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Bug 3321

Summary: SDL_Init(SDL_INIT_VIDEO) always sets error on Win 7 and below
Product: SDL Reporter: Jason Francis <jason>
Component: videoAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: minor    
Priority: P2 CC: amaranth72
Version: 2.0.4   
Hardware: x86_64   
OS: Windows 7   
Attachments: Remove SDL_LoadObject and SDL_LoadFunction calls in WIN_CreateDevice
Test case

Description Jason Francis 2016-05-05 22:25:47 UTC 
Created attachment 2446 [details]
Remove SDL_LoadObject and SDL_LoadFunction calls in WIN_CreateDevice

Calling SDL_GetError() after SDL_Init(SDL_INIT_VIDEO) on Windows 7 or earlier will always return an error about a missing SHCORE.DLL. The error is coming from use of SDL_LoadObject, which sets an error when it fails to load a file. The SHCORE.DLL file only exists on Win8 or above,.

The WIN_CreateDevice function in src/video/windows/SDL_windowsvideo.c tries to dynamically load SHCORE.DLL in order to use the "GetDpiForMonitor" function. This function additionally only exists on Win8.1 or above, as per the MSDN documentation: https://msdn.microsoft.com/en-us/library/windows/desktop/dn280510(v=vs.85).aspx

The use of GetDpiForMonitor in SDL is optional and only comes as an added bonus for when the user is on a system that has it available. For this reason I think it is not appropriate to use SDL_LoadObject or SDL_LoadFunction in order to access optional DLLs. I've attached a patch that fixes this issue by just using LoadLibrary and GetProcAddress in the WIN_CreateDevice function. This retains the optional functionality but prevents SDL_Init from setting an error when an optional DLL does not exist.
Comment 1 Jason Francis 2016-05-05 22:33:23 UTC 
Created attachment 2447 [details]
Test case

Attached a test case. This will always print an error about SHCORE.DLL missing on Win7, but will print nothing on Win8.1 or Win10. I don't have access to a Win8 machine to test.
Comment 2 Ozkan Sezer 2016-05-24 08:00:05 UTC 
A much simpler solution would be clearing the error, e.g.:

diff -r 7cbfd97f1430 src/video/windows/SDL_windowsvideo.c
--- a/src/video/windows/SDL_windowsvideo.c	Mon May 23 15:29:25 2016 -0300
+++ b/src/video/windows/SDL_windowsvideo.c	Tue May 24 10:55:10 2016 +0300
@@ -119,6 +119,9 @@ WIN_CreateDevice(int devindex)
     if (data->shcoreDLL) {
         data->GetDpiForMonitor = (HRESULT (WINAPI *)(HMONITOR, MONITOR_DPI_TYPE, UINT *, UINT *)) SDL_LoadFunction(data->shcoreDLL, "GetDpiForMonitor");
     }
+    else {
+        SDL_ClearError();
+    }
 
     /* Set the function pointers */
     device->VideoInit = WIN_VideoInit;
Comment 3 Alex Szpakowski 2016-05-24 16:47:51 UTC 
Note that SDL_GetError should *only* be used when a SDL function returns indicating an error (-1 or NULL, usually). SDL_GetError should not be used for checking whether an error has occurred at all - it may return anything it wants.
Comment 4 Sam Lantinga 2017-08-12 03:19:31 UTC 
You should only check the SDL error if an API function fails. That said, this was cleaned up in this commit:
https://hg.libsdl.org/SDL/rev/f94279990934