realloc function - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the realloc function in C?

The realloc function changes the size of a previously allocated memory block. It can increase or decrease the size without losing the existing data up to the minimum of old and new sizes.

intermediate

What happens if realloc cannot enlarge the existing memory block in place?

If realloc cannot extend the current block, it allocates a new block of the requested size, copies the old data to the new block, frees the old block, and returns a pointer to the new block.

intermediate

What does realloc(ptr, 0) do?

Calling realloc with size 0 is implementation-defined: it may either free the memory pointed to by ptr and return NULL, or return a unique pointer that should not be dereferenced.

intermediate

How should you safely use realloc to avoid memory leaks?

Always assign the result of realloc to a temporary pointer first. If it returns NULL, the original pointer is still valid. Only update the original pointer if realloc succeeds.

beginner

What header file must be included to use realloc?

You must include <stdlib.h> to use the realloc function in C.

What does realloc do if the new size is smaller than the old size?

A It reduces the memory block size and keeps the first part of the data.

B It frees the old block and returns NULL.

C It increases the memory block size.

D It leaves the memory block unchanged.

realloc reduces the block size and preserves the data up to the new smaller size.

If realloc fails to allocate new memory, what happens to the original pointer?

A It remains valid and unchanged.

B It points to freed memory.

C It becomes NULL.

D It points to the new memory block.

The original pointer remains valid if realloc fails; only the returned pointer is NULL.

Which header file is required to use realloc?

A <code>&lt;stdio.h&gt;</code>

B <code>&lt;stdlib.h&gt;</code>

C <code>&lt;string.h&gt;</code>

D <code>&lt;memory.h&gt;</code>

realloc is declared in <stdlib.h>.

What is the correct way to avoid losing the original pointer when using realloc?

A Use <code>malloc</code> instead of <code>realloc</code>.

B Assign <code>realloc</code> result directly to the original pointer.

C Call <code>free</code> before <code>realloc</code>.

D Use a temporary pointer to store <code>realloc</code> result first.

Using a temporary pointer prevents losing the original pointer if realloc fails.

What does realloc(NULL, size) do?

A Returns NULL.

B Frees memory.

C Acts like <code>malloc(size)</code>.

D Causes a runtime error.

Passing NULL to realloc behaves like malloc and allocates new memory.

Explain how the realloc function works and when you would use it.

Think about changing the size of a box without losing what is inside.

Describe the safe way to use realloc to avoid memory leaks or losing data.

Imagine you want to move your stuff to a bigger box but keep the old box until the new one is ready.