Is there really no version of realloc() supporting alignment?

Question

There exist several aligned versions of the venerable malloc(), e.g.:

#include <stdlib.h>
int posix_memalign(void **memptr, size_t alignment, size_t size);
void *aligned_alloc(size_t alignment, size_t size);

#include <malloc.h>
void *memalign(size_t alignment, size_t size);

(originating in POSIX, glibc and Linux libc respectively). But - I can't seem to find any mention of a version of realloc() which supports alignment. Has it really never been implemented? It seems pretty trivial to combine the functionality of non-aligned realloc() with the search for an aligned chunk of memory in the aligned malloc() variants.

Related:

Does realloc keep the memory alignment of posix_memalign?

Undersigned

Yes. There is no for many reasons. One is that the alligned allocated functions do not save information about the data size of the alligned block and no copy of data is possible. It was left to the application developer. There were many reasons for it, very actively discussed many years ago - see the appropriate mailing list archives for details – Benthos

@PeterJ: Are you saying the aligned allocation functions save less information than what malloc() saves? I highly doubt it... anyway, if you can flesh this out some more, preferable with a link to some of that discussion, please post an answer. – Undersigned

Not less. The topic is too complicated for the forum. Browse the mailing list archives for explanation. – Benthos

@PeterJ: Which list though?... – Undersigned

@Undersigned I would expect this to be around in the posix / iso-c mailing list archive. This is a guess - but I'll check in the evening (GMT) – Navarre

It seems pretty trivial to combine the functionality of non-aligned realloc() with the search for an aligned chunk of memory in the aligned malloc() variants. And that would be a very simple reason for not implementing it. It's "pretty trivial". – Alguire

@AndrewHenle: Pretty trivial when you're on the inside of the allocator, not on the outside. – Undersigned

@TonyTannous: Thanks for the bounty, friend :-) – Undersigned

@TonyTannous: I just asked this – Undersigned

@Undersigned Pretty trivial when you're on the inside of the allocator, not on the outside. I'd consider a function that consists of little more than calls such as posix_memalign(), memcpy(), free(), and a bit of error checking pretty trivial even from the outside of the allocator. – Alguire

For (large) aligned memory requirements, applications normally use anonymous memory maps via mmap(). Even though they aren't standard C or even POSIX, they are supported by most systems (excluding Windows, of course). (For large amounts of memory, allocating a copy at the same time, for @AndrewHenle's workaround, may not be possible at all. Consider e.g. a 2GB+ object on a 32-bit system.) – Banyan

Answer 1

Aligned realloc is only implemented in Microsoft with the _aligned_realloc function. There is no POSIX version defined and no implementation in Linux. I never understood why though, because it does not seem so complicated to code in glibc. I think it's a matter of time before someone implement it considering the advent of wide SIMD instructions.

At the moment, just allocate a new block of aligned memory, copy the content and free the old pointer. This should not slow down your application anyway.

Wildon