Apparently I'm on a quest to mislead you.
I did some stepping through code and it's the growable memfiles that use
direct buffer access, not the user-supplied buffer, non growable ones, as
you saw in the documentation.
Sorry about that!
Anyway, CArchive doesn't wipe out the buffer passed to CMemFile -
Carchive creates a buffer to read from the CMemFile with FillBuffer()
(happens on the first read, not in the constructor).
The archive doesn't get unbuffered (direct) access to the memfile data, but
the data is still there (not wiped out).
Also, the example CArchive constructor params I gave you isn't a good idea
since CArchive's default minimum buffer size generally isn't very
appropriate (128 bytes).
Again, sorry for the confusion!
Cheers,
Mark
--
Mark Salsbery
Microsoft MVP - Visual C++
"XirisJohn" <XirisJohn@discussions.microsoft.com> wrote in message
news: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