Add GetLastErrorCode() to expose errno-based failures

Author: sdottaka

src/WinIMerge/WinIMerge.cpp

@@ -154,6 +154,13 @@ void UpdateWindowTitle(HWND hWnd)
 		SetWindowTextW(hWnd, title);
 }
 
+void ShowErrorMessageBox(HWND hWnd, const std::string& msg)
+{
+	std::error_code ec(m_pImgMergeWindow->GetLastErrorCode(), std::generic_category());
+	std::string msg2 = msg + ": " + ec.message();
+	MessageBoxA(hWnd, msg2.c_str(), NULL, MB_OK | MB_ICONERROR);
+}
+
 bool NewImages(HWND hWnd, int nImages)
 {
 	bool bSucceeded;
@@ -173,6 +180,8 @@ bool OpenImages(HWND hWnd, int nImages, const std::wstring filename[3])
 		bSucceeded = m_pImgMergeWindow->OpenImages(filename[0].c_str(), filename[1].c_str(), filename[2].c_str());
 	if (bSucceeded)
 		UpdateWindowTitle(hWnd);
+	else
+		ShowErrorMessageBox(hWnd, "Failed to open files");
 	InvalidateRect(hWnd, NULL, TRUE);
 	return bSucceeded;
 }
@@ -199,9 +208,7 @@ void SaveImageAs(HWND hWnd, int pane)
 	if (GetSaveFileNameW(&ofn) != 0)
 	{
 		if (!m_pImgMergeWindow->SaveImageAs(pane, ofn.lpstrFile))
-		{
-			MessageBoxW(hWnd, L"Failed to save file", nullptr, MB_OK | MB_ICONERROR);
-		}
+			ShowErrorMessageBox(hWnd, "Failed to save file");
 	}
 }
 
@@ -830,7 +837,7 @@ LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
 		case ID_HELP_ABOUT:
 			MessageBoxW(hWnd, 
 				L"WinIMerge\n\n"
-				L"(c) 2014-2021 sdottaka@users.sourceforge.net All rights reserved.\n\n"
+				L"(c) 2014-2025 sdottaka@users.sourceforge.net All rights reserved.\n\n"
 				L"This software uses the FreeImage open source image library. \n"
 				L"See http://freeimage.sourceforge.net for details.\n"
 				L"FreeImage is used under the GNU GPL version.\n", L"WinIMerge", MB_OK | MB_ICONINFORMATION);

src/WinIMerge/WinIMerge.rc

@@ -328,7 +328,7 @@ BEGIN
             VALUE "FileDescription", "WinIMerge"
             VALUE "FileVersion", STRFILEVER
             VALUE "InternalName", "WinIMerge.exe"
-            VALUE "LegalCopyright", "Copyright (C) 2014-2021 sdottaka@users.sourceforge.net"
+            VALUE "LegalCopyright", "Copyright (C) 2014-2025 sdottaka@users.sourceforge.net"
             VALUE "OriginalFilename", "WinIMerge.exe"
             VALUE "ProductName", "WinIMerge"
             VALUE "ProductVersion", STRPRODUCTVER

src/WinIMergeLib/ImgDiffBuffer.hpp

@@ -518,6 +518,7 @@ class CImgDiffBuffer
 		, m_diffAlgorithm(MYERS_DIFF)
 		, m_blinkInterval(BLINK_INTERVAL)
 		, m_overlayAnimationInterval(OVERLAY_ANIMATION_INTERVAL)
+		, m_lastErrorCode(0)
 	{
 		for (int i = 0; i < 3; ++i)
 			m_currentPage[i] = 0;
@@ -1190,7 +1191,14 @@ class CImgDiffBuffer
 	{
 		if (pane < 0 || pane >= m_nImages)
 			return false;
-		return !!m_imgDiff[pane].save(filename);
+		int savedErrno = errno;
+		errno = 0;
+		bool result = !!m_imgDiff[pane].save(filename);
+		if (!result)
+			m_lastErrorCode = errno != 0 ? errno : ENOTSUP;
+		if (errno == 0)
+			errno = savedErrno;
+		return result;
 	}
 
 	int  GetImageWidth(int pane) const
@@ -1496,9 +1504,16 @@ class CImgDiffBuffer
 		m_imgOrig32[pane].copySubImage(image, x, y, x2, y2);
 	}
 
+	int GetLastErrorCode() const
+	{
+		return m_lastErrorCode;
+	}
+
 protected:
 	bool LoadImages()
 	{
+		int savedErrno = errno;
+		errno = 0;
 		bool bSucceeded = true;
 		for (int i = 0; i < m_nImages; ++i)
 		{
@@ -1523,6 +1538,7 @@ class CImgDiffBuffer
 					if (!m_imgConverter[i].isValid())
 					{
 						bSucceeded = false;
+						m_lastErrorCode = (errno != 0) ? errno : ENOTSUP;
 					}
 				}
 				m_imgOrig32[i] = m_imgOrig[i];
@@ -1554,6 +1570,8 @@ class CImgDiffBuffer
 			}
 			m_imgOrig32[i].convertTo32Bits();
 		}
+		if (errno == 0)
+			errno = savedErrno;
 		return bSucceeded;
 	}
 
@@ -2279,4 +2297,5 @@ class CImgDiffBuffer
 	DIFF_ALGORITHM m_diffAlgorithm;
 	int m_blinkInterval;
 	int m_overlayAnimationInterval;
+	int m_lastErrorCode;
 };

src/WinIMergeLib/ImgMergeBuffer.hpp

@@ -441,16 +441,28 @@ class CImgMergeBuffer : public CImgDiffBuffer
 		if (pane < 0 || pane >= m_nImages)
 			return false;
 		m_imgOrig[pane].pullImageKeepingBPP(m_imgOrig32[pane]);
+		int savedErrno = errno;
+		errno = 0;
 		if (m_imgOrigMultiPage[pane].isValid())
 		{
 			m_imgOrigMultiPage[pane].replacePage(m_currentPage[pane], m_imgOrig[pane]);
 			if (!m_imgOrigMultiPage[pane].save(filename))
+			{
+				m_lastErrorCode = errno != 0 ? errno : ENOTSUP;
+				if (errno == 0)
+					errno = savedErrno;
 				return false;
+			}
 		}
 		else
 		{
 			if (!m_imgOrig[pane].save(filename))
+			{
+				m_lastErrorCode = errno != 0 ? errno : ENOTSUP;
+				if (errno == 0)
+					errno = savedErrno;
 				return false;
+			}
 		}
 		m_undoRecords.save(pane);
 		m_filename[pane] = filename;

src/WinIMergeLib/ImgMergeWindow.hpp

@@ -1229,6 +1229,11 @@ class CImgMergeWindow : public IImgMergeWindow
 		}
 	}
 
+	int GetLastErrorCode() const
+	{
+		return m_buffer.GetLastErrorCode();
+	}
+
 private:
 
 	ATOM MyRegisterClass(HINSTANCE hInstance)

src/WinIMergeLib/WinIMergeLib.h

@@ -189,6 +189,7 @@ struct IImgMergeWindow
 	virtual void SetOverlayAnimationInterval(int interval) = 0;
 	virtual bool IsDarkBackgroundEnabled() const = 0;
 	virtual void SetDarkBackgroundEnabled(bool enabled) = 0;
+	virtual int GetLastErrorCode() const = 0;
 };
 
 struct IImgToolWindow

src/WinIMergeLib/WinIMergeLib.rc

@@ -144,7 +144,7 @@ BEGIN
             VALUE "FileDescription", "WInIMergeLib"
             VALUE "FileVersion", STRFILEVER
             VALUE "InternalName", "WinIMergeLib.dll"
-            VALUE "LegalCopyright", "Copyright (C) 2014-2021 sdottaka@users.sourceforge.net"
+            VALUE "LegalCopyright", "Copyright (C) 2014-2025 sdottaka@users.sourceforge.net"
             VALUE "OriginalFilename", "WinIMergeLib.dll"
             VALUE "ProductName", "WinIMergeLib"
             VALUE "ProductVersion", STRPRODUCTVER