c - When do we need to use posix_memalign instead of malloc?

Question

Seems posix_memalign let you choose a customized alignment,but when is that necessary?

malloc has already done the alignment work internally.

UPDATE

The exact reason I ask this is because I see nginx does this,ngx_memalign(NGX_POOL_ALIGNMENT, size, log);,here NGX_POOL_ALIGNMENT is defined as 16, nginxs.googlecode.com/svn-history/trunk/src/core/ngx_palloc.c

Answer 1

Basically, if you need tougher alignment than malloc will give you. Malloc generally returns a pointer aligned such, that it may be used with any of the primitive types (often, 8 bytes on common desktop machines).

However, sometimes you need memory aligned on other boundaries, for example 4K-aligned, etc. In this case, you would need memalign.

You would need this, for example,

• when writing a memory manager (such as a garbage collector). In this case, it is sometimes handy to work with memory aligned on larger block sizes. This way, you can store meta-data common to all objects in a given block at the bottom of the allocated area, and access this simply by masking the least significant bits of the object pointer.
• when interfacing with hardware (never done this myself, but IIRC, certain kinds of block-devices require aligned memory). See n.m.'s answer for details.