aligned malloc

[gadget]

Hi everyone, just wondering what the default behavior of malloc() is with respect to alignment is in bionic.

Here it is mentioned that "GNU libc malloc() always returns 8-byte aligned memory addresses"
http://stackoverflow.com/questions/5061392/aligned-memory-management

what about bionic?

[David Turner]

According to https://android.googlesource.com/platform/bionic/+/master/libc/upstream-dlmalloc/malloc.c this is 8 byte minimum as well (see line 43).

However, this is only the current revision of the sources. Previous iterations might be slightly different, and future ones as well. I.e. treat this as an implementation detail, not a guarantee.

Generally speaking, any allocator *must* return an 8-byte aligned address if the requested size is >= 8 (otherwise lots of things would break, because malloc() doesn't know if the caller is going to store 8-byte aligned values like doubles in the corresponding memory block).

It's what happens for sizes < 8 that could change, and I could easily imagine an allocator implementation that would want to "optimize" these allocations into smaller aligned slots (e.g. in larger page-allocated pools), so I would _not_ rule that out for this case.

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[gadget]

David, thank you very much for a perfect answer!