5 files changed, 63 insertions, 8 deletions
diff --git a/stdwndclass_windows.go b/stdwndclass_windows.go
index d57cc20..44af126 100644
--- a/stdwndclass_windows.go
+++ b/stdwndclass_windows.go
@@ -31,8 +31,53 @@ func defWindowProc(hwnd _HWND, uMsg uint32, wParam _WPARAM, lParam _LPARAM) _LRE
 	return _LRESULT(r1)
 }
 
-func stdWndProc(s *sysData) func(hwnd _HWND, uMsg uint32, wParam _WPARAM, lParam _LPARAM) _LRESULT {
+// don't worry about error returns from GetWindowLongPtr()/SetWindowLongPtr()
+// see comments of http://blogs.msdn.com/b/oldnewthing/archive/2014/02/03/10496248.aspx
+
+func getWindowLongPtr(hwnd _HWND, what uintptr) uintptr {
+	r1, _, _ := _getWindowLongPtr.Call(
+		uintptr(hwnd),
+		what)
+	return r1
+}
+
+func setWindowLongPtr(hwnd _HWND, what uintptr, value uintptr) {
+	_setWindowLongPtr.Call(
+		uintptr(hwnd),
+		what,
+		value)
+}
+
+// we can store a pointer in extra space provided by Windows
+// we'll store sysData there
+// see http://blogs.msdn.com/b/oldnewthing/archive/2005/03/03/384285.aspx
+func getSysData(hwnd _HWND) *sysData {
+	return (*sysData)(unsafe.Pointer(getWindowLongPtr(hwnd, negConst(_GWLP_USERDATA))))
+}
+
+func storeSysData(hwnd _HWND, uMsg uint32, wParam _WPARAM, lParam _LPARAM) _LRESULT {
+	// we can get the lpParam from CreateWindowEx() in WM_NCCREATE and WM_CREATE
+	// we can freely skip any messages that come prior
+	// see http://blogs.msdn.com/b/oldnewthing/archive/2005/04/22/410773.aspx and http://blogs.msdn.com/b/oldnewthing/archive/2014/02/03/10496248.aspx (note the date on the latter one!)
+	if uMsg == _WM_NCCREATE {
+		// the lpParam to CreateWindowEx() is the first uintptr of the CREATESTRUCT
+		// so rather than create that whole structure, we'll just grab the uintptr at the address pointed to by lParam
+		cs := (*uintptr)(unsafe.Pointer(lParam))
+		saddr := *cs
+		setWindowLongPtr(hwnd, negConst(_GWLP_USERDATA), saddr)
+		// don't set s; we return here
+	}
+	// TODO is this correct for WM_NCCREATE? I think the above link does it but I'm not entirely sure...
+	return defWindowProc(hwnd, uMsg, wParam, lParam)
+}
+
+func stdWndProc(unused *sysData) func(hwnd _HWND, uMsg uint32, wParam _WPARAM, lParam _LPARAM) _LRESULT {
 	return func(hwnd _HWND, uMsg uint32, wParam _WPARAM, lParam _LPARAM) _LRESULT {
+		s := getSysData(hwnd)
+		if s == nil {		// not yet saved
+			return storeSysData(hwnd, uMsg, wParam, lParam)
+		}
+
 		switch uMsg {
 		case _WM_COMMAND:
 			id := _HMENU(wParam.LOWORD())
diff --git a/sysdata_windows.go b/sysdata_windows.go
index b9df277..f813de3 100644
--- a/sysdata_windows.go
+++ b/sysdata_windows.go
@@ -30,6 +30,7 @@ type classData struct {
 	xstyle			uint32
 	mkid				bool
 	altStyle			uint32
+	storeSysData		bool
 	doNotLoadFont	bool
 	appendMsg		uintptr
 	insertBeforeMsg	uintptr
@@ -48,6 +49,7 @@ var classTypes = [nctypes]*classData{
 		register:			registerStdWndClass,
 		style:			_WS_OVERLAPPEDWINDOW,
 		xstyle:			0,
+		storeSysData:		true,
 		doNotLoadFont:	true,
 	},
 	c_button:			&classData{
@@ -157,6 +159,10 @@ func (s *sysData) make(window *sysData) (err error) {
 	if s.alternate {
 		style = uintptr(ct.altStyle)
 	}
+	lpParam := uintptr(_NULL)
+	if ct.storeSysData {
+		lpParam = uintptr(unsafe.Pointer(s))
+	}
 	uitask <- &uimsg{
 		call:		_createWindowEx,	
 		p:		[]uintptr{
@@ -171,7 +177,7 @@ func (s *sysData) make(window *sysData) (err error) {
 			pwin,
 			uintptr(cid),
 			uintptr(hInstance),
-			uintptr(_NULL),
+			lpParam,
 		},
 		ret:	ret,
 	}
diff --git a/tools/windowsconstgen.go b/tools/windowsconstgen.go
index 6be1b59..777324d 100644
--- a/tools/windowsconstgen.go
+++ b/tools/windowsconstgen.go
@@ -110,8 +110,8 @@ func preamble(pkg string) string {
 // for backwards compatibiilty reasons, Windows defines GetWindowLongPtr()/SetWindowLongPtr() as a macro which expands to GetWindowLong()/SetWindowLong() on 32-bit systems
 // we'll just simulate that here
 var gwlpNames = map[string]string{
-	"386":		"etWindowLong",
-	"amd64":		"etWindowLongPtr",
+	"386":		"etWindowLongW",
+	"amd64":		"etWindowLongPtrW",
 }
 
 func printConst(f *os.File, goconst string, winconst string) {
diff --git a/zconstants_windows_386.go b/zconstants_windows_386.go
index c1d88c2..fa0ab76 100644
--- a/zconstants_windows_386.go
+++ b/zconstants_windows_386.go
@@ -28,6 +28,7 @@ const _ERROR = 0
 const _ES_AUTOHSCROLL = 128
 const _ES_PASSWORD = 32
 const _FALSE = 0
+const _GWLP_USERDATA = -21
 const _GWL_STYLE = -16
 const _ICC_PROGRESS_CLASS = 32
 const _LBS_EXTENDEDSEL = 2048
@@ -140,6 +141,7 @@ const _WM_MBUTTONDOWN = 519
 const _WM_MBUTTONUP = 520
 const _WM_MOUSEACTIVATE = 33
 const _WM_MOUSEMOVE = 512
+const _WM_NCCREATE = 129
 const _WM_PAINT = 15
 const _WM_RBUTTONDOWN = 516
 const _WM_RBUTTONUP = 517
@@ -162,5 +164,5 @@ const _IDC_ARROW = 32512
 const _IDI_APPLICATION = 32512
 const _INVALID_HANDLE_VALUE = 4294967295
 const _NULL = 0
-var _getWindowLongPtr = user32.NewProc("GetWindowLong")
-var _setWindowLongPtr = user32.NewProc("SetWindowLong")
+var _getWindowLongPtr = user32.NewProc("GetWindowLongW")
+var _setWindowLongPtr = user32.NewProc("SetWindowLongW")
diff --git a/zconstants_windows_amd64.go b/zconstants_windows_amd64.go
index 4d3c1c3..54bbae8 100644
--- a/zconstants_windows_amd64.go
+++ b/zconstants_windows_amd64.go
@@ -28,6 +28,7 @@ const _ERROR = 0
 const _ES_AUTOHSCROLL = 128
 const _ES_PASSWORD = 32
 const _FALSE = 0
+const _GWLP_USERDATA = -21
 const _GWL_STYLE = -16
 const _ICC_PROGRESS_CLASS = 32
 const _LBS_EXTENDEDSEL = 2048
@@ -140,6 +141,7 @@ const _WM_MBUTTONDOWN = 519
 const _WM_MBUTTONUP = 520
 const _WM_MOUSEACTIVATE = 33
 const _WM_MOUSEMOVE = 512
+const _WM_NCCREATE = 129
 const _WM_PAINT = 15
 const _WM_RBUTTONDOWN = 516
 const _WM_RBUTTONUP = 517
@@ -162,5 +164,5 @@ const _IDC_ARROW = 32512
 const _IDI_APPLICATION = 32512
 const _INVALID_HANDLE_VALUE = 18446744073709551615
 const _NULL = 0
-var _getWindowLongPtr = user32.NewProc("GetWindowLongPtr")
-var _setWindowLongPtr = user32.NewProc("SetWindowLongPtr")
+var _getWindowLongPtr = user32.NewProc("GetWindowLongPtrW")
+var _setWindowLongPtr = user32.NewProc("SetWindowLongPtrW")