Aligned Memory Blocks - The GNU C Library

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##### 3.2.2.7 Allocating Aligned Memory Blocks

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`. `memalign` is declared in `malloc.h` and `posix_memalign` is 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 such blocks.

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

> 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:

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

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