malloc函数实现过程

// malloc函数原型
// void *malloc( size_t size );//（MSDN中的定义）
type* temp=(type*)malloc(sizeof(type*n));
// type表示数据类型；
// n表示需要创建几个该类型的成员
// sizeof(type*n)表示成员所占空间

// 该段代码拷贝于vs2015的malloc.h
// malloc.h
#pragma once
#define _INC_MALLOC

#include <corecrt.h>
#include <corecrt_malloc.h>

_CRT_BEGIN_C_HEADER

// 查看是win32还是win64的平台

#ifdef _WIN64
    #define _HEAP_MAXREQ 0xFFFFFFFFFFFFFFE0
#else
    #define _HEAP_MAXREQ 0xFFFFFFE0
#endif

// 定义相关的宏常量
#define _HEAPEMPTY (-1)//堆空
#define _HEAPOK (-2)
#define _HEAPBADBEGIN (-3)
#define _HEAPBADNODE (-4)
#define _HEAPEND (-5)//堆尾
#define _HEAPBADPTR (-6)
#define _FREEENTRY 0
#define _USEDENTRY 1

//定义一个结构体
typedef struct _heapinfo
{
    int* _pentry;
    size_t _size;
    int _useflag;
} _HEAPINFO;

//定义相关宏
#define _mm_free(a) _aligned_free(a)
#define _mm_malloc(a, b) _aligned_malloc(a, b)

// 未完待续。。。

_Ret_notnull_ _Post_writable_byte_size_(_Size)
void* __cdecl _alloca(_In_ size_t _Size);



#if !defined __midl && !defined RC_INVOKED

    _ACRTIMP intptr_t __cdecl _get_heap_handle(void);

    _Check_return_
    _DCRTIMP int __cdecl _heapmin(void);

    #if defined _DEBUG || defined _CRT_USE_WINAPI_FAMILY_DESKTOP_APP || defined _CORECRT_BUILD
        _ACRTIMP int __cdecl _heapwalk(_Inout_ _HEAPINFO* _EntryInfo);
    #endif

    #ifdef _CRT_USE_WINAPI_FAMILY_DESKTOP_APP
        _Check_return_ _DCRTIMP int __cdecl _heapchk(void);
        _DCRTIMP int __cdecl _resetstkoflw(void);
    #endif

    #define _ALLOCA_S_THRESHOLD 1024
    #define _ALLOCA_S_STACK_MARKER 0xCCCC
    #define _ALLOCA_S_HEAP_MARKER 0xDDDD

    #ifdef _WIN64
        #define _ALLOCA_S_MARKER_SIZE 16
    #else
        #define _ALLOCA_S_MARKER_SIZE 8
    #endif

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


    #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;
        }
        return _Ptr;
    }

    __inline size_t _MallocaComputeSize(size_t _Size)
    {
        size_t _MarkedSize = _Size + _ALLOCA_S_MARKER_SIZE;
        return _MarkedSize > _Size ? _MarkedSize : 0;
    }

    #pragma warning(pop)

#endif



#ifdef _DEBUG

    #ifndef _CRTDBG_MAP_ALLOC
        #undef _malloca
        #define _malloca(size) \
            __pragma(warning(suppress: 6255 6386))                                       \
            (_MallocaComputeSize(size) != 0                                              \
                ? _MarkAllocaS(malloc(_MallocaComputeSize(size)), _ALLOCA_S_HEAP_MARKER) \
                : NULL)
    #endif

#else

    #undef _malloca
    #define _malloca(size) \
        __pragma(warning(suppress: 6255 6386))                                             \
        (_MallocaComputeSize(size) != 0                                                    \
            ? (((_MallocaComputeSize(size) <= _ALLOCA_S_THRESHOLD)                         \
                ? _MarkAllocaS(_alloca(_MallocaComputeSize(size)), _ALLOCA_S_STACK_MARKER) \
                : _MarkAllocaS(malloc(_MallocaComputeSize(size)), _ALLOCA_S_HEAP_MARKER))) \
            : NULL)

#endif



#if defined __midl && !defined RC_INVOKED
#elif defined _DEBUG && defined _CRTDBG_MAP_ALLOC
#else

    #undef _freea

    #pragma warning(push)
    #pragma warning(disable: 6014)
    __inline void __CRTDECL _freea(_Pre_maybenull_ _Post_invalid_ 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", 0));
            }
            #endif
        }
    }
    #pragma warning(pop)

#endif



#if !__STDC__
    #define alloca _alloca
#endif



_CRT_END_C_HEADER