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##### 3.2.2.7 Allocating Aligned Memory Blocks

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`. `aligned_alloc` and `posix_memalign` are declared in `stdlib.h`.

— Function: void * aligned_alloc (size_t alignment, size_t size)

> Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

> The `aligned_alloc` function allocates a block of size bytes whose address is a multiple of alignment. The alignment must be a power of two and size must be a multiple of alignment.

> The `aligned_alloc` function returns a null pointer on error and sets `errno` to one of the following values:

> `ENOMEM`
>   There was insufficient memory available to satisfy the request.
> `EINVAL`
>   alignment is not a power of two.

>   This function was introduced in ISO C11 and hence may have better portability to modern non-POSIX systems than `posix_memalign`.

— Function: void * memalign (size_t boundary, size_t size)

> Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

> 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.

> The `memalign` function returns a null pointer on error and sets `errno` to one of the following values:

> `ENOMEM`
>   There was insufficient memory available to satisfy the request.
> `EINVAL`
>   alignment is not a power of two.

> The `memalign` function is obsolete and `aligned_alloc` or `posix_memalign` should be used instead.

— Function: int posix_memalign (void **memptr, size_t alignment, size_t size)

> Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

> 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. The possible error values returned are:

> `ENOMEM`
>   There was insufficient memory available to satisfy the request.
> `EINVAL`
>   alignment is not a power of two multiple of `sizeof (void *)`.

> This function was introduced in POSIX 1003.1d. Although this function is superseded by `aligned_alloc`, it is more portable to older POSIX systems that do not support ISO C11.

— Function: void * valloc (size_t size)

> Preliminary: | MT-Unsafe init | AS-Unsafe init lock | AC-Unsafe init lock fd mem | See POSIX Safety Concepts.

> Using `valloc` is like using `memalign` and passing the page size as the value of the second argument. It is implemented like this:

>               void *
>               valloc (size_t size)
>               {
>                 return memalign (getpagesize (), size);
>               }

> Query Memory Parameters for more information about the memory subsystem.

> The `valloc` function is obsolete and `aligned_alloc` or `posix_memalign` should be used instead.