[geocar]

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.

[simiones]

Well, even that doesn't really work, because optimized allocators typicallly don't allocate exactly the amount that you ask for, they allocate some larger chunk to help with both alignment and fragmentation.

So, most likely, there are two sizes in reality: the size of your user data that you care about; and the size of the memory chunk in which your user data resides, that free() cares about. So, unless you're willing to go for an API like this, you can't rely on the consumer:

  int* ptr_to_dest = NULL;
  size_t size = 10 * sizeof(int);
  size_t allocated = malloc(&ptr_to_dest, size);
  if (allocated <= 0 || !ptr_to_dest) {
    //handle allocation error
  }
  //... use ptr_to_dest, size
  free(ptr_to_dest, allocated); //careful not to pass size here!

[byroot]

That's why C23 introduce free_sized [0].

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

[Someone]

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

> That's why C23 introduce free_sized

Is it? C23 still has free, so there’s no guarantee callers wil use free_sized, so the allocator still has to be able to obtain a block’s size from a pointer.

Or do I overlook something?

[inigyou]

I suppose by default it will just call free, but you can substitute different malloc implementations, maybe one with a malloc_no_header function

[Someone]

Because free still exists and there is no “allocate this number of bytes of memory; I promise to free it with a call to free_size” call, you can’t have a malloc implementation that doesn’t track block sizes.

I think the only thing free_size adds is robustness. Allocators can check the passed in size with what they know and abort the program if they do not match. That can thwart some security issues.

[xxs]

> keep track of that metadata themselves you obviously don’t need to

likely it'd a be perf. hit in most cases. They'd have to copy to 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.

[inigyou]

munmap has metadata in a separate data structure in the kernl