What does posix_memalign/memalign do

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I'm trying to understand what functions memalign() and posix_memalign() do. Reading the available documentation didn't help.

Can someone help me understand how it works and what is it used for? Or, perhaps provide a usage example?

I'm trying to understand how linux memory works, I need to write my own simple memory pool (low-fragmentation heap).

memory-pool memory c
6 Answers

When you use posix_memalign in GNU C, you should be careful that second parameter should not only be a power of two, but also be a multiple of sizeof (void*). Note that this requirement is different from the one in the memalign function, which requires only power of two.

int
__posix_memalign (void **memptr, size_t alignment, size_t size)
{
  void *mem;

  /* Test whether the SIZE argument is valid.  It must be a power of
     two multiple of sizeof (void *).  */
  if (alignment % sizeof (void *) != 0
      || !powerof2 (alignment / sizeof (void *))
      || alignment == 0)
    return EINVAL;


  void *address = RETURN_ADDRESS (0);
  mem = _mid_memalign (alignment, size, address);

  if (mem != NULL)
    {
      *memptr = mem;
      return 0;
    }

  return ENOMEM;
}
weak_alias (__posix_memalign, posix_memalign)

When we look at the first if condition in the posix_memalign implementation, it checks whether alignment is multiple of sizeof(void*).

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