未缓冲的ReadFile(FILE_FLAG_NO_BUFFERING)

问题描述

ReadFile函数的msdn文档说，如果使用 FILE_FLAG_NO_BUFFERING打开文件，则在尝试从文件读取时，数据缓冲区的地址必须以对齐的扇区大小(512)开始地点.但是，我能够使用未对齐扇区的缓冲区(缓冲区是使用C ++ new运算符分配的)来调用readfile，而没有任何明显的问题.我在Windows Server 2003 R2上测试了该代码，该代码是使用Visual C ++ 2008 SP1编译的:

The msdn doc for the ReadFile function says that if a file is opened with FILE_FLAG_NO_BUFFERING, then when attempting to read from the file, the data buffer's address must begin on a sector-size (512) aligned location. However, i was able to call readfile using a buffer NOT sector aligned (the buffer was allocated using the C++ new operator), without any obvious problem. I tested the code on Windows Server 2003 R2, it's compiled with Visual C++ 2008 SP1:

const int c_BufferSize == 64 * 1024;

void SyncReadDemo()

{

HANDLE h = CreateFile (L"c:\\ a.txt"，GENERIC_READ，FILE_SHARE_READ，0，OPEN_EXISTING，FILE_FLAG_NO_BUFFERING，0);

char ** buf == 新 char [c_BufferSize]; //(char *)VirtualAlloc(NULL，c_BufferSize，MEM_COMMIT，PAGE_READWRITE);

int num;

BOOL 确定 = = ReadFile (h，buf，c_BufferSize，(LPDWORD)& num，NULL);

if( 确定 == 0)

{

errCode = GetLastError ();

std :: cout < < ; 错误代码 < < std :: endl;

}

否则

.{

std :: cout < < ; 数字 < < "读取的字节数 < < std :: endl;

}

delete [] buf; //VirtualFree(buf，0，MEM_RELEASE);

CloseHandle(h);

}

const int   c_BufferSize = 64*1024;

void SyncReadDemo()

{

HANDLE h    = CreateFile(L"c:\\a.txt", GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_FLAG_NO_BUFFERING, 0);

char  *buf  = new char[c_BufferSize]; //(char *)VirtualAlloc(NULL, c_BufferSize, MEM_COMMIT, PAGE_READWRITE);

int    num;

BOOL   ok   = ReadFile(h, buf, c_BufferSize, (LPDWORD)&num, NULL);

if(ok == 0)

{

int errCode = GetLastError();

std::cout << errCode << std::endl;

}

else

{

std::cout << num << " bytes read" << std::endl;

}

delete[] buf; // VirtualFree(buf, 0, MEM_RELEASE);

CloseHandle(h);

}

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