>>276
コメントにそう書いてある。
/* Some constants coerced to size_t */
/* Annoying but necessary to avoid errors on some platforms */
#define SIZE_T_ZERO ((size_t)0)
#define SIZE_T_ONE ((size_t)1)
#define SIZE_T_TWO ((size_t)2)
#define SIZE_T_FOUR ((size_t)4)

/* The bit mask value corresponding to MALLOC_ALIGNMENT */
#define CHUNK_ALIGN_MASK (MALLOC_ALIGNMENT - SIZE_T_ONE)