C programming interview question about dynamic memory allocation

If you want to allocated 8 bytes of memory dynamically in the heap but this 8 bytes of memory is not available continuously. It is available as 5 bytes and 3 bytes separately in the heap. Then can you allocate the memory? if yes, how?

If yes, Can we use only malloc or malloc with calloc

The granularity of the heap is typically 8 or 16 bytes. It is rather unlikely that there be 2 blocks available for respectively 5 and 3 bytes and neither of them be available for an allocation of 8 bytes. If memory is so scarce you encounter this situation and allocating an 8 byte block with malloc() or calloc() fails, you should not try to handle the requirement with 2 separate blocks, but rather signal an out of memory condition and handle it gracefully.

> If you want to allocated 8 bytes of memory dynamically in the heap but this 8 bytes of memory is not available continuously.
It is available as 5 bytes and 3 bytes separately in the heap.
Then can you allocate the memory?

Each successful call to malloc() (or calloc()) allocates a contiguous block of memory. If there is no large enough block available then malloc() (calloc()) will fail. malloc() does not have any way to return discontinuous memory blocks. And it must not move allocated blocks, so it cannot coalesce blocks that are not already contiguous.

So, can you allocate an 8-byte chunk? No, you cannot.

Can you allocate two chunks, with a combined size of 8 bytes? If we understand the question as asserting that a 5-byte chunk and a separate 3-byte chunk can be allocated separately, then yes: that is stipulated by the question. If that's what is actually being asked, then the underlying issue is probably how you can discover that the 5-byte and 3-byte chunks are available. There is no memory-introspection mechanism by which you could look for a combination of available blocks, but you could simply try smaller and smaller sizes until you find one that works, and then see whether you can allocate the remainder. For example:

void allocate8(void **ret1, void **ret2) {
    void *p1 = NULL;
    void *p2 = NULL;

    for (int i = 8; i >= 4; i--) {
        p1 = malloc(i);
        if (p1) {
            if (i < 8) {
                p2 = malloc(8 - i);
                if (!p2) {
                    free(p1);
                    p1 = NULL;
                }
            }
            break;
        }
    }

    *ret1 = p1;
    *ret2 = p2;
}

But is that useful? Almost surely not.

If I understood correctly the problem you described, it concerns memory fragmentation. Unfortunately as a developer you can't remedy this especially being limited to only malloc() and calloc() functions, the best you can do is be aware of the problem and try to catch it. The solution to the problem depends on the memory allocator used in the system.