malloc_state 相關函數¶

/*
   Initialize a malloc_state struct.
   This is called only from within malloc_consolidate, which needs
   be called in the same contexts anyway.  It is never called directly
   outside of malloc_consolidate because some optimizing compilers try
   to inline it at all call points, which turns out not to be an
   optimization at all. (Inlining it in malloc_consolidate is fine though.)
 */

static void malloc_init_state(mstate av) {
    int     i;
    mbinptr bin;

    /* Establish circular links for normal bins */
    for (i = 1; i < NBINS; ++i) {
        bin     = bin_at(av, i);
        bin->fd = bin->bk = bin;
    }

#if MORECORE_CONTIGUOUS
    if (av != &main_arena)
#endif
        set_noncontiguous(av);
    if (av == &main_arena) set_max_fast(DEFAULT_MXFAST);
    // 設置 flags 標記目前沒有fast chunk
    av->flags |= FASTCHUNKS_BIT;
    // 就是 unsorted bin
    av->top = initial_top(av);
}

static void malloc_consolidate(mstate av) {
    mfastbinptr *fb;             /* current fastbin being consolidated */
    mfastbinptr *maxfb;          /* last fastbin (for loop control) */
    mchunkptr    p;              /* current chunk being consolidated */
    mchunkptr    nextp;          /* next chunk to consolidate */
    mchunkptr    unsorted_bin;   /* bin header */
    mchunkptr    first_unsorted; /* chunk to link to */

    /* These have same use as in free() */
    mchunkptr       nextchunk;
    INTERNAL_SIZE_T size;
    INTERNAL_SIZE_T nextsize;
    INTERNAL_SIZE_T prevsize;
    int             nextinuse;
    mchunkptr       bck;
    mchunkptr       fwd;

    /*
      If max_fast is 0, we know that av hasn't
      yet been initialized, in which case do so below
    */
    // 說明 fastbin 已經初始化
    if (get_max_fast() != 0) {
        // 清空 fastbin 標記
        // 因爲要合併 fastbin 中的 chunk 了。
        clear_fastchunks(av);
        //
        unsorted_bin = unsorted_chunks(av);

        /*
          Remove each chunk from fast bin and consolidate it, placing it
          then in unsorted bin. Among other reasons for doing this,
          placing in unsorted bin avoids needing to calculate actual bins
          until malloc is sure that chunks aren't immediately going to be
          reused anyway.
        */
        // 按照 fd 順序遍歷 fastbin 的每一個 bin，將 bin 中的每一個 chunk 合併掉。
        maxfb = &fastbin(av, NFASTBINS - 1);
        fb    = &fastbin(av, 0);
        do {
            p = atomic_exchange_acq(fb, NULL);
            if (p != 0) {
                do {
                    check_inuse_chunk(av, p);
                    nextp = p->fd;

                    /* Slightly streamlined version of consolidation code in
                     * free() */
                    size      = chunksize(p);
                    nextchunk = chunk_at_offset(p, size);
                    nextsize  = chunksize(nextchunk);

                    if (!prev_inuse(p)) {
                        prevsize = prev_size(p);
                        size += prevsize;
                        p = chunk_at_offset(p, -((long) prevsize));
                        unlink(av, p, bck, fwd);
                    }

                    if (nextchunk != av->top) {
                        // 判斷 nextchunk 是否是空閒的。
                        nextinuse = inuse_bit_at_offset(nextchunk, nextsize);

                        if (!nextinuse) {
                            size += nextsize;
                            unlink(av, nextchunk, bck, fwd);
                        } else
                         // 設置 nextchunk 的 prev inuse 爲0，以表明可以合併當前 fast chunk。
                            clear_inuse_bit_at_offset(nextchunk, 0);

                        first_unsorted     = unsorted_bin->fd;
                        unsorted_bin->fd   = p;
                        first_unsorted->bk = p;

                        if (!in_smallbin_range(size)) {
                            p->fd_nextsize = NULL;
                            p->bk_nextsize = NULL;
                        }

                        set_head(p, size | PREV_INUSE);
                        p->bk = unsorted_bin;
                        p->fd = first_unsorted;
                        set_foot(p, size);
                    }

                    else {
                        size += nextsize;
                        set_head(p, size | PREV_INUSE);
                        av->top = p;
                    }

                } while ((p = nextp) != 0);
            }
        } while (fb++ != maxfb);

    } else {
        malloc_init_state(av);
        // 在非調試情況下沒有什麼用，在調試情況下，做一些檢測。
        check_malloc_state(av);
    }