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：
1 #include<cstdio>
2 #include<iostream>
3 #include<malloc.h>
4 #include<assert.h>
5 #include<ctime>
6 using namespace std;
7
8 #define size 16
9
10 int main(void)
11 {
12 void * p = NULL;
13 srand(time(0));
14 int a = 10;
15 while (a--)
16 {
17 int n = rand() % 10000;
18 p = malloc(n);
19 size_t w = *((size_t*)((char*)p - size));
20 cout << "w=" << w << endl;
21 cout << "n=" << n << endl;
22 assert(w == n);
23 free(p);
24 }
25 return 0;
26 }
（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：
1 //这儿是根据不同的硬件平台的宏定义
2 #if defined (_M_IX86)
3 #define _ALLOCA_S_MARKER_SIZE 8
4 #elif defined (_M_X64)
5 #define _ALLOCA_S_MARKER_SIZE 16
6 #elif defined (_M_ARM)
7 #define _ALLOCA_S_MARKER_SIZE 8
8 #elif !defined (RC_INVOKED)
9 #error Unsupported target platform.
10 #endif /* !defined (RC_INVOKED) */
11
12 _STATIC_ASSERT(sizeof(unsigned int) <= _ALLOCA_S_MARKER_SIZE);
13
14 #if !defined (__midl) && !defined (RC_INVOKED)
15 #pragma warning(push)
16 #pragma warning(disable:6540)
17 __inline void *_MarkAllocaS(_Out_opt_ __crt_typefix(unsigned int*) void *_Ptr, unsigned int _Marker)
18 {
19 if (_Ptr)
20 {
21 *((unsigned int*)_Ptr) = _Marker;
22 //
23 _Ptr = (char*)_Ptr + _ALLOCA_S_MARKER_SIZE;
24 //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。
25 }
26 return _Ptr;
27 }
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：
1 /* _freea must be in the header so that its allocator matches _malloca */
2 #if !defined (__midl) && !defined (RC_INVOKED)
3 #if !(defined (_DEBUG) && defined (_CRTDBG_MAP_ALLOC))
4 #undef _freea
5 __pragma(warning(push))
6 __pragma(warning(disable: 6014))
7 _CRTNOALIAS __inline void __CRTDECL _freea(_Pre_maybenull_ _Post_invalid_ void * _Memory)
8 {
9 unsigned int _Marker;
10 if (_Memory)
11 {
12 _Memory = (char*)_Memory - _ALLOCA_S_MARKER_SIZE;
13 //获得原始指针
14 _Marker = *(unsigned int *)_Memory;//得到指针所指区块的大小
15 if (_Marker == _ALLOCA_S_HEAP_MARKER)
16 {
17 free(_Memory);
18 }
19 #if defined (_ASSERTE)
20 else if (_Marker != _ALLOCA_S_STACK_MARKER)
21 {
22 #pragma warning(suppress: 4548) /* expression before comma has no effect */
23 _ASSERTE(("Corrupted pointer passed to _freea", 0));
24 }
25 #endif /* defined (_ASSERTE) */
26 }
27 }
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：
1 // Allocator adaptor to check size arguments for debugging.
2 // Reports errors using assert. Checking can be disabled with
3 // NDEBUG, but it‘s far better to just use the underlying allocator
4 // instead when no checking is desired.
5 // There is some evidence that this can confuse Purify.
6 template <class _Alloc>
7 class debug_alloc {
8
9 private:
10
11 enum {_S_extra = 8}; // Size of space used to store size. Note
12 // that this must be large enough to preserve
13 // alignment.
14
15 //这儿就像它所说的那样
16 public:
17
18 static void* allocate(size_t __n)
19 {
20 //
21 这里实际申请的内存大小要多 8 个字节
22 char* __result = (char*)_Alloc::allocate(__n + (int) _S_extra);
23 *(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，==。
24 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
25 }
26
27 static void deallocate(void* __p, size_t __n)
28 {
29 char* __real_p = (char*)__p - (int) _S_extra;//获得原始指针
30 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
31 _Alloc::deallocate(__real_p, __n + (int) _S_extra);
32 }
33
34 static void* reallocate(void* __p, size_t __old_sz, size_t __new_sz)
35 {
36 char* __real_p = (char*)__p - (int) _S_extra;
37 assert(*(size_t*)__real_p == __old_sz);
38 char* __result = (char*)
39 _Alloc::reallocate(__real_p, __old_sz + (int) _S_extra,
40 __new_sz + (int) _S_extra);
41 *(size_t*)__result = __new_sz;
42 return __result + (int) _S_extra;
43 }
44
45 };
再来看看 gcc 下，其实也有类似的设计：
1 #if(defined(_X86_) && !defined(__x86_64))
2 #define _ALLOCA_S_MARKER_SIZE 8
3 #elif defined(__ia64__) || defined(__x86_64)
4 #define _ALLOCA_S_MARKER_SIZE 16
5 #endif
6
7 #if !defined(RC_INVOKED)
8 static __inline void *_MarkAllocaS(void *_Ptr,unsigned int _Marker) {
9 if(_Ptr) {
10 *((unsigned int*)_Ptr) = _Marker;
11 _Ptr = (char*)_Ptr + _ALLOCA_S_MARKER_SIZE;
12 }
13 return _Ptr;
14 }
15 #endif
1 #ifndef RC_INVOKED
2 #undef _freea
3 static __inline void __cdecl _freea(void *_Memory) {
4 unsigned int _Marker;
5 if(_Memory) {
6 _Memory = (char*)_Memory - _ALLOCA_S_MARKER_SIZE;
7 _Marker = *(unsigned int *)_Memory;
8 if(_Marker==_ALLOCA_S_HEAP_MARKER) {
9 free(_Memory);
10 }
11 #ifdef _ASSERTE
12 else if(_Marker!=_ALLOCA_S_STACK_MARKER) {
13 _ASSERTE(("Corrupted pointer passed to _freea",0));
14 }
15 #endif
16 }
17 }
18 #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 :)
时间： 2024-08-05 07:08:42