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 or valloc. These functions are declared in `stdlib.h'.

With the GNU library, you can use free to free the blocks that 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: 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);
}

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