free() 是如何释放不同内存区块大小的指针？

最初是在知乎上看到这个问题的C++ delete[] 是如何知道数组大小的？，我也挺好奇，所以就作了一番工作。

申请内存时，指针所指向区块的大小这一信息，其实就记录在该指针的周围
看下面这段代码：

 #include<cstdio>
 #include<iostream>
 #include<malloc.h>
 #include<assert.h>
 #include<ctime>
 using namespace std;

 #define size 16

 int main(void)
 {
     void * p = NULL;
     srand(time());
     int a = ;
     while (a--)
     {
         int n = rand() % ;
         p = malloc(n);
         size_t w = *((size_t*)((char*)p - size));
         cout << "w=" << w << endl;
         cout << "n=" << n << endl;
         assert(w == n);
         free(p);
     }
     return ;
 }

（注：如果是X86的CPU，请将 size 改为 8）

你会发现 w 和 n 始终是一致的，，这样其实不是巧合，来看 M$ 编译器 \ vc \include\ 目录下 malloc.h这一头文件 中 184 到 209 行的代码：

 //这儿是根据不同的硬件平台的宏定义
 #if defined (_M_IX86)
 #define _ALLOCA_S_MARKER_SIZE   8
 #elif defined (_M_X64)
 #define _ALLOCA_S_MARKER_SIZE   16
 #elif defined (_M_ARM)
 #define _ALLOCA_S_MARKER_SIZE   8
 #elif !defined (RC_INVOKED)
 #error Unsupported target platform.
 #endif  /* !defined (RC_INVOKED) */

 _STATIC_ASSERT(sizeof(unsigned int) <= _ALLOCA_S_MARKER_SIZE);

 #if !defined (__midl) && !defined (RC_INVOKED)
 #pragma warning(push)
 #pragma warning(disable:6540)
 __inline void *_MarkAllocaS(_Out_opt_ __crt_typefix(unsigned int*) void *_Ptr, unsigned int _Marker)
 {
     if (_Ptr)
     {
         *((unsigned int*)_Ptr) = _Marker;
  //
         _Ptr = (char*)_Ptr + _ALLOCA_S_MARKER_SIZE;
  //最后返回给调用者的指针，是原始指针偏移了_ALLOCA_S_MARKER_SIZE的新指针，这也是刚才我将指针向后偏移，就能得到该指针所指向内存区块的大小的原因。
     }
     return _Ptr;
 }

再来看看在 M$ 编译器中它是如何释放的，同样在 mallloc.h 文件249行到274行：

 /* _freea must be in the header so that its allocator matches _malloca */
 #if !defined (__midl) && !defined (RC_INVOKED)
 #if !(defined (_DEBUG) && defined (_CRTDBG_MAP_ALLOC))
 #undef _freea
 __pragma(warning(push))
 __pragma(warning(disable: ))
 _CRTNOALIAS __inline void __CRTDECL _freea(_Pre_maybenull_ _Post_invalid_ void * _Memory)
 {
     unsigned int _Marker;
     if (_Memory)
     {
         _Memory = (char*)_Memory - _ALLOCA_S_MARKER_SIZE;
 //获得原始指针
         _Marker = *(unsigned int *)_Memory;//得到指针所指区块的大小
         if (_Marker == _ALLOCA_S_HEAP_MARKER)
         {
             free(_Memory);
         }
 #if defined (_ASSERTE)
         else if (_Marker != _ALLOCA_S_STACK_MARKER)
         {
             #pragma warning(suppress: 4548) /* expression before comma has no effect */
             _ASSERTE(("Corrupted pointer passed to _freea", ));
         }
 #endif  /* defined (_ASSERTE) */
     }
 }

再来看看 SGI　STL标准库源码 stl_alloc.h 文件209 行到 246行 debug_alloc类模板的设计：

 // Allocator adaptor to check size arguments for debugging.
 // Reports errors using assert.  Checking can be disabled with
 // NDEBUG, but it's far better to just use the underlying allocator
 // instead when no checking is desired.
 // There is some evidence that this can confuse Purify.
 template <class _Alloc>
 class debug_alloc {

 private:

   enum {_S_extra = };  // Size of space used to store size.  Note
                         // that this must be large enough to preserve
                         // alignment.

                         //这儿就像它所说的那样
 public:

   static void* allocate(size_t __n)
   {
     //
 这里实际申请的内存大小要多  个字节
     char* __result = (char*)_Alloc::allocate(__n + (int) _S_extra);
     *(size_t*)__result = __n;//前 4 个字节用于存储区块大小，可以看到，它预留了4个字节的空白区，具体原由 还望大牛能指出，==。
     return __result + (int) _S_extra;//最后返回相对于原始指针偏移8个字节的新指针
   }

   static void deallocate(void* __p, size_t __n)
   {
     char* __real_p = (char*)__p - (int) _S_extra;//获得原始指针
     assert(*(size_t*)__real_p == __n);//这里增加了一个断言，防止析构了被破坏的指针
     _Alloc::deallocate(__real_p, __n + (int) _S_extra);
   }

   static void* reallocate(void* __p, size_t __old_sz, size_t __new_sz)
   {
     char* __real_p = (char*)__p - (int) _S_extra;
     assert(*(size_t*)__real_p == __old_sz);
     char* __result = (char*)
       _Alloc::reallocate(__real_p, __old_sz + (int) _S_extra,
                                    __new_sz + (int) _S_extra);
     *(size_t*)__result = __new_sz;
     return __result + (int) _S_extra;
   }

 };

再来看看 gcc 下，其实也有类似的设计：

 #if(defined(_X86_) && !defined(__x86_64))
 #define _ALLOCA_S_MARKER_SIZE 8
 #elif defined(__ia64__) || defined(__x86_64)
 #define _ALLOCA_S_MARKER_SIZE 16
 #endif

 #if !defined(RC_INVOKED)
   static __inline void *_MarkAllocaS(void *_Ptr,unsigned int _Marker) {
     if(_Ptr) {
       *((unsigned int*)_Ptr) = _Marker;
       _Ptr = (char*)_Ptr + _ALLOCA_S_MARKER_SIZE;
     }
     return _Ptr;
   }
 #endif

 #ifndef RC_INVOKED
 #undef _freea
   static __inline void __cdecl _freea(void *_Memory) {
     unsigned int _Marker;
     if(_Memory) {
       _Memory = (char*)_Memory - _ALLOCA_S_MARKER_SIZE;
       _Marker = *(unsigned int *)_Memory;
       if(_Marker==_ALLOCA_S_HEAP_MARKER) {
     free(_Memory);
       }
 #ifdef _ASSERTE
       else if(_Marker!=_ALLOCA_S_STACK_MARKER) {
     _ASSERTE(("Corrupted pointer passed to _freea",));
       }
 #endif
     }
   }
 #endif /* RC_INVOKED */

其实，很多在实际写代码中困惑我们的问题，都可以通过 阅读相关源代码来得到 答案。

所以，经常阅读那些开源代码，还是相当有好处的 :)