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#memory#pointer#alignment#metadata#malloc#allocation#size#padding#need#allocator

Discussion (19 Comments)Read Original on HackerNews

pjmlp•2 minutes ago
Old magazines like The C/C++ Users' Journal and DDJ used to have ads for companies selling malloc()/free() replacement libraries, exactly because a single implementation isn't adequate to all scenarios.
CodesInChaos•about 2 hours ago
Your bump allocator suffers from integer overflows turned into buffer overflows when the requested allocation is big enough:

  if (a->cursor + size > a->limit) return NULL; // out of memory
I'd rewrite it like this:

  if (size > a->limit - a->cursor) return NULL; // out of memory
dn3500•13 minutes ago
Your test is backwards. I would write it like this:

  if (size > a->limit - a->cursor) return NULL;
CodesInChaos•10 minutes ago
You're right. Fixed.
lexicality•about 1 hour ago
I'm a little confused. We start with

  [ Header ][ ...variable padding... ][ Back Pointer ][ User Memory ]
                                       ^ always exactly sizeof(void*)
                                         bytes before User Memory,
                                         no matter how much padding
                                         came before it
and then it says we don't need to align the back pointer and we end up with

  [ Header ][ Back Pointer ][ Padding ][ User Memory ]
without a clear explanation of how we now get to the back pointer if it's behind the variable alignment.
torh•34 minutes ago
This puzzled me as well.
phire•about 2 hours ago
> so alloc() doesn’t just need to hand back a pointer. it needs to hand back a pointer that’s correctly aligned for whatever type the caller is about to store there.

Malloc doesn't know the required alignment (because has no idea what the type is, everything is cast through void). So all malloc implementations have a minimum alignment guarantee. Typically 16 bytes these days on x86, as that means even 128bit SSE values will end up aligned by default.

You couldn't go below the sizeof(void ) anyway, the backpointer needs to aligned too.

The padding only happens when you use memalign or aligned_malloc to specify a much larger alignment.

CodesInChaos•about 2 hours ago
There is no reason an allocation needs to contain any inline metadata. And even if it does, the allocator could choose to make it unalined, and pay the cost of an unalined access on de-allocation.
phire•about 1 hour ago
True.

But most C code out there assumes malloc will always return something that is at least aligned to sizeof(void *), it's very rare to see aligned_alloc. So how is your alloc allocation going to know when it can get away with a smaller alignment?

Even if you are on a cpu that doesn't fault on unaligned memory access, any malloc implementation that doesn't align by default will have serious disadvantages in any benchmarks. IMO, There is no good reason to use an unaligned backpointer.

CodesInChaos•11 minutes ago
Yes, large allocations need to be aligned to `alignof(max_align_t)`. But small allocations could have a smaller alignment. For example a single byte allocation can actually be a single byte with no alignment, since types can't have an alignment larger than their size.
drivebyhooting•about 2 hours ago
Why bother with dynamic padding and a back pointer? That wastes at least 8 bytes.

You might as well always align to 8 bytes and make your header a multiple of 8.

jraph•12 minutes ago
The back pointer could also be a 1 byte number giving the size of the padding.
irenaeus•3 days ago
I've read some allocator walkthroughs before but I thought that this line stood out:

"to get individual free() working, the allocator needs to remember something about every allocation it handed out. and that’s the moment metadata stops being optional."

That's just a very nice distillation of an important concept.

HexDecOctBin•about 2 hours ago
This becomes particularly important when the allocation and metadata cannot (or should not) be stored in the same address space. An example of this is allocating memory on GPUs.

The conventional approach for allocating memory on GPUs for games and other applications is to use a real-time allocator such as TLSF. However, it is not usually discussed that TLSF is real-time because it stores metadata in-band. It is possible to create a variant of TLSF that preserves its real-time properties while storing metadata out-of-band, but this requires careful consideration.

geocar•about 2 hours ago
Oh? How do you think munmap does it?

If you can convince the caller to keep track of that metadata themselves you obviously don’t need to. That can be important.

Something I noticed is that _very often_ the code that is calling malloc(n) is keeping track of n somehow for its own reasons (bounds checking, grow/gap pointers, etc) so merging the value halves stack churn and it’s an easy win.

xxs•6 minutes ago
> keep track of that metadata themselves you obviously don’t need to

likely it'd be perf. hit in most cases. They'd have to copy at the tail end (likely) of the allocated area or the start and offset the pointer. They'd need to know the size of the metadata and account for that. including aligning it, hence the tail feels 'nicer'

It's possible to manually use mmap and forgo malloc entirely, rolling your own arena manager.

byroot•about 2 hours ago
That's why C23 introduce free_sized [0].

[0] https://en.cppreference.com/c/memory/free_sized

lifthrasiir•about 2 hours ago
In the other words, free() is a flawed API. :-)
nathaah3•3 days ago
glad you found it useful :)