What is aligned memory allocation?

Question

I also want to know whether glibc malloc() does this.

Answer 1

Alignment requirements specify what address offsets can be assigned to what types. This is completely implementation-dependent, but is generally based on word size. For instance, some 32-bit architectures require all int variables start on a multiple of four. On some architectures, alignment requirements are absolute. On others (e.g. x86) flouting them only comes with a performance penalty.

malloc is required to return an address suitable for any alignment requirement. In other words, the returned address can be assigned to a pointer of any type. From C99 §7.20.3 (Memory management functions):

The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated).