The address of a block returned by malloc or realloc in
the GNU system 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 memalign, posix_memalign, or
valloc. These functions are declared in stdlib.h.
With the GNU library, you can use free to free the blocks that
memalign, posix_memalign, and valloc return. That
does not work in BSD, however—BSD does not provide any way to free
The memalign function allocates a block of size bytes whose
address is a multiple of boundary. The boundary must be a
power of two! The function memalign works by allocating a
somewhat larger block, and then returning an address within the block
that is on the specified boundary.
— Function: int posix_memalign (void **memptr, size_t alignment, size_t size)
The posix_memalign function is similar to the memalign
function in that it returns a buffer of size bytes aligned to a
multiple of alignment. But it adds one requirement to the
parameter alignment: the value must be a power of two multiple of
sizeof (void *).
If the function succeeds in allocation memory a pointer to the allocated
memory is returned in *memptr and the return value is zero.
Otherwise the function returns an error value indicating the problem.
This function was introduced in POSIX 1003.1d.
— Function: void * valloc (size_t size)
Using valloc is like using memalign and passing the page size
as the value of the second argument. It is implemented like this: