Why does a call to GetDIBits fail on Win64? - delphi

Question

I have a call to GetDIBits that works perfectly in 32-bit, but fails on 64-bit. Despite the different values for the handles the content of the bitmapinfo structure are the same.
Here is the smallest (at least slightly structured) code example I could come up with to reproduce the error. I tested with Delphi 10 Seattle Update 1, but the error seems to occur even with other Delphi versions.
program Project1;
{$APPTYPE CONSOLE}
{$R *.res}
uses
Winapi.Windows,
System.SysUtils,
Vcl.Graphics;
type
TRGBALine = array[Word] of TRGBQuad;
PRGBALine = ^TRGBALine;
type
{ same structure as TBitmapInfo, but adds space for two more entries in bmiColors }
TMyBitmapInfo = record
bmiHeader: TBitmapInfoHeader;
bmiColors: array[0..2] of TRGBQuad;
public
constructor Create(AWidth, AHeight: Integer);
end;
constructor TMyBitmapInfo.Create(AWidth, AHeight: Integer);
begin
FillChar(bmiHeader, Sizeof(bmiHeader), 0);
bmiHeader.biSize := SizeOf(bmiHeader);
bmiHeader.biWidth := AWidth;
bmiHeader.biHeight := -AHeight; //Otherwise the image is upside down.
bmiHeader.biPlanes := 1;
bmiHeader.biBitCount := 32;
bmiHeader.biCompression := BI_BITFIELDS;
bmiHeader.biSizeImage := 4*AWidth*AHeight; // 4 = 32 Bits/Pixel div 8 Bits/Byte
bmiColors[0].rgbRed := 255;
bmiColors[1].rgbGreen := 255;
bmiColors[2].rgbBlue := 255;
end;
procedure Main;
var
bitmap: TBitmap;
res: Cardinal;
Bits: PRGBALine;
buffer: TMyBitmapInfo;
BitmapInfo: TBitmapInfo absolute buffer;
BitsSize: Cardinal;
icon: TIcon;
IconInfo: TIconInfo;
begin
bitmap := TBitmap.Create;
try
icon := TIcon.Create;
try
icon.LoadFromResourceID(0, Integer(IDI_WINLOGO));
if not GetIconInfo(icon.Handle, IconInfo) then begin
Writeln('Error GetIconInfo: ', GetLastError);
Exit;
end;
bitmap.PixelFormat := pf32bit;
bitmap.Handle := IconInfo.hbmColor;
BitsSize := BytesPerScanline(bitmap.Width, 32, 32) * bitmap.Height;
Bits := AllocMem(BitsSize);
try
ZeroMemory(Bits, BitsSize);
buffer := TMyBitmapInfo.Create(bitmap.Width, bitmap.Height);
res := GetDIBits(bitmap.Canvas.Handle, bitmap.Handle, 0, bitmap.Height, Bits, BitmapInfo, DIB_RGB_COLORS);
if res = 0 then begin
Writeln('Error GetDIBits: ', GetLastError);
Exit;
end;
Writeln('Succeed');
finally
FreeMem(Bits);
end;
finally
icon.Free;
end;
finally
bitmap.Free;
end;
end;
begin
try
Main;
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
Readln;
end.

Answer 1

Update A comment to this answer points the way as to why your code is failing. The order of evaluation of bitmap.Handle and bitmap.Canvas.Handle matters. Since parameter evaluation order is undefined, your program has undefined behaviour. And that explains why the x86 and x64 programs differ in behaviour.
So you could resolve the issue by assigning the bitmap handle and device context to local variables in the appropriate order, and then passing these as the arguments to GetDIBits. But I still think that the code is far better to avoid the VCL TBitmap class and use GDI calls directly, as in the code below.
I believe that your mistake is to pass the bitmap handle, and its canvas handle. Instead you should pass, for example, a device context obtained by calling CreateCompatibleDC(0). Or pass IconInfo.hbmColor to GetDIBits. But don't pass the handle of the TBitmap and the handle of its canvas.
I also cannot see any purpose to the TBitmap that you create. All you do with it is obtain the width and height of IconInfo.hbmColor. You don't need to create TBitmap to do that.
So if I were you I would remove the TBitmap, and use CreateCompatibleDC(0) to obtain the device context. This should greatly simplify the code.
You will also need to delete the bitmaps returned by the call to GetIconInfo, but I guess that you know this already and removed that code from the question for simplicity.
Frankly, the VCL objects are just getting in the way here. It's actually much simpler to call the GDI functions directly. Perhaps something like this:
procedure Main;
var
res: Cardinal;
Bits: PRGBALine;
bitmap: Winapi.Windows.TBitmap;
DC: HDC;
buffer: TMyBitmapInfo;
BitmapInfo: TBitmapInfo absolute buffer;
BitsSize: Cardinal;
IconInfo: TIconInfo;
begin
if not GetIconInfo(LoadIcon(0, IDI_WINLOGO), IconInfo) then begin
Writeln('Error GetIconInfo: ', GetLastError);
Exit;
end;
try
if GetObject(IconInfo.hbmColor, SizeOf(bitmap), #bitmap) = 0 then begin
Writeln('Error GetObject');
Exit;
end;
BitsSize := BytesPerScanline(bitmap.bmWidth, 32, 32) * abs(bitmap.bmHeight);
Bits := AllocMem(BitsSize);
try
buffer := TMyBitmapInfo.Create(bitmap.bmWidth, abs(bitmap.bmHeight));
DC := CreateCompatibleDC(0);
res := GetDIBits(DC, IconInfo.hbmColor, 0, abs(bitmap.bmHeight), Bits, BitmapInfo,
DIB_RGB_COLORS);
DeleteDC(DC);
if res = 0 then begin
Writeln('Error GetDIBits: ', GetLastError);
Exit;
end;
Writeln('Succeed');
finally
FreeMem(Bits);
end;
finally
DeleteObject(IconInfo.hbmMask);
DeleteObject(IconInfo.hbmColor);
end;
end;