typedef struct ngx_slab_page_s  ngx_slab_page_t;

struct ngx_slab_page_s {
    uintptr_t         slab;
    ngx_slab_page_t  *next;         //下一个page页
    uintptr_t         prev;         //上一个page页
};

// slab状态结构体
typedef struct {
    ngx_uint_t        total;
    ngx_uint_t        used;

    ngx_uint_t        reqs;
    ngx_uint_t        fails;
} ngx_slab_stat_t;


typedef struct {
    ngx_shmtx_sh_t    lock;         //mutex锁

    size_t            min_size;     //设定的最小内存块长度；
    size_t            min_shift;    //ngx_init_zone_pool中默认为3

    ngx_slab_page_t  *pages;        //每一页对应一个ngx_slab_page_t页描述结构体，所有的ngx_slab_page_t存放在连续的内存中构成数组，而pages就是数组首地址
    ngx_slab_page_t  *last;         //最后页面地址
    ngx_slab_page_t   free;         //所有的空闲页组成一个链表挂在free成员上

    ngx_slab_stat_t  *stats;        
    ngx_uint_t        pfree;        // 剩余页数

    u_char           *start;        //第一页的首地址
    u_char           *end;          //指向这段共享内存的尾部

    ngx_shmtx_t       mutex;        //Nginx封装的互斥锁

    u_char           *log_ctx;      // slab操作失败时会记录日志，为区别是哪个slab共享内存出错，可以在slab中分配一段内存存放描述的字符串，然后再用
log_ctx指向这个字符串
    u_char            zero;         // 表示空字符串防止出错

    unsigned          log_nomem:1;

    void             *data;     //slab的模块自由使用，slab管理内存时不会用到它
    void             *addr;     //指向所属的ngx_shm_zone_t里的ngx_shm_t成员的addr成员，一般用于指示一段共享内存块的起始位置
} ngx_slab_pool_t;

void
ngx_slab_sizes_init(void)
{
    ngx_uint_t  n;

    ngx_slab_max_size = ngx_pagesize / 2;
    ngx_slab_exact_size = ngx_pagesize / (8 * sizeof(uintptr_t));
    for (n = ngx_slab_exact_size; n >>= 1; ngx_slab_exact_shift++) {
        /* void */
    }
}

void
ngx_slab_init(ngx_slab_pool_t *pool)
{
    u_char           *p;
    size_t            size;
    ngx_int_t         m;
    ngx_uint_t        i, n, pages;
    ngx_slab_page_t  *slots, *page;
    //最小分配的空间是8byte 
    pool->min_size = (size_t) 1 << pool->min_shift;

    slots = ngx_slab_slots(pool);

    p = (u_char *) slots;
    size = pool->end - p;

    ngx_slab_junk(p, size);

    n = ngx_pagesize_shift - pool->min_shift;

    for (i = 0; i < n; i++) {
        /* only "next" is used in list head */
        slots[i].slab = 0;
        slots[i].next = &slots[i];
        slots[i].prev = 0;
    }
    //跳过上面那些slab page
    p += n * sizeof(ngx_slab_page_t);

    pool->stats = (ngx_slab_stat_t *) p;
    ngx_memzero(pool->stats, n * sizeof(ngx_slab_stat_t));

    p += n * sizeof(ngx_slab_stat_t);

    size -= n * (sizeof(ngx_slab_page_t) + sizeof(ngx_slab_stat_t));

    pages = (ngx_uint_t) (size / (ngx_pagesize + sizeof(ngx_slab_page_t)));

    pool->pages = (ngx_slab_page_t *) p;
    ngx_memzero(pool->pages, pages * sizeof(ngx_slab_page_t));

    page = pool->pages;

    /* only "next" is used in list head */
    //初始化free，free.next是下次分配页时候的入口  
    pool->free.slab = 0;
    pool->free.next = page;
    pool->free.prev = 0;
    //更新第一个slab page的状态，这儿slab成员记录了整个缓存区的页数目  
    page->slab = pages;
    page->next = &pool->free;
    page->prev = (uintptr_t) &pool->free;
    //实际缓存区(页)的开头，对齐   
    //因为对齐的原因,使得m_page数组和数据区域之间可能有些内存无法使用
    pool->start = ngx_align_ptr(p + pages * sizeof(ngx_slab_page_t),
                                ngx_pagesize);

    m = pages - (pool->end - pool->start) / ngx_pagesize;
    if (m > 0) {
        pages -= m;
        page->slab = pages;
    }
    //跳过pages * sizeof(ngx_slab_page_t)
    pool->last = pool->pages + pages;
    pool->pfree = pages;

    pool->log_nomem = 1;
    pool->log_ctx = &pool->zero;
    pool->zero = '\0';
}

void *
ngx_slab_alloc(ngx_slab_pool_t *pool, size_t size)
{
    void  *p;
    //由于是共享内存，所以在进程间需要用锁来保持同步
    ngx_shmtx_lock(&pool->mutex);

    p = ngx_slab_alloc_locked(pool, size);

    ngx_shmtx_unlock(&pool->mutex);

    return p;
}

void *
ngx_slab_alloc_locked(ngx_slab_pool_t *pool, size_t size)
{
    size_t            s;
    uintptr_t         p, m, mask, *bitmap;
    ngx_uint_t        i, n, slot, shift, map;
    ngx_slab_page_t  *page, *prev, *slots;

    if (size > ngx_slab_max_size) {

        ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                       "slab alloc: %uz", size);

        page = ngx_slab_alloc_pages(pool, (size >> ngx_pagesize_shift)
                                          + ((size % ngx_pagesize) ? 1 : 0));
        if (page) {
            p = ngx_slab_page_addr(pool, page);

        } else {
            p = 0;
        }

        goto done;
    }

    if (size > pool->min_size) {
        shift = 1;
        for (s = size - 1; s >>= 1; shift++) { /* void */ }
        slot = shift - pool->min_shift;

    } else {
        shift = pool->min_shift;
        slot = 0;
    }

    pool->stats[slot].reqs++;

    ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                   "slab alloc: %uz slot: %ui", size, slot);

    slots = ngx_slab_slots(pool);
    page = slots[slot].next;

    if (page->next != page) {

        if (shift < ngx_slab_exact_shift) {

            bitmap = (uintptr_t *) ngx_slab_page_addr(pool, page);

            map = (ngx_pagesize >> shift) / (8 * sizeof(uintptr_t));

            for (n = 0; n < map; n++) {

                if (bitmap[n] != NGX_SLAB_BUSY) {

                    for (m = 1, i = 0; m; m <<= 1, i++) {
                        if (bitmap[n] & m) {
                            continue;
                        }

                        bitmap[n] |= m;

                        i = (n * 8 * sizeof(uintptr_t) + i) << shift;

                        p = (uintptr_t) bitmap + i;

                        pool->stats[slot].used++;

                        if (bitmap[n] == NGX_SLAB_BUSY) {
                            for (n = n + 1; n < map; n++) {
                                if (bitmap[n] != NGX_SLAB_BUSY) {
                                    goto done;
                                }
                            }

                            prev = ngx_slab_page_prev(page);
                            prev->next = page->next;
                            page->next->prev = page->prev;

                            page->next = NULL;
                            page->prev = NGX_SLAB_SMALL;
                        }

                        goto done;
                    }
                }
            }

        } else if (shift == ngx_slab_exact_shift) {

            for (m = 1, i = 0; m; m <<= 1, i++) {
                if (page->slab & m) {
                    continue;
                }

                page->slab |= m;

                if (page->slab == NGX_SLAB_BUSY) {
                    prev = ngx_slab_page_prev(page);
                    prev->next = page->next;
                    page->next->prev = page->prev;

                    page->next = NULL;
                    page->prev = NGX_SLAB_EXACT;
                }

                p = ngx_slab_page_addr(pool, page) + (i << shift);

                pool->stats[slot].used++;

                goto done;
            }

        } else { /* shift > ngx_slab_exact_shift */

            mask = ((uintptr_t) 1 << (ngx_pagesize >> shift)) - 1;
            mask <<= NGX_SLAB_MAP_SHIFT;

            for (m = (uintptr_t) 1 << NGX_SLAB_MAP_SHIFT, i = 0;
                 m & mask;
                 m <<= 1, i++)
            {
                if (page->slab & m) {
                    continue;
                }

                page->slab |= m;

                if ((page->slab & NGX_SLAB_MAP_MASK) == mask) {
                    prev = ngx_slab_page_prev(page);
                    prev->next = page->next;
                    page->next->prev = page->prev;

                    page->next = NULL;
                    page->prev = NGX_SLAB_BIG;
                }

                p = ngx_slab_page_addr(pool, page) + (i << shift);

                pool->stats[slot].used++;

                goto done;
            }
        }

        ngx_slab_error(pool, NGX_LOG_ALERT, "ngx_slab_alloc(): page is busy");
        ngx_debug_point();
    }
    //分出一页加入到m_slot数组对应元素中  
    page = ngx_slab_alloc_pages(pool, 1);

    if (page) {
        if (shift < ngx_slab_exact_shift) {
            bitmap = (uintptr_t *) ngx_slab_page_addr(pool, page);

            n = (ngx_pagesize >> shift) / ((1 << shift) * 8);

            if (n == 0) {
                n = 1;
            }

            /* "n" elements for bitmap, plus one requested */

            for (i = 0; i < (n + 1) / (8 * sizeof(uintptr_t)); i++) {
                bitmap[i] = NGX_SLAB_BUSY;
            }

            m = ((uintptr_t) 1 << ((n + 1) % (8 * sizeof(uintptr_t)))) - 1;
            bitmap[i] = m;

            map = (ngx_pagesize >> shift) / (8 * sizeof(uintptr_t));

            for (i = i + 1; i < map; i++) {
                bitmap[i] = 0;
            }

            page->slab = shift;
            page->next = &slots[slot];
            page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_SMALL;

            slots[slot].next = page;

            pool->stats[slot].total += (ngx_pagesize >> shift) - n;

            p = ngx_slab_page_addr(pool, page) + (n << shift);

            pool->stats[slot].used++;

            goto done;

        } else if (shift == ngx_slab_exact_shift) {

            page->slab = 1;
            page->next = &slots[slot];
            page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_EXACT;

            slots[slot].next = page;

            pool->stats[slot].total += 8 * sizeof(uintptr_t);

            p = ngx_slab_page_addr(pool, page);

            pool->stats[slot].used++;

            goto done;

        } else { /* shift > ngx_slab_exact_shift */

            page->slab = ((uintptr_t) 1 << NGX_SLAB_MAP_SHIFT) | shift;
            page->next = &slots[slot];
            page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_BIG;

            slots[slot].next = page;

            pool->stats[slot].total += ngx_pagesize >> shift;

            p = ngx_slab_page_addr(pool, page);

            pool->stats[slot].used++;

            goto done;
        }
    }

    p = 0;

    pool->stats[slot].fails++;

done:

    ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                   "slab alloc: %p", (void *) p);

    return (void *) p;
}

/* 由于是共享内存，所以在进程间需要用锁来保持同步 */
void *
ngx_slab_calloc(ngx_slab_pool_t *pool, size_t size)
{
    void  *p;

    ngx_shmtx_lock(&pool->mutex);

    p = ngx_slab_calloc_locked(pool, size);

    ngx_shmtx_unlock(&pool->mutex);

    return p;
}

void *
ngx_slab_calloc_locked(ngx_slab_pool_t *pool, size_t size)
{
    void  *p;

    p = ngx_slab_alloc_locked(pool, size);
    if (p) {
        ngx_memzero(p, size);
    }

    return p;
}


static ngx_slab_page_t *
ngx_slab_alloc_pages(ngx_slab_pool_t *pool, ngx_uint_t pages)
{
    ngx_slab_page_t  *page, *p;
    //初始化的时候pool->free.next默认指向第一个pool->pages
    //从pool->free.next开始，每次取(slab page) page = page->next
    for (page = pool->free.next; page != &pool->free; page = page->next) {

        if (page->slab >= pages) {

            if (page->slab > pages) {
                page[page->slab - 1].prev = (uintptr_t) &page[pages];

                page[pages].slab = page->slab - pages;
                page[pages].next = page->next;
                page[pages].prev = page->prev;

                p = (ngx_slab_page_t *) page->prev;
                p->next = &page[pages];
                page->next->prev = (uintptr_t) &page[pages];

            } else {//page页不够用了，则free的next和prev都指向自己
                p = (ngx_slab_page_t *) page->prev;
                p->next = page->next;
                page->next->prev = page->prev;
            }
            //NGX_SLAB_PAGE_START标记page是分配的pages个页的第一个页，并在第一个页page中记录出其后连续的pages个页是一起分配的
            page->slab = pages | NGX_SLAB_PAGE_START;
            page->next = NULL;
            page->prev = NGX_SLAB_PAGE;

            pool->pfree -= pages;
            //pages为1。则直接返回该page
            if (--pages == 0) {
                return page;
            }

            for (p = page + 1; pages; pages--) {
                //如果分配的页数pages>1，更新后面page slab的slab成员为NGX_SLAB_PAGE_BUSY
                p->slab = NGX_SLAB_PAGE_BUSY;
                p->next = NULL;
                p->prev = NGX_SLAB_PAGE;
                p++;
            }

            return page;
        }
    }

    if (pool->log_nomem) {
        ngx_slab_error(pool, NGX_LOG_CRIT,
                       "ngx_slab_alloc() failed: no memory");
    }

    return NULL;
}

void
ngx_slab_free(ngx_slab_pool_t *pool, void *p)
{
    ngx_shmtx_lock(&pool->mutex);

    ngx_slab_free_locked(pool, p);

    ngx_shmtx_unlock(&pool->mutex);
}

void
ngx_slab_free_locked(ngx_slab_pool_t *pool, void *p)
{
    size_t            size;
    uintptr_t         slab, m, *bitmap;
    ngx_uint_t        i, n, type, slot, shift, map;
    ngx_slab_page_t  *slots, *page;

    ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0, "slab free: %p", p);

    if ((u_char *) p < pool->start || (u_char *) p > pool->end) {
        ngx_slab_error(pool, NGX_LOG_ALERT, "ngx_slab_free(): outside of pool");
        goto fail;
    }
    //根据p找到需要释放的m_page元素  
    n = ((u_char *) p - pool->start) >> ngx_pagesize_shift;
    page = &pool->pages[n];
    //如果分配的时候一次性分配多个page，则第一个page的slab指定本次一次性分配了多少个页page
    slab = page->slab;
    type = ngx_slab_page_type(page);

    switch (type) {

    case NGX_SLAB_SMALL:

        shift = slab & NGX_SLAB_SHIFT_MASK;
        size = (size_t) 1 << shift;

        if ((uintptr_t) p & (size - 1)) {
            goto wrong_chunk;
        }

        n = ((uintptr_t) p & (ngx_pagesize - 1)) >> shift;
        m = (uintptr_t) 1 << (n % (8 * sizeof(uintptr_t)));
        n /= 8 * sizeof(uintptr_t);
        bitmap = (uintptr_t *)
                             ((uintptr_t) p & ~((uintptr_t) ngx_pagesize - 1));

        if (bitmap[n] & m) {
            slot = shift - pool->min_shift;

            if (page->next == NULL) {
                slots = ngx_slab_slots(pool);

                page->next = slots[slot].next;
                slots[slot].next = page;

                page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_SMALL;
                page->next->prev = (uintptr_t) page | NGX_SLAB_SMALL;
            }

            bitmap[n] &= ~m;

            n = (ngx_pagesize >> shift) / ((1 << shift) * 8);

            if (n == 0) {
                n = 1;
            }

            i = n / (8 * sizeof(uintptr_t));
            m = ((uintptr_t) 1 << (n % (8 * sizeof(uintptr_t)))) - 1;

            if (bitmap[i] & ~m) {
                goto done;
            }

            map = (ngx_pagesize >> shift) / (8 * sizeof(uintptr_t));

            for (i = i + 1; i < map; i++) {
                if (bitmap[i]) {
                    goto done;
                }
            }

            ngx_slab_free_pages(pool, page, 1);

            pool->stats[slot].total -= (ngx_pagesize >> shift) - n;

            goto done;
        }

        goto chunk_already_free;

    case NGX_SLAB_EXACT:

        m = (uintptr_t) 1 <<
                (((uintptr_t) p & (ngx_pagesize - 1)) >> ngx_slab_exact_shift);
        size = ngx_slab_exact_size;

        if ((uintptr_t) p & (size - 1)) {
            goto wrong_chunk;
        }
        //slab(位图)中对应的位为1  
        if (slab & m) {
            slot = ngx_slab_exact_shift - pool->min_shift;

            if (slab == NGX_SLAB_BUSY) {
                slots = ngx_slab_slots(pool);

                page->next = slots[slot].next;
                slots[slot].next = page;

                page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_EXACT;
                page->next->prev = (uintptr_t) page | NGX_SLAB_EXACT;
            }

            page->slab &= ~m;

            if (page->slab) {
                goto done;
            }

            ngx_slab_free_pages(pool, page, 1);

            pool->stats[slot].total -= 8 * sizeof(uintptr_t);

            goto done;
        }

        goto chunk_already_free;

    case NGX_SLAB_BIG:
        //slab的高16位是slot块的位图,低16位用于存储slot块大小的偏移
        shift = slab & NGX_SLAB_SHIFT_MASK;
        size = (size_t) 1 << shift;

        if ((uintptr_t) p & (size - 1)) {
            goto wrong_chunk;
        }

        m = (uintptr_t) 1 << ((((uintptr_t) p & (ngx_pagesize - 1)) >> shift)
                              + NGX_SLAB_MAP_SHIFT);
        //该slab块确实正在被使用  
        if (slab & m) {
            slot = shift - pool->min_shift;

            if (page->next == NULL) {
                slots = ngx_slab_slots(pool);

                page->next = slots[slot].next;
                slots[slot].next = page;

                page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_BIG;
                page->next->prev = (uintptr_t) page | NGX_SLAB_BIG;
            }

            page->slab &= ~m;

            if (page->slab & NGX_SLAB_MAP_MASK) {
                goto done;
            }
            //如果page页中所有slab块都不在使用就将该页面链入free中
            ngx_slab_free_pages(pool, page, 1);

            pool->stats[slot].total -= ngx_pagesize >> shift;

            goto done;
        }

        goto chunk_already_free;
    //用户归还整个页面 
    case NGX_SLAB_PAGE:

        if ((uintptr_t) p & (ngx_pagesize - 1)) {
            goto wrong_chunk;
        }

        if (!(slab & NGX_SLAB_PAGE_START)) {
            ngx_slab_error(pool, NGX_LOG_ALERT,
                           "ngx_slab_free(): page is already free");
            goto fail;
        }

        if (slab == NGX_SLAB_PAGE_BUSY) {
            ngx_slab_error(pool, NGX_LOG_ALERT,
                           "ngx_slab_free(): pointer to wrong page");
            goto fail;
        }
        //计算归还page的个数
        size = slab & ~NGX_SLAB_PAGE_START;
        //归还页面 
        ngx_slab_free_pages(pool, page, size);

        ngx_slab_junk(p, size << ngx_pagesize_shift);

        return;
    }

    /* not reached */

    return;

done:

    pool->stats[slot].used--;

    ngx_slab_junk(p, size);

    return;

wrong_chunk:

    ngx_slab_error(pool, NGX_LOG_ALERT,
                   "ngx_slab_free(): pointer to wrong chunk");

    goto fail;

chunk_already_free:

    ngx_slab_error(pool, NGX_LOG_ALERT,
                   "ngx_slab_free(): chunk is already free");

fail:

    return;
}

Syntax: limit_conn_zone key zone=name:size;
Default: —
Context: http

Syntax:	limit_conn zone number;
Default: —
Context: http, server, location

static char *
ngx_http_limit_conn_zone(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    u_char                            *p;
    ssize_t                            size;
    ngx_str_t                         *value, name, s;
    ngx_uint_t                         i;
    ngx_shm_zone_t                    *shm_zone;
    ngx_http_limit_conn_ctx_t         *ctx;
    ngx_http_compile_complex_value_t   ccv;

    value = cf->args->elts;

    ctx = ngx_pcalloc(cf->pool, sizeof(ngx_http_limit_conn_ctx_t));
    if (ctx == NULL) {
        return NGX_CONF_ERROR;
    }

    ngx_memzero(&ccv, sizeof(ngx_http_compile_complex_value_t));

    ccv.cf = cf;
    ccv.value = &value[1];
    ccv.complex_value = &ctx->key;

    if (ngx_http_compile_complex_value(&ccv) != NGX_OK) {
        return NGX_CONF_ERROR;
    }

    size = 0;
    name.len = 0;

    for (i = 2; i < cf->args->nelts; i++) {

        if (ngx_strncmp(value[i].data, "zone=", 5) == 0) {

            name.data = value[i].data + 5;

            p = (u_char *) ngx_strchr(name.data, ':');

            if (p == NULL) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid zone size \"%V\"", &value[i]);
                return NGX_CONF_ERROR;
            }

            name.len = p - name.data;

            s.data = p + 1;
            s.len = value[i].data + value[i].len - s.data;

            size = ngx_parse_size(&s);

            if (size == NGX_ERROR) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid zone size \"%V\"", &value[i]);
                return NGX_CONF_ERROR;
            }

            if (size < (ssize_t) (8 * ngx_pagesize)) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "zone \"%V\" is too small", &value[i]);
                return NGX_CONF_ERROR;
            }

            continue;
        }

        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "invalid parameter \"%V\"", &value[i]);
        return NGX_CONF_ERROR;
    }

    if (name.len == 0) {
        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "\"%V\" must have \"zone\" parameter",
                           &cmd->name);
        return NGX_CONF_ERROR;
    }

    shm_zone = ngx_shared_memory_add(cf, &name, size,
                                     &ngx_http_limit_conn_module);
    if (shm_zone == NULL) {
        return NGX_CONF_ERROR;
    }

    if (shm_zone->data) {
        ctx = shm_zone->data;

        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "%V \"%V\" is already bound to key \"%V\"",
                           &cmd->name, &name, &ctx->key.value);
        return NGX_CONF_ERROR;
    }

    shm_zone->init = ngx_http_limit_conn_init_zone;
    shm_zone->data = ctx;

    return NGX_CONF_OK;
}

static char *
ngx_http_limit_conn(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_shm_zone_t               *shm_zone;
    ngx_http_limit_conn_conf_t   *lccf = conf;
    ngx_http_limit_conn_limit_t  *limit, *limits;

    ngx_str_t  *value;
    ngx_int_t   n;
    ngx_uint_t  i;

    value = cf->args->elts;

    shm_zone = ngx_shared_memory_add(cf, &value[1], 0,
                                     &ngx_http_limit_conn_module);
    if (shm_zone == NULL) {
        return NGX_CONF_ERROR;
    }

    limits = lccf->limits.elts;

    if (limits == NULL) {
        if (ngx_array_init(&lccf->limits, cf->pool, 1,
                           sizeof(ngx_http_limit_conn_limit_t))
            != NGX_OK)
        {
            return NGX_CONF_ERROR;
        }
    }

    for (i = 0; i < lccf->limits.nelts; i++) {
        if (shm_zone == limits[i].shm_zone) {
            return "is duplicate";
        }
    }

    // 解析配置种被限制的连接数
    n = ngx_atoi(value[2].data, value[2].len);
    if (n <= 0) {
        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "invalid number of connections \"%V\"", &value[2]);
        return NGX_CONF_ERROR;
    }

    if (n > 65535) {
        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "connection limit must be less 65536");
        return NGX_CONF_ERROR;
    }

    limit = ngx_array_push(&lccf->limits);
    if (limit == NULL) {
        return NGX_CONF_ERROR;
    }

    limit->conn = n;
    limit->shm_zone = shm_zone;

    return NGX_CONF_OK;
}

static ngx_int_t
ngx_http_limit_conn_handler(ngx_http_request_t *r)
{
    size_t                          n;
    uint32_t                        hash;
    ngx_str_t                       key;
    ngx_uint_t                      i;
    ngx_slab_pool_t                *shpool;
    ngx_rbtree_node_t              *node;
    ngx_pool_cleanup_t             *cln;
    ngx_http_limit_conn_ctx_t      *ctx;
    ngx_http_limit_conn_node_t     *lc;
    ngx_http_limit_conn_conf_t     *lccf;
    ngx_http_limit_conn_limit_t    *limits;
    ngx_http_limit_conn_cleanup_t  *lccln;

    if (r->main->limit_conn_set) {
        return NGX_DECLINED;
    }

    lccf = ngx_http_get_module_loc_conf(r, ngx_http_limit_conn_module);
    limits = lccf->limits.elts;

    for (i = 0; i < lccf->limits.nelts; i++) {
        ctx = limits[i].shm_zone->data;

        //获取limit_conn_zone指令后key的值
        if (ngx_http_complex_value(r, &ctx->key, &key) != NGX_OK) {
            return NGX_HTTP_INTERNAL_SERVER_ERROR;
        }

        if (key.len == 0) {
            continue;
        }

        if (key.len > 255) {
            ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                          "the value of the \"%V\" key "
                          "is more than 255 bytes: \"%V\"",
                          &ctx->key.value, &key);
            continue;
        }

        r->main->limit_conn_set = 1;
        //对key进行crc计算，用于在红黑树中查找结点
        hash = ngx_crc32_short(key.data, key.len);

        shpool = (ngx_slab_pool_t *) limits[i].shm_zone->shm.addr;

        ngx_shmtx_lock(&shpool->mutex);
        // 查找符合要求的结点
        node = ngx_http_limit_conn_lookup(ctx->rbtree, &key, hash);
        // 没有找到对应的结点，将此次请求插入到红黑树中
        if (node == NULL) {
            //计算结点所需内存
            n = offsetof(ngx_rbtree_node_t, color)
                + offsetof(ngx_http_limit_conn_node_t, data)
                + key.len;
            // 申请结点内存
            node = ngx_slab_alloc_locked(shpool, n);

            if (node == NULL) {
                ngx_shmtx_unlock(&shpool->mutex);
                ngx_http_limit_conn_cleanup_all(r->pool);
                return lccf->status_code;
            }

            lc = (ngx_http_limit_conn_node_t *) &node->color;
            // 赋值key，用于下次请求到来查询
            node->key = hash;
            lc->len = (u_char) key.len;
            lc->conn = 1;
            ngx_memcpy(lc->data, key.data, key.len);

            ngx_rbtree_insert(ctx->rbtree, node);

        } else {

            lc = (ngx_http_limit_conn_node_t *) &node->color;
            // 判断key标记的connections是否超过配置，如果超过则返回错误码
            if ((ngx_uint_t) lc->conn >= limits[i].conn) {

                ngx_shmtx_unlock(&shpool->mutex);

                ngx_log_error(lccf->log_level, r->connection->log, 0,
                              "limiting connections by zone \"%V\"",
                              &limits[i].shm_zone->shm.name);

                ngx_http_limit_conn_cleanup_all(r->pool);
                return lccf->status_code;
            }
            //连接数量+1
            lc->conn++;
        }

        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                       "limit conn: %08Xi %d", node->key, lc->conn);

        ngx_shmtx_unlock(&shpool->mutex);
        //注册内存池清理函数
        cln = ngx_pool_cleanup_add(r->pool,
                                   sizeof(ngx_http_limit_conn_cleanup_t));
        if (cln == NULL) {
            return NGX_HTTP_INTERNAL_SERVER_ERROR;
        }

        // 绑定内存清理函数
        cln->handler = ngx_http_limit_conn_cleanup;
        lccln = cln->data;

        lccln->shm_zone = limits[i].shm_zone;
        lccln->node = node;
    }

    return NGX_DECLINED;
}

gcc -g

proxy_cache_path /dev/sda1/data inactive=60m max_size=10G;

return value;

static char *
ngx_stream_return(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_stream_return_srv_conf_t *rscf = conf;

    ngx_str_t                           *value;
    ngx_stream_core_srv_conf_t          *cscf;
    ngx_stream_compile_complex_value_t   ccv;

    if (rscf->text.value.data) {
        return "is duplicate";
    }

    value = cf->args->elts;

    ngx_memzero(&ccv, sizeof(ngx_stream_compile_complex_value_t));

    ccv.cf = cf;
    ccv.value = &value[1];
    ccv.complex_value = &rscf->text;

    if (ngx_stream_compile_complex_value(&ccv) != NGX_OK) {
        return NGX_CONF_ERROR;
    }

    cscf = ngx_stream_conf_get_module_srv_conf(cf, ngx_stream_core_module);

    cscf->handler = ngx_stream_return_handler;

    return NGX_CONF_OK;
}

static void
ngx_stream_return_handler(ngx_stream_session_t *s)
{
    ngx_str_t                      text;
    ngx_buf_t                     *b;
    ngx_connection_t              *c;
    ngx_stream_return_ctx_t       *ctx;
    ngx_stream_return_srv_conf_t  *rscf;

    c = s->connection;

    c->log->action = "returning text";

    rscf = ngx_stream_get_module_srv_conf(s, ngx_stream_return_module);

    // 获取变量值
    if (ngx_stream_complex_value(s, &rscf->text, &text) != NGX_OK) {
        ngx_stream_finalize_session(s, NGX_STREAM_INTERNAL_SERVER_ERROR);
        return;
    }

    ngx_log_debug1(NGX_LOG_DEBUG_STREAM, c->log, 0,
                   "stream return text: \"%V\"", &text);

    if (text.len == 0) {
        ngx_stream_finalize_session(s, NGX_STREAM_OK);
        return;
    }

    // 分配上下文内存
    ctx = ngx_pcalloc(c->pool, sizeof(ngx_stream_return_ctx_t));
    if (ctx == NULL) {
        ngx_stream_finalize_session(s, NGX_STREAM_INTERNAL_SERVER_ERROR);
        return;
    }

    // 设置模块上下文
    ngx_stream_set_ctx(s, ctx, ngx_stream_return_module);

    b = ngx_calloc_buf(c->pool);
    if (b == NULL) {
        ngx_stream_finalize_session(s, NGX_STREAM_INTERNAL_SERVER_ERROR);
        return;
    }

    b->memory = 1;
    b->pos = text.data;
    b->last = text.data + text.len;
    b->last_buf = 1;

    ctx->out = ngx_alloc_chain_link(c->pool);
    if (ctx->out == NULL) {
        ngx_stream_finalize_session(s, NGX_STREAM_INTERNAL_SERVER_ERROR);
        return;
    }

    // 挂载输出buf
    ctx->out->buf = b;
    ctx->out->next = NULL;

    // 设置写回调函数
    c->write->handler = ngx_stream_return_write_handler;

    // 调用写回调函数
    ngx_stream_return_write_handler(c->write);
}

geo $remote_addr $var {
	
}

static char *
ngx_http_geo_block(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    char                     *rv;
    size_t                    len;
    ngx_str_t                *value, name;
    ngx_uint_t                i;
    ngx_conf_t                save;
    ngx_pool_t               *pool;
    ngx_array_t              *a;
    ngx_http_variable_t      *var;
    ngx_http_geo_ctx_t       *geo;
    ngx_http_geo_conf_ctx_t   ctx;
#if (NGX_HAVE_INET6)
    static struct in6_addr    zero;
#endif

    value = cf->args->elts;

    geo = ngx_palloc(cf->pool, sizeof(ngx_http_geo_ctx_t));
    if (geo == NULL) {
        return NGX_CONF_ERROR;
    }

    name = value[1];

    if (name.data[0] != '$') {
        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "invalid variable name \"%V\"", &name);
        return NGX_CONF_ERROR;
    }

    name.len--;
    name.data++;

    if (cf->args->nelts == 3) {

        geo->index = ngx_http_get_variable_index(cf, &name);
        if (geo->index == NGX_ERROR) {
            return NGX_CONF_ERROR;
        }

        name = value[2];

        if (name.data[0] != '$') {
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "invalid variable name \"%V\"", &name);
            return NGX_CONF_ERROR;
        }

        name.len--;
        name.data++;

    } else {
        geo->index = -1;
    }

    var = ngx_http_add_variable(cf, &name, NGX_HTTP_VAR_CHANGEABLE);
    if (var == NULL) {
        return NGX_CONF_ERROR;
    }

    pool = ngx_create_pool(NGX_DEFAULT_POOL_SIZE, cf->log);
    if (pool == NULL) {
        return NGX_CONF_ERROR;
    }

    ngx_memzero(&ctx, sizeof(ngx_http_geo_conf_ctx_t));

    ctx.temp_pool = ngx_create_pool(NGX_DEFAULT_POOL_SIZE, cf->log);
    if (ctx.temp_pool == NULL) {
        return NGX_CONF_ERROR;
    }

    ngx_rbtree_init(&ctx.rbtree, &ctx.sentinel, ngx_str_rbtree_insert_value);

    ctx.pool = cf->pool;
    ctx.data_size = sizeof(ngx_http_geo_header_t)
                  + sizeof(ngx_http_variable_value_t)
                  + 0x10000 * sizeof(ngx_http_geo_range_t *);
    ctx.allow_binary_include = 1;

    save = *cf;
    cf->pool = pool;
    cf->ctx = &ctx;
    cf->handler = ngx_http_geo;
    cf->handler_conf = conf;

    rv = ngx_conf_parse(cf, NULL);

    *cf = save;

    geo->proxies = ctx.proxies;
    geo->proxy_recursive = ctx.proxy_recursive;

    if (ctx.ranges) {

        if (ctx.high.low && !ctx.binary_include) {
            for (i = 0; i < 0x10000; i++) {
                a = (ngx_array_t *) ctx.high.low[i];

                if (a == NULL || a->nelts == 0) {
                    continue;
                }

                len = a->nelts * sizeof(ngx_http_geo_range_t);

                ctx.high.low[i] = ngx_palloc(cf->pool, len + sizeof(void *));
                if (ctx.high.low[i] == NULL) {
                    return NGX_CONF_ERROR;
                }

                ngx_memcpy(ctx.high.low[i], a->elts, len);
                ctx.high.low[i][a->nelts].value = NULL;
                ctx.data_size += len + sizeof(void *);
            }

            if (ctx.allow_binary_include
                && !ctx.outside_entries
                && ctx.entries > 100000
                && ctx.includes == 1)
            {
                ngx_http_geo_create_binary_base(&ctx);
            }
        }

        if (ctx.high.default_value == NULL) {
            ctx.high.default_value = &ngx_http_variable_null_value;
        }

        geo->u.high = ctx.high;

        var->get_handler = ngx_http_geo_range_variable;
        var->data = (uintptr_t) geo;

        ngx_destroy_pool(ctx.temp_pool);
        ngx_destroy_pool(pool);

    } else {
        if (ctx.tree == NULL) {
            ctx.tree = ngx_radix_tree_create(cf->pool, -1);
            if (ctx.tree == NULL) {
                return NGX_CONF_ERROR;
            }
        }

        geo->u.trees.tree = ctx.tree;

#if (NGX_HAVE_INET6)
        if (ctx.tree6 == NULL) {
            ctx.tree6 = ngx_radix_tree_create(cf->pool, -1);
            if (ctx.tree6 == NULL) {
                return NGX_CONF_ERROR;
            }
        }

        geo->u.trees.tree6 = ctx.tree6;
#endif

        var->get_handler = ngx_http_geo_cidr_variable;
        var->data = (uintptr_t) geo;

        ngx_destroy_pool(ctx.temp_pool);
        ngx_destroy_pool(pool);

        if (ngx_radix32tree_insert(ctx.tree, 0, 0,
                                   (uintptr_t) &ngx_http_variable_null_value)
            == NGX_ERROR)
        {
            return NGX_CONF_ERROR;
        }

        /* NGX_BUSY is okay (default was set explicitly) */

#if (NGX_HAVE_INET6)
        if (ngx_radix128tree_insert(ctx.tree6, zero.s6_addr, zero.s6_addr,
                                    (uintptr_t) &ngx_http_variable_null_value)
            == NGX_ERROR)
        {
            return NGX_CONF_ERROR;
        }
#endif
    }

    return rv;
}