Why Does My Qt Application Fail to Load with a 'windows' Plugin Error?

DLL Loading Failure: Qt Application Fails with "windows" Plugin Error

Introduction

When attempting to start a Qt application, an error message might arise indicating a failure to find or load the "windows" platform plugin. This error can be encountered in various scenarios, leading to frustration and disruption.

Common Setup and Scenario

Often, this issue occurs in applications developed with Qt and built using Visual Studio. The setup typically involves Windows 7, Visual Studio 2012, Qt 5.2.0, and a 32-bit application using QtSingleApplication.

Debugging and Troubleshooting

To resolve the error, several steps can be taken:

1. Verify VC Redistributables: Ensure that all necessary Visual C Redistributables are installed on the target machine, including the VC 2010 and 2012 redistributables.
2. Add Platform Plugin: Create a ".platforms/" directory and place the qwindows.dll file inside it. This ensures that the application can locate the platform plugin.
3. Set Path to DLL: Add the following line at the beginning of the main() function:

QCoreApplication::addLibraryPath("./");

This line tells the application to search for libraries in the current directory, ensuring that it can find qwindows.dll.

Potential Dependencies and Errors

In some cases, the error message might be accompanied by additional errors, such as:

• "GetProcAddress(...) called from (...) returned NULL"
• "This application failed to start because it could not find or load the Qt platform plugin "windows""
• "Available platform plugins are: offscreen"

These errors can indicate that the required DLLs are not available or accessible to the application.

Conclusion

By following the steps outlined above, you can resolve the error related to Qt platform plugin loading. Verifying redistributables, adding the platform plugin, and setting the library path are crucial steps in ensuring that your Qt application can start successfully.

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