CODEWITHSUNDEEP
ubuntugccmallocubuntu-16.04c11
blackcafe
blackcafe

Reputation: 73

Difference between aligned malloc and standard malloc?

I'm new to malloc and aligned malloc. I know how to use them. However, I don't know exactly in which case we should use aligned malloc instead of standard malloc. Can you explain it to me please?

Upvotes: 6

Views: 5949

Answers (1)

paxdiablo
paxdiablo

Reputation: 882686

The glibc documentation makes it reasonably clear where you should use aligned_alloc:

The address of a block returned by malloc or realloc in GNU systems is always a multiple of eight (or sixteen on 64-bit systems). If you need a block whose address is a multiple of a higher power of two than that, use aligned_alloc or posix_memalign.

The C standard already guarantees that malloc will return a suitably aligned memory block for any of the standard types but there may be situations in which you want or need stricter alignment.

As one example, I seem to recall that SSE2 (SIMD) instructions need their data aligned on 16-byte boundaries so you could use aligned_alloc to give you that even on systems where malloc only guarantees alignment to an 8-byte boundary.

Upvotes: 6

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