Function dlmalloc

system/lib/dlmalloc.c:4630–4768

#if !ONLY_MSPACES

void* dlmalloc(size_t bytes) {
    /*
     Basic algorithm:
     If a small request (< 256 bytes minus per-chunk overhead):
     1. If one exists, use a remainderless chunk in associated smallbin.
     (Remainderless means that there are too few excess bytes to
     represent as a chunk.)
     2. If it is big enough, use the dv chunk, which is normally the
     chunk adjacent to the one used for the most recent small request.
     3. If one exists, split the smallest available chunk in a bin,
     saving remainder in dv.
     4. If it is big enough, use the top chunk.
     5. If available, get memory from system and use it
     Otherwise, for a large request:
     1. Find the smallest available binned chunk that fits, and use it
     if it is better fitting than dv chunk, splitting if necessary.
     2. If better fitting than any binned chunk, use the dv chunk.
     3. If it is big enough, use the top chunk.
     4. If request size >= mmap threshold, try to directly mmap this chunk.
     5. If available, get memory from system and use it
     
     The ugly goto's here ensure that postaction occurs along all paths.
     */
    
#if USE_LOCKS
    ensure_initialization(); /* initialize in sys_alloc if not using locks */
#endif
    
    if (!PREACTION(gm)) {
        void* mem;
        size_t nb;
        if (bytes <= MAX_SMALL_REQUEST) {
            bindex_t idx;
            binmap_t smallbits;
            nb = (bytes < MIN_REQUEST)? MIN_CHUNK_SIZE : pad_request(bytes);
            idx = small_index(nb);
            smallbits = gm->smallmap >> idx;
            
            if ((smallbits & 0x3U) != 0) { /* Remainderless fit to a smallbin. */
                mchunkptr b, p;
                idx += ~smallbits & 1;       /* Uses next bin if idx empty */
                b = smallbin_at(gm, idx);
                p = b->fd;
                assert(chunksize(p) == small_index2size(idx));
                unlink_first_small_chunk(gm, b, p, idx);
                set_inuse_and_pinuse(gm, p, small_index2size(idx));
                mem = chunk2mem(p);
                check_malloced_chunk(gm, mem, nb);
                goto postaction;
            }
            
            else if (nb > gm->dvsize) {
                if (smallbits != 0) { /* Use chunk in next nonempty smallbin */
                    mchunkptr b, p, r;
                    size_t rsize;
                    bindex_t i;
                    binmap_t leftbits = (smallbits << idx) & left_bits(idx2bit(idx));
                    binmap_t leastbit = least_bit(leftbits);