C Academy · Lesson

Alignment and Splitting

Make blocks usable and tidy.

Why Alignment Matters

Every type has an alignment requirement: its address must be a multiple of some power of two. A double typically needs 8-byte alignment.

Because malloc does not know what you will store, it must return pointers aligned for the strictest type, usually 16 bytes on 64-bit systems.

#include <stdalign.h>
/* the strictest fundamental alignment */
size_t strict = alignof(max_align_t);   /* often 16 */

Rounding a size up to the next multiple of a power-of-two alignment is a classic bit trick: add a - 1, then mask off the low bits.

This works only when a is a power of two, which all real alignments are.

static size_t align_up(size_t n, size_t a) {
    return (n + a - 1) & ~(a - 1);
}
/* align_up(13, 8) == 16, align_up(16, 8) == 16 */