[ric129]

>It's applicable to C because most realistic implementations of malloc(3) store the size of the malloc'd area to the nearest word[2]

This is not so true anymore, and I'd be very careful with that sort of tricks. While the default memory allocator on most distributions (ptmalloc2) still probably works like that, there are other memory allocators in use like jemalloc/tcmalloc, which store metadata for a group of blocks (e.g all equal-sized blocks in a page) rather than for every single block.

Like Asooka wrote, you can malloc_usable_size(), if available. But have in mind that malloc_usable_size() returns the size of the block that malloc has given you, which can be _larger_ than the size you requested, as many memory allocators round block sizes up.

I actually wrote a memory allocator that works that way: https://github.com/ricleite/lrmalloc