allocate an array with overflow-aware size calculation and check failure.
Use realloc without losing the original allocation on failure.

Lifetime chosen at runtime

Dynamic allocation is useful when size or lifetime is not known at compile time. malloc reserves uninitialized storage, calloc reserves and zero-initializes bytes, and both return null on failure. In C, do not cast malloc's return; include stdlib.h.

Check count * sizeof element for overflow before allocation when count is untrusted. Use sizeof *pointer so the allocation remains correct if the pointed-to type changes.

Allocate a runtime-sized int array

if (count > SIZE_MAX / sizeof *values) {
    fprintf(stderr, "Requested array is too large\n");
    return 1;
}

int *values = malloc(count * sizeof *values);
if (values == NULL && count != 0) {
    perror("malloc");
    return 1;
}

/* use values */
free(values);
values = NULL;

Ownership ends with free

Every successful allocation needs one clear owner responsible for free. A leak loses the last pointer without freeing. A dangling pointer still holds an address after free. Double-free and use-after-free are undefined behavior.

realloc may move the block and returns null without freeing the original on failure. Assign the result to a temporary pointer first. Only replace the owner's pointer after success.

Failure-safe resize

int *resized = realloc(values, newCount * sizeof *values);
if (resized == NULL && newCount != 0) {
    /* values is still valid */
} else {
    values = resized;
}

Try it yourself

Explain why values = realloc(values, newSize); can leak memory.

Need a hint?

What happens to the only original pointer when realloc returns null?

Check the worked solution

The assignment overwrites the original pointer with null, losing access to the still-allocated block. Use a temporary pointer.

Quick check

After free(p), which action is valid?

Select an answer to check your thinking.

Why this lesson exists

Syllabus mapping

Dynamic memory allocation · Allocating and freeing values and arrays

Maps to course outcomes CO3, CO4, CO5.