__mt_alloc源码分析（10）

__pool<true>::_M_reserve_block

最后，当线程自己的空闲块没有的时候，__mt_alloc必须向内存池的全局空闲块链表求援，于是进入了_M_reserve_block函数。

 

<mt_allocator.cc>

312    char*

313    __pool<true>::_M_reserve_block(size_t __bytes, const size_t __thread_id)

 

_M_reserve_block函数原型。返回值是直接给用户使用的块的数据区地址，参数__bytes是用户要求分配的内存的字节大小，__thread_id是分配内存的线程的id。

 

314    {

315      // Round up to power of 2 and figure out which bin to use.

316      const size_t __which = _M_binmap[__bytes];

317      const _Tune& __options = _M_get_options();

318      const size_t __bin_size = ((__options._M_min_bin << __which)

319                    + __options._M_align);

320      size_t __block_count = __options._M_chunk_size - sizeof(_Block_address);

321      __block_count /= __bin_size;     

 

进行一些前期计算，__which是负责分配内存的bin的索引，而__bin_size 是这个bin负责的最大的内存字节数。__options包含当前内存池的一些参数，__block_count表示从OS申请的一块内存（__pool每次总是从OS申请大小为__options._M_chunk_size的内存块）能建造多少个空闲内存块。

 

322     

323      // Are we using threads?

324      // - Yes, check if there are free blocks on the global

325      //   list. If so, grab up to __block_count blocks in one

326      //   lock and change ownership. If the global list is

327      //   empty, we allocate a new chunk and add those blocks

328      //   directly to our own freelist (with us as owner).

329      // - No, all operations are made directly to global pool 0

330      //   no need to lock or change ownership but check for free

331      //   blocks on global list (and if not add new ones) and