Saving Bitmap image data to HGLOBAL [modified]

[Mark Salsbery]

Oops wrong constructor :)

unsigned char *pMyGlobalMemory = (unsigned char *)GlobalLock(MyHGlobal);
UnmanagedMemoryStream^ ms = gcnew UnmanagedMemoryStream(pMyGlobalMemory, GlobalSize(MyHGlobal));

Note you're still resonsible for making sure the allocated global memory is large enough to write to. GlobalSize() will give you the allocated size, but if you don't know how many bytes will be written, you'll need to write to a growable memory stream first, check how many bytes were written, compare it to the globally allocated size, and reallocate if you need more room.

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

I made a small change: void* pMyGlobal = GlobalLock(MyHGlobal); UnmanagedMemoryStream^ ms = gcnew UnmanagedMemoryStream((unsigned char*)(pMyGlobal), GlobalSize(gPtrImage)); MyImage->Save(ms, System::Drawing::Imaging::ImageFormat::Bmp); It builds but then I get ExternalException was unhandled in GDI+

[Mark Salsbery]

alleyes wrote:

It builds but then I get ExternalException was unhandled in GDI+

I can't debug that for you from here, but I suspect not enough allocated on the HGLOBAL. So maybe something like this:

MemoryStream^ ms = gcnew MemoryStream();
MyImage->Save(ms, System::Drawing::Imaging::ImageFormat::Bmp);
if (GlobalSize(MyHGlobal) < ms->Length)
{
    GlobalRealloc(MyHGlobal, ms->Length, 0);
}
void \*pMyGlobalMemory = GlobalLock(MyHGlobal);
UnmanagedMemoryStream^ ums = gcnew UnmanagedMemoryStream((unsigned char \*)pMyGlobalMemory, GlobalSize(MyHGlobal));
ms->WriteTo(ums);
GlobalUnlock(MyHGlobal);

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

Mark Salsbery wrote:

I can't debug that for you from here

No of course not. I was suspecting perhaps an image already in memory was the issue and maybe that needed a dispose method called, not sure. Thanks for the re-size tip :-D

[alleyes 0]

NotSupportedException unhandled. Stream does not support writing? Since when? :confused:

[Mark Salsbery]

Sorry try another constructor: ...gcnew UnmanagedMemoryStream((unsigned char *)pMyGlobalMemory, GlobalSize(MyHGlobal), GlobalSize(MyHGlobal), FileAccess.ReadWrite);

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

Ugh! Still getting an AccessViolation Exception. That can either mean not enough memory available or is that this memory can't be written to. I see the CanWrite, CanSeek flags are TRUE. Capacity and Length are both the same - 1658934

modified on Thursday, January 21, 2010 9:22 AM

[Mark Salsbery]

Did you use FileAccess.ReadWrite? I'm only guessing here since I don't know what line the exception occurs on...

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

Hi, This is what I have so far:

MemoryStream^ ms = gcnew MemoryStream();
MyImage->Save(ms, System::Drawing::Imaging::ImageFormat::Bmp);
ms->Seek(0, SeekOrigin::Begin);

if (GlobalSize(MyGlobal) < ms->Length)
{
GlobalReAlloc(MyGlobal, (LONG)ms->Length, 0);
}

void* pMyGlobal = (UCHAR*)MyGlobal;
UnmanagedMemoryStream^ ums = gcnew UnmanagedMemoryStream((UCHAR*)pMyGlobal, GlobalSize(MyGlobal), GlobalSize(MyGlobal), FileAccess::ReadWrite);
ums->Position = 0;
ms->WriteTo(ums);

It occurs on the line: ms->WriteTo(ums); This is even without setting the Position of the streams. I did this because just before the Call to WriteTo, the position was set to Length.

[Mark Salsbery]

Where did this come from? void* pMyGlobal = (UCHAR*)MyGlobal; Casting a handle to a pointer is bad. For an HGLOBAL it's bad unless you know for sure GMEM_FIXED was used to allocate the memory. Much safer to use GlobalLock to get the pointer.

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

Sorry I threw you there. The lock is already done earlier during the creating of the original images. Didn't make sense to call another GlobalLock on it again. The call to GlobalAlloc is GlobalAlloc(GMEM_MOVEABLE, Size) You have to call GlobalLock on the existing memory again?

[Mark Salsbery]

alleyes wrote:

The lock is already done earlier during the creating of the original images. Didn't make sense to call another GlobalLock on it again. The call to GlobalAlloc is GlobalAlloc(GMEM_MOVEABLE, Size)

Then you need to use the pointer returned by the GlobalLock() call.

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

alleyes wrote:

Didn't make sense to call another GlobalLock on it again.

There's nothing wrong with locking again as long as you have an equal number of unlock calls. GlobalRealloc on a locked handle can only reallocate memory in the same place so has more chance of failing, since there may not be enough room in that place. For best results the handle should be unlocked before reallocating then locked again when you need the pointer.

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

Sincere thanks Mark :-D When I have the time, I think I'll do all this in managed code by Marshalling the native HGLOBAL. But for now it works well.