memory-alignment for allocation in C

mem.c

/*
 * In this example, we don't used fixed-length types, which bugs me, but the idea is that it has to work on memory-length numbers.
 *
 * So it takes long, which is memory address length.  So, you can change it to uint64_t or uint32_t, or just ints or size_t if you want
 * and it should mostly work.  However, for this exmaple to be complete, I needed the rounding to support memory addresses as input.
 */

/* 4KiB for this example. */
#define ALIGNMENT_SIZE 0x1000
#define NULL ((void*)0)

/*
 * Round a value down to the alignment.
 */
static long
roundDown(long size) {
  return (size & (~(ALIGNMENT_SIZE - 1)));
}

/*
 * Round up a number to be in ALIGNMENT_SIZE blocks.
 */
static long
roundUp(long size) {
  if (0 ==  size) {
    return (ALIGNMENT_SIZE);
  }

  return ((size + ALIGNMENT_SIZE - 1) & (~(ALIGNMENT_SIZE - 1)));
}

/*
 * If you can't guarantee 4KiB alignment, but need it for whatever interface.
 *
 * This will return a guaranteed 4KiB aligned address. 
 */
static void *
alignMalloc(long size) {
  void *m = malloc(roundUp(size));
  if (m) {
    return (void *)roundDown(m);
  } else {
    return NULL;
  }
}

Just trying out gist by publishing code that I find I often need.