Application failed to start because it could not find or load the QT platform plugin "windows"

Question

I have looked through all of the questions that appear to be related on stack overflow, and none of the solutions seem to help me.

I am building a Qt application with this setup:

• Windows 7 Professional x64
• Visual Studio 2012
• Qt 5.2.0 built with configure -developer-build -debug-and-release -opensource -nomake examples -nomake tests -platform win32-msvc2012 -no-opengl
• Project uses QtSingleApplication (qt-solutions)
• Application is a 32 bit application
• qmake run with the following: -makefile -spec win32-msvc2012
• .pri uses QMAKE_CXX += /D_USING_V110_SDK71_

I can build and run my program fine on my development machine (noted above); I can also install and run the package from Program Files directory on dev machine.

When I install and run on a Windows Vista machine (multiple machines)

• VC++ redist 2012 11.0.61030.0 installed
• VC++ redist 2010 10.0.40219 installed
• plus 2005, 2008 versions of redist

(also fails on a clean install of Windows 7)

I get:

Application failed to start because it could not find or load the QT platform plugin "windows"

So I followed the instructions and added a .platforms/ directory, and added qwindows.dll (also added qminimal.dll and qoffscreen.dll); I also added libEGL.dll, libGLESv2.dll (even though I shouldn't need them I don't think)

Once I added qoffscreen.dll I now get the additional message: Available platform plugins are: offscreen

If I run through Dependency Walker I get this error listed:

GetProcAddress(0x76CA0000 [KERNEL32.DLL], "GetCurrentPackageId") called from "MSVCR110.DLL" at address 0x6AC6FDFA and returned NULL. Error: The specified procedure could not be found (127).

and then further down get the:

GetProcAddress(0x745A0000 [UXTHEME.DLL], "BufferedPaintUnInit") called from "COMCTL32.DLL" at address 0x745FFBF8 and returned 0x745AE18C.
This application failed to start because it could not find or load the Qt platform plugin "windows".

Available platform plugins are: offscreen.

Reinstalling the application may fix this problem.

Any ideas how to fix this dll issue?

Answer 1

I'm seeing this question in various forms getting verbose non-answers, complicated answers, etc., across StackOverflow, Microsoft forums, and Qt forums. Just to clear through everything ... the relevant text in the Qt documentation is this:

All Qt GUI applications require a plugin that implements the Qt Platform Abstraction (QPA) layer in Qt. For Windows, the name of the platform plugin is qwindows.dll. This file must be located within a specific subdirectory (by default, platforms) under your distribution directory. [...]

The search paths for Qt plugins are relative to the location of the QtCore library and no further steps are required to ensure plugins are found after installing the application on the target machine.

So relative to where your executable is finding QtCore.dll you need to have a directory called "plugins" with contents that Qt expects. So a minimal Windows Qt installation looks like this:

+ <your installation dir>
   o  <your .exe>
   o  Qt6Core.dll
   o  Qt6Gui.dll
   o  Qt6Widgets.dll
   +  plugins
      + platforms
         qwindows.dll

The documentation is less than clear about the necessity of the "platforms" directory but you can see this is the way they lay out the "plugins" directory in their distribution.

Answer 2

You need to add environmental variable QT_QPA_PLATFORM_PLUGIN_PATH to the system which points to the platforms directory in QT plugins. In my case, I was using Anaconda and PySide2. Therefore my directory path was C:\ProgramData\Anaconda3\envs\cv\Lib\site-packages\PySide2\plugins. This fixes the issue for every QT project. Otherwise, you have copy platforms directory to every QT project.

Answer 3

I fixed this by placing qt.conf in my application's exe folder:

[Paths]
Prefix=C:/Qt/Qt5.11.2/5.11.2/msvc2017

Where:

1. I have installed a custom Qt kit in C:\Qt\Qt5.11.2\5.11.2\msvc2017
2. qt.conf informs the app where the custom kit via the Prefix property. Note use forward slashes not backslashes (!)
3. And, optionally, the Qt kit's bin folder is included in my PATH environment variable

Defining Prefix in your qt.conf file allows it to find the qwindows.dll platform plugin when your app starts.

Answer 4

I had the same issue "Application failed to start because it could not find or load the QT platform plugin "windows" I fixed this by copying below files to the app.exe (my app executable) folder,

Qt5Core.dll, Qt5Gui.dll, Qt5Widgets.dll and a "platforms" directory with qminimal.dll, qoffscreen.dll, qwindows.dll.

I hope this will help someone

Answer 5

For me, I needed to set QT_QPA_PLATFORM_PLUGIN_PATH to the platforms directory and then it worked.

For what it's worth, this solution was also mentioned on GitHub.

Answer 6

Tried all the above - turned out for me it was simply because I didn't have the main Qt dlls in the apps folder

• Qt5Core.dll
• Qt5Widgets.dll
• etc

Answer 7

For the people who have this problem in the future - I have a dirty little hack, worked for me. Try at your own risk.

Follow all the steps in Initial deployment (Quick and dirty) [http://wiki.qt.io/Deploy_an_Application_on_Windows]

1. Close Qt Creator.
2. Copy the following into C:\Deployment\ The release version of MyApp.exe All the .dll files from C:\Qt\5.2.1\mingw48_32\bin\ All the folders from C:\Qt\5.2.1\mingw48_32\plugins\
3. (If you used QML) All the folders from C:\Qt\5.2.1\mingw48_32\qml\ Rename C:\Qt\ to C:\QtHidden\ (This turns your PC into a clean environment, just like one that doesn't have Qt installed).
4. Launch C:\Deployment\MyApp.exe.

Now for the hack -

1. Duplicate your folder for safety
2. If your file was in /cat/Deployment, go to /cat
3. Now, delete the Deployment folder while the .exe is still running.
4. It will tell you that it cannot delete certain files, so say Skip(or skip all)
5. What you're left with is the list of all the .dll files that your .exe was actually using and could not delete: the list of all the files and only the files that you need to keep.

You can close the .exe file now. To check whether it is deploying okay, go into the folder where you installed it, say C:/Qt and rename it to C:/NotQt (basically make Qt invisible to the system). If it works now, it will deploy on other systems more often than not.

Answer 8

The application is able to run on the host system, since the Qt bin path is in the system PATH variable.

There is a standard Qt tool for deployment of Qt applications on Windows windeployqt to be able to run the application on target machines that do not have Qt installed.

That tool takes care about Qt DLL dependencies, makes a copy of platforms\qwindows.dll and also it makes a copy of libraries that you cannot detect with the Dependency Walker, since image plugins and some other DLLs are loaded at runtime.

You do not even need to have your Qt bin folder in your environment PATH. The simplest deployment:

• copy built exe binary to a new folder
• open cmd console in that folder
• call windeployqt using the full path (if it is not in the system PATH) and provide your executable, for example:

  c:\Qt\Qt5.2.1\5.2.1\msvc2010_opengl\bin\windeployqt.exe application.exe
  

As a result you have in that folder all needed Qt DLLs to run the application.

The tool windeployqt has various options. It can also take care about deployment of qml related files.

Of course you can have also issues with MSVC redistributables, but those should be deployed separately and installed once per system.

Only some 3rd party libraries should be copied manually if they are used, for example OpenSSL.

Answer 9

It's missing qwindows.dll, which normally should be in platforms, unless you add:

QCoreApplication::addLibraryPath("<yourpath>");

If you don't do this btw, and put your qwindows.dll somewhere else, Qt will search your PATH for the DLL, which may take a LOT of time (10s - several minutes)!

Answer 10

Note this issue can also be caused if the search path for qwindows.dll that is encoded in your app includes the path where you installed Qt. Consider the following scenario:

1. I install Qt to c:\Qt\...
2. I develop an app and deploy it correctly somewhere else.
3. It runs on any computer properly because it includes qwindows.dll in a subdirectory.
4. I upgrade my local Qt to a new version.
5. I try to run my app again.

The result is this error, because the qwindows.dll in c:\Qt\... is found before the one in its local directory and it is incompatible with it. Very annoying.

A solution is to place a file qt.conf in the same directory as your exe file. I don't know how to avoid this. If you used the tool windeployqt.exe to deploy your app, so you have a subdirectory called platforms, then this is sufficient:

[Paths]
Plugins=.

Answer 11

The error is caused because the program can't find qwindows.dll

qwindows.dll has to be in a folder named platforms so that the path from your executable to the dll is platforms/qwindows.dll

Whereas this wasn't enough in my case. I had also to add following line at the beginning of my main()

QCoreApplication::addLibraryPath("./");

Then everything worked.

Answer 12

I got the same issue: 1. it can run in VS2010; 2. it can run in a folder with files as: app.exe \platforms\qwindows.dll ...

1. but it failed to load qwindows on a clean machine with same OS as the developing one.

Solved simply by move the platform folder to plugins: app.exe plugins\platforms\qwindows.dll

---

plus: qwindows.dll can be renamed as any you like as it is queried by an plugin interafce: qt_plugin_query_metadata()

Answer 13

I got this issue and how I solved it:

1. Used dependency walker(http://www.dependencywalker.com/) to see the exact path of the dlls needed. Try it because both QtCreator and QT framework both have the same dlls and you must pinpoint the exact ones used. I copied all dlls needed in the same folder as the app.

2. I have copied the folder platforms from QT framework /plugins and copied it in the same folder as the app. Now the app comtained also plugin/platform/ folder with all its dlls

3. And the most important step in my case is to create a file named qt.conf in the same folder as the app . This file should contain the path to the plugins. My qt.conf file contains:

   [Paths]
   Libraries=../lib/qtcreator
   Plugins=plugins
   Imports=imports
   Qml2Imports=qml
   

Answer 14

Well I solved my issue, although I'm not sure what the difference is:

I copied every dll from my qt directory into both ./ and ./platforms of my application directory.

The application got past the error, but then crashed.

VERSION.dll was causing the crash (noted in dependency walker), so I removed it from both places.

The Application started up, so I systematically removed all unneeded dll's.

This got me back to the same state I had originally.

I then uninstalled my application and re-installed (with only the ./platforms/qwindows.dll file remaining), application works correctly.

So all I can assume is that I had an incorrect version of qwindows.dll in the platforms directory.