關於使用valgrind檢測內存泄露的問題

用valgrind檢測內存泄露的時候，發現一個問題，就是程序總會報一個泄露點

1 blocks are still reachable

查了程序發現並沒有泄露，最後百度才知道我對內存泄露的知識理解並不全面

以前我的理解是，只要分配的內存但是沒釋放就會導致內存泄露

但是這養理解是不全面的。只要分配了內存，如果它的指針沒有丟失，那麼就不算是泄露

所以來說，static的指針或者全局指針，如果沒有釋放它，那也算是not-freed blocks，也就是reachable的，但是現代的OS會得到這些指針並去釋放它

下面內容摘抄於https://bbs.csdn.net/topics/60443491 @天笑2001

1. 使用全局指針或靜態指針:

int main()
{
    static int  *pi; // use static pointer: not a real memory leak!!!      
    pi = new int;
    return 0;
}

使用valgrind檢查:

 valgrind --leak-check=yes --show-reachable=yes -q ./memtest  

==13782== searching for pointers to 1 not-freed blocks. 
==13782== checked 4484352 bytes. 
==13782== 
==13782== 4 bytes in 1 blocks are definitely lost in loss record 1 of 1 
==13782==    at 0xB74D27AB: __builtin_new (vg_replace_malloc.c:172) 
==13782==    by 0xB74D2802: operator new(unsigned) (vg_replace_malloc.c:185) 
==13782==    by 0x8048491: main (in /home/prog/valgrind/memtest) 
==13782==    by 0xB71C4BA6: __libc_start_main (in /lib/libc-2.3.2.so) 
==13782== 
==13782== LEAK SUMMARY: 
==13782==    definitely lost: 4 bytes in 1 blocks. 
==13782==    possibly lost:   0 bytes in 0 blocks. 
==13782==    still reachable: 0 bytes in 0 blocks. 
==13782==         suppressed: 0 bytes in 0 blocks. 
==13782==

報告說有一個"not-freed block",但這個block是reachable的,所以它不算是真正的memory leak.

2. 如果使用的是局部指針:

int main()
{
    int *pi; // use local pointer: real memory leak!!!    
    pi = new int[100];
    return 0;
}

==13782== searching for pointers to 1 not-freed blocks. 
==13782== checked 4484352 bytes. 
==13782== 
==13782== 4 bytes in 1 blocks are definitely lost in loss record 1 of 1 
==13782==    at 0xB74D27AB: __builtin_new (vg_replace_malloc.c:172) 
==13782==    by 0xB74D2802: operator new(unsigned) (vg_replace_malloc.c:185) 
==13782==    by 0x8048491: main (in /home/prog/valgrind/memtest) 
==13782==    by 0xB71C4BA6: __libc_start_main (in /lib/libc-2.3.2.so) 
==13782== 
==13782== LEAK SUMMARY: 
==13782==    definitely lost: 4 bytes in 1 blocks. 
==13782==    possibly lost:   0 bytes in 0 blocks. 
==13782==    still reachable: 0 bytes in 0 blocks. 
==13782==         suppressed: 0 bytes in 0 blocks. 
==13782==

報告說是"definitely lost",這纔是真正的memory leak!

2 當然,我們一般情況下是不會分配內存而不釋放的. 它的典型應用便是著名的memory pool . 其要領是爲了提高內存管理的效率,針對特殊需要(如要分配的內存是等大的並且很小)去重載new,delete操作符. 它會避免每次申請很小的內存,而是一下子得到很大的一塊,再自己去定義和使用.(出於效率的考慮,它最後並不釋放) 我花了一下午時間分析的我用的SGI STL的內存分配源碼(我是在LINUX下使用g++的), 下面一段是在/usr/include/c++/3.2.3/bits/stl_alloc.h中找到的STL源碼:

template <bool __threads, int __inst>     class __default_alloc_template     
{
  private:
    enum {_ALIGN = 8};
    enum {_MAX_BYTES = 128};
    enum {_NFREELISTS = _MAX_BYTES / _ALIGN};
    union _Obj
    {
        union _Obj * _M_free_list_link;
        char _M_client_data[1]; // The client sees this.    
           
    };
    static _Obj
        * volatile _S_free_list[_NFREELISTS]; // Chunk allocation state.      
    static char * _S_start_free;
    static size_t _S_heap_size;
    static _STL_mutex_lock _S_node_allocator_lock;
    ....
}

可以看到,在關鍵的數據結構上,它使用了靜態的指針,並且定義了一些常量來控制內存塊的大小等.

3 對於這種技術是不是"隱式的內存泄漏" , 我認爲不是. 隱式的內存泄漏指分配內存後,雖然也釋放,但沒有及時釋放,導致內存消耗無法及時回收.在服務器編程時特別忌諱. 一般說來, 內存池的大小是可控, 不會一直增長下去. 從應用的層面上來說,它也是"釋放"了內存了的.