Re: CMemFile::GetBufferPtr

From:

=?Utf-8?B?WGlyaXNKb2hu?= <XirisJohn@discussions.microsoft.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 10 Sep 2007 14:22:08 -0700

Message-ID:

<B2478DF6-F9A0-493E-A5FE-EB10F8366E0B@microsoft.com>

Thanks Mark!

That certainly works, but it seems like a workaround to me. The CMemFile
will still report that it does not support direct buffering, and thus there
will be an extra memmove_s in CArchive::FillBuffer. Not a big deal, perhaps,
but I'm intrigued to know if there is a better way :-)

John

"Mark Salsbery [MVP]" wrote:

Instead of

CArchive archive(&buffer, CArchive::load);

try

CArchive archive(&buffer, CArchive::load, 0, NULL);

This is the only thing I do different than your code....I just can't
remember why I had to do this, but the problem was similar :)

Mark

--
Mark Salsbery
Microsoft MVP - Visual C++

"XirisJohn" <XirisJohn@discussions.microsoft.com> wrote in message
news:096E7ECA-B267-46B4-8038-1D6A01332657@microsoft.com...

There has been a change in CMemFile::GetBufferPtr from Studio 6 to Studio
8.

Studio 6:

if (nCommand == bufferCheck)
{
return 1;

Studio 8:

if (nCommand == bufferCheck)
{
// only allow direct buffering if we're
// growable
if (m_nGrowBytes > 0)
return bufferDirect;
else
return 0;

This appears to have broken some existing code of mine and I'm trying to
figure out why.

My code (actually somebody else's who is no longer with the company)
pretty
much looks like this:

Saving:
CMemFile buffer;
CArchive archive(&buffer, CArchive::store);
object-->SaveToArchive(archive);
buffer.SeekToBegin();
buffer.Read( // into some static memory...

Loading:
BYTE *pBuffer;
pBuffer = // the static memory...
CMemFile buffer((LPBYTE) pBuffer, bufferSize);
CArchive archive(&buffer, CArchive::load);
object-->LoadFromArchive(archive);

The default value of nGrowSize ( the omitted value in the CMemFile::ctor)
is
zero. The documentation states (and the code supports the statement) that
if
you use a nGrowSize > 0 that CMemFile won't use the memory that you pass
in.
So, therefore, nGrowSize must be zero.

Because I want CArchive to use the direct memory access mode. As near as
I
can tell, CArchive calls CMemFile::GetBufferPtr to determine if direct
access
is supported:

m_bDirectBuffer =
m_pFile->GetBufferPtr(CFile::bufferCheck)&CFile::bufferDirect;
if (!m_bDirectBuffer)
{
// no support for direct buffering, allocate new buffer
m_lpBufStart = new BYTE[m_nBufSize];
m_bUserBuf = FALSE;
}
etc.

So, under the new MFC, GetBufferPtr returns 0, and my data is now wiped
out.
Under the old MFC, GetBuffferPtr returns 1 and I can use my data via
FillBuffer.

Am I missing something here?

Thanks in advance,
John