关于CDib类的问题。

这是技术内幕里面的原码
BOOL CDib::Read(CFile* pFile)
{
// 1. read file header to get size of info hdr + color table
// 2. read info hdr (to get image size) and color table
// 3. read image
// can't use bfSize in file header
Empty();
int nCount, nSize;
BITMAPFILEHEADER bmfh;
try {
nCount = pFile->Read((LPVOID) &bmfh, sizeof(BITMAPFILEHEADER));
if(nCount != sizeof(BITMAPFILEHEADER)) {
throw new CException;
}
if(bmfh.bfType != 0x4d42) {
throw new CException;
}
nSize = bmfh.bfOffBits - sizeof(BITMAPFILEHEADER);
m_lpBMIH = (LPBITMAPINFOHEADER) new char[nSize];
m_nBmihAlloc = m_nImageAlloc = crtAlloc;
nCount = pFile->Read(m_lpBMIH, nSize); // info hdr & color table
ComputeMetrics();
ComputePaletteSize(m_lpBMIH->biBitCount);
MakePalette();
m_lpImage = (LPBYTE) new char[m_dwSizeImage];
nCount = pFile->Read(m_lpImage, m_dwSizeImage); // image only
}
catch(CException* pe) {
AfxMessageBox("Read error");
pe->Delete();
return FALSE;
}
return TRUE;
}



HBITMAP CDib::CreateSection(CDC* pDC /* = NULL */)
{
if(m_lpBMIH == NULL) return NULL;
if(m_lpImage != NULL) return NULL; // can only do this if image doesn't exist
m_hBitmap = ::CreateDIBSection(pDC->GetSafeHdc(), (LPBITMAPINFO) m_lpBMIH,
DIB_RGB_COLORS, (LPVOID*) &m_lpImage, NULL, 0);
ASSERT(m_lpImage != NULL);
return m_hBitmap;
}


LPBYTE m_lpImage; // starting address of DIB bits
=========================================================================
我用read打开一个位图，然后想用CreateSection得到其句柄，但是m_lpImage != NULL，CreateSection返回为空，为什么m_lpImage会非空？如果强制清空则程序错误，应该怎么解决？。
技术内幕上说CreateSection调用CreateDIBSection函数来创建一个DIB段（section），什么叫DIB段？