Google·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Google SWE interview with a memory management question. Pretty standard C fundamentals stuff but easy to fumble if you mix up the details under pressure.

Questions Asked (1)

Can you explain the differences between malloc, calloc, and realloc?

Technical Trade-offs

Author's notes

Thought I had this cold but mid-explanation I second-guessed whether calloc zeroes out memory or just allocates it.

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AI HintsAI Generated

Suggested Approach

Start by defining each function's purpose and signature, then compare their initialization behavior, arguments, and return values. Highlight practical use cases and trade-offs, such as performance and safety, to show deeper understanding.

Pro tip: Mention that calloc can be more efficient than malloc+memset because some systems allocate zeroed pages lazily, and always check for allocation failures to demonstrate production-ready coding habits.

1. Define each function

Briefly state what malloc, calloc, and realloc do, including their function signatures and return types.

2. Compare initialization and arguments

Explain that malloc leaves memory uninitialized, calloc zero-initializes and takes two arguments (count and size), and realloc resizes an existing block.

3. Discuss return values and error handling

Note that all return NULL on failure, and realloc returns a new pointer that may differ from the original; always check for NULL and handle carefully.

4. Highlight use cases and trade-offs

Give scenarios where each is preferred: malloc for general allocation, calloc for arrays needing zeroing, realloc for dynamic resizing; mention performance and safety considerations.

5. Summarize with best practices

Conclude with tips like checking for allocation failures, avoiding memory leaks, and using realloc safely with temporary pointers.

Key Points to Mention

malloc(size_t size) allocates uninitialized memory.
calloc(size_t nmemb, size_t size) allocates zero-initialized memory and checks for overflow.
realloc(void *ptr, size_t size) resizes a previously allocated block, may move it, and returns NULL on failure.
All three return NULL on failure and require checking.
calloc can be more efficient than malloc+memset due to lazy zeroing.
realloc should be used with a temporary pointer to avoid losing the original pointer on failure.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.