I ran an overnight stress test of my application, and when i came in this morning the software had crashed from a Windows error.

The error occurred because the process was obviously out of GDI handles:

Process Explorer

Task Manager

The next thing is to figure out exactly which kind of GDI resource (e.g. Pen, Brush, Bitmap, Font, Region, DC) i'm leaking. For that i turned to NirSoft's GDIView:

• Pen: 0
• Ext Pen: 0
• Brush: 4
• Bitmap: 35
• Font: 19
• Palette: 1
• Region: 3
• DC: 11
• Metafile DC: 0
• Enhanced Metafile DC: 0
• Other GDI: 0
• GDI Total: 0
• All GDI: 10,000

What could a GDI handle be, one that is not any known GDI type?

The answer was GDI HFONT handles.

It is a Windows 8 issue that GDIView cannot show the font handles.

I used hooking to intercept every call to:

• CreateFont
• DestroyFont

and logged every handle creation, along with its stack trace of when it was allowed. At the end i created a report of all undeleted HFONTs.

How did i do it?

I used the Detours library for Delphi.

Step 1 - For every GDI function there is that creates something, we ask Detours to intercept the function.

• We pass the address of our replacement function
• and it returns the address of the original function (so we can call it)

var
    CreateFontIndirectAOriginal: function (const p1: TLogFontA): HFONT; stdcall = nil;
    DeleteObjectOriginal: function (p1: HGDIOBJ): BOOL; stdcall = nil;

CreateFontIndirectAOriginal := InterceptCreate(@CreateFontIndirectA, @CreateFontIndirectAIntercept);
DeleteObjectOriginal := InterceptCreate(@DeleteObject, @DeleteObjectIntercept);

Step 2 - Declare our versions of the GDI functions:

function CreateFontIndirectAIntercept(const p1: TLogFontA): HFONT; stdcall;
begin
    Result := CreateFontIndirectAOriginal(p1);
end;

function DeleteObjectIntercept(p1: HGDIOBJ): BOOL; stdcall;
begin
    Result := DeleteObjectOriginal(p1);
end;

Step 3 - Add code to track every font created by CreateFont, and every font destruction by DestroyObject

function CreateFontIndirectAIntercept(const p1: TLogFontA): HFONT; stdcall;
begin
    Result := TrampolineCreateFontIndirectA(p1);
    GdiLeakTrackerSvc.AddFont(Result);
end;

function DeleteObjectIntercept(p1: HGDIOBJ): BOOL; stdcall;
var
    objType: DWORD;
begin
    objType := GetObjectType(p1);
    Result := TrampolineDeleteObject(p1);

    case objType of
    OBJ_FONT:  GdiLeakTrackerSvc.RemoveObject(p1);
    end;
end;

And then the GdiLeakTrackerSvc service tracks all font creations, font destructions, and can let us know during program shutdown if anything leaked.