static void *find_fit(size_t asize)
{
    /* next fit search */
    for (;GET_SIZE(HDRP(next_fit)) > 0; next_fit = NEXT_BLKP(next_fit))
		if(!GET_ALLOC(HDRP(next_fit)) && asize <= GET_SIZE(HDRP(next_fit)))
           return next_fit;
    return NULL;
}