关于ULARGE_INTEGER 在计算磁盘容量时候的运算

ULARGE_INTEGER Union

http://blog.csdn.net/erenno1/article/details/4672219

The ULARGE_INTEGER structure is used to specify a 64-bit unsigned integer value.

typedef union _ULARGE_INTEGER {  struct {    DWORD LowPart;    DWORD HighPart;  };  struct {    DWORD LowPart;    DWORD HighPart;  } u;  ULONGLONG QuadPart;} ULARGE_INTEGER,  *PULARGE_INTEGER;

Members

LowPart

Low-order 32 bits.

HighPart

High-order 32 bits.

u

LowPart

Low-order 32 bits.

HighPart

High-order 32 bits.

QuadPart

Unsigned 64-bit integer.

Remarks

The ULARGE_INTEGER structure is actually a union. If your compiler has built-in support for 64-bit integers, use theQuadPart member to store the 64-bit integer. Otherwise, use theLowPart and HighPart members to store the 64-bit integer.

Header

Declared in Winnt.h; include Windows.h

上面是MSDN上的说明！

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由于在32位PC上无法int最大值为2^32，所以，对于64位数的存放采取了这样一种数据结构，假如我们要对这个64位的数字进行运算的话应该怎么办呢？

例如 使用BOOL GetDiskFreeSpaceEx() 获得磁盘信息时候

PULARGE_INTEGER FreeBytesAvailableToCaller;

……

BOOL GetDiskFreeSpaceEx(
  LPCWSTR lpDirectoryName,
  PULARGE_INTEGER lpFreeBytesAvailableToCaller,
  PULARGE_INTEGER lpTotalNumberOfBytes,
  PULARGE_INTEGER lpTotalNumberOfFreeBytes
)

获取了磁盘空间之后，由于得到的值是以字节为单位的，这里，我们想将其转化为以G为单位的。。

方法1：

(float)FreeBytesAvailableToCaller。HighPart<<2+(float)FreeBytesAvailableToCaller。LowPart/(1024*1024*1024);

原理： 首先，在计算机中所有数字都是以二进制存储的，HighPart是高32位，LowPart是低32位，摒弃你脑袋里的10进制概念，将这两部分均以2进制的视角来看。。。1G是2^30字节，所以低32位再除以2^30后，低32位的高2位仍未改变（切记是二进制的视角）。。同理，高32位的每位在计算后也肯定不会改变，即30位以后的都不作改变，所以将30位之后的所有二进制位整合，就可以得到整个64位数除以2^30的二进制整数部分结果了，这里(float)FreeBytesAvailableToCaller。HighPart<<2即空出俩位与后32位的高2位整合（HighPart<<2可以用*4代替）。。。

方法2：(float)FreeBytesAvailableToCaller。.QuadPart/(1024*1024*1024)直接计算。。。

 

 

 

 

以下是我的实际代码:

 

 

void GetDiskFreeSpaceEx(LPCTSTR lpDirectoryName){BOOL fResult=::GetDiskFreeSpaceEx(lpDirectoryName,(PULARGE_INTEGER)&m_i64FreeBytesToCaller,(PULARGE_INTEGER)&m_i64TotalBytes,(PULARGE_INTEGER)&m_i64FreeBytes);if(fResult){m_i64FreeBytesToCaller=(float)(((PULARGE_INTEGER)&m_i64FreeBytesToCaller)->HighPart<<2)+(float)((PULARGE_INTEGER)&m_i64FreeBytesToCaller)->LowPart/(1024*1024*1024);m_i64TotalBytes=(float)(((PULARGE_INTEGER)&m_i64TotalBytes)->HighPart<<2)+(float)((PULARGE_INTEGER)&m_i64TotalBytes)->LowPart/(1024*1024*1024);cout<<"totalspace:"<<m_i64TotalBytes<<"GB"<<endl;//磁盘总容量cout<<"freespace:"<<m_i64FreeBytesToCaller<<"GB";//磁盘剩余容量}elsecout<<"设备为准备好";cout<<endl;}