Dynamic Memory Allocation in C: malloc and free: 25 flashcards
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1. What is dynamic memory allocation?
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A technique where a running C program requests memory from the heap at runtime instead of having the compiler fix the size beforehand.
2. Which header declares malloc, calloc, realloc, and free?
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`<stdlib.h>`
3. What does `malloc(size)` return on success?
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A `void*` pointer to a block of uninitialized memory of `size` bytes.
4. What does `malloc` return when allocation fails?
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`NULL`
5. What is the state of memory returned by `malloc`?
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Uninitialized — it contains garbage values from previous use.
6. What does `calloc(n, size)` do differently from `malloc`?
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It allocates space for `n` items of `size` bytes each and initializes every byte to zero.
7. What does `realloc(ptr, new_size)` do?
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Resizes a previously allocated block, preserving existing contents up to the smaller of old/new size; may move the block and return a new pointer.
8. What does `free(ptr)` actually do?
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It releases the heap block back to the allocator; it does not erase data or set the pointer to `NULL`.
9. Why must you cast the `void*` from `malloc`?
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Because `void*` has no type; casting to the target pointer type (e.g., `(int *)`) lets the compiler treat the block correctly.
10. What is the correct way to compute allocation size for an array of `n` integers?
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`n * sizeof(int)` — always multiply the count by `sizeof(type)`.
11. What is a memory leak?
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Memory that was allocated but never freed, so the heap shrinks over time until the program may crash.
12. What is a dangling pointer?
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A pointer that still holds the address of a block after it has been freed; using it is undefined behavior.
13. What is a double free and why is it dangerous?
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Calling `free` twice on the same block; it corrupts the heap and is undefined behavior.
14. Why assign `realloc`'s result to a temporary pointer first?
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If `realloc` fails and returns `NULL`, the original pointer is still valid and must be freed; assigning directly would lose it and leak the old block.
15. What happens to the original block if `realloc` fails?
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It remains valid and unchanged — you must still free it later.
16. Why set a pointer to `NULL` after `free`?
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It prevents accidental reuse of the dangling pointer, which would be undefined behavior.
17. Can you `free` a pointer to a stack variable?
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No — `free` only accepts pointers returned by `malloc`, `calloc`, or `realloc`.
18. Where does dynamically allocated memory live?
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On the heap, which persists until explicitly freed or the program exits.
19. How does stack memory differ from heap memory in lifetime?
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Stack memory is automatically reclaimed when a function returns; heap memory is not — it must be freed manually.
20. When is `calloc` preferred over `malloc`?
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When you need zero-initialized memory, such as an array of counters that should start at 0.
21. When is `malloc` preferred over `calloc`?
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When every byte will be overwritten immediately, so zeroing would be wasted work.
22. Name two tools that detect memory leaks and invalid accesses.
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Valgrind and AddressSanitizer.
23. What real data structures rely on dynamic memory allocation?
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Dynamic arrays, linked lists, trees, and graphs — anything whose size is unknown at compile time.
24. What is the complete rhythm for safe dynamic memory use?
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Allocate, check for `NULL`, use, free, and set the pointer to `NULL`.
25. Why is `malloc(5)` wrong when you meant five integers?
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It allocates only 5 bytes, not 20; writing `arr[3]` then corrupts memory beyond the block.
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