Скачать или смотреть Resolving stdin Input Blockage When Using _setmode in a JNI Embedded Java VM

Discover how to solve the issue of unwanted `Enter` key waits when using `_setmode` in a JNI embedded Java VM within a Windows application.
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This video is based on the question https://stackoverflow.com/q/63085456/ asked by the user '7Hazard' ( https://stackoverflow.com/u/8122837/ ) and on the answer https://stackoverflow.com/a/63089900/ provided by the user '7Hazard' ( https://stackoverflow.com/u/8122837/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving stdin Input Blockage When Using _setmode in a JNI Embedded Java VM

When working with Java Native Interface (JNI) to embed the Java Virtual Machine (JVM) within a native application, particularly on Windows, developers may encounter some unexpected behaviors, especially with standard input. One common issue relates to the _setmode function, which changes how the input streams function. Specifically, this function can lead to the process waiting for user input through the Enter key, an undesirable side effect during execution. Let’s explore this issue in more detail and how to resolve it effectively.

The Problem: Unwanted User Input Wait

In the scenario where a native application like altv-server.exe uses JNI to create a Java VM, the following lines of code are often present:

[[See Video to Reveal this Text or Code Snippet]]

While this line should successfully change the mode of standard input from text to binary (_O_BINARY), it results in the application waiting for an Enter key input. This behavior may arise when the application is compiled with the /MD flag, leading to certain threads reading from stdin inadvertently causing the application to pause.

Conditional Behavior Based on Compilation Flags

Compiled with /MD: The application waits for user input when _setmode is called.

Compiled with /MDd: This does not trigger the same behavior, but it isn't a viable option if compliance with the /MD flag is required for your DLL.

As such, the challenge here is twofold:

Understanding why this happens when compiled with /MD.

Finding a way to resolve the problem without changing the compilation flags.

The Solution: Adjust Thread Management

The unexpected input wait stems from a thread actively reading from stdin before _setmode is called. Fortunately, there is a straightforward solution. The aim is to manage the thread's operation relative to stdin and the mode-setting function effectively.

Steps to Fix the Issue

Reorder Thread Initialization: Instead of initializing or starting your thread that handles stdin input before you call _setmode, do it after the mode has been set.

[[See Video to Reveal this Text or Code Snippet]]

Ensure Clean Threading Practices: It’s essential to keep the threading model neat and ensure that no threads attempt to interact with standard input until after the mode has been set. This minimizes disruption and accidental user prompts.

Test Your Implementation: After implementing the changes, thoroughly test the application to confirm that it no longer waits for Enter key input while still functioning as intended.

Conclusion

By carefully managing when your threads interact with standard input and strategically calling _setmode, you can effectively avoid unwanted waits for user input in your JNI-embedded Java VM. This solution allows you to maintain your compilation flags and ensures a seamless experience for users of your application. Happy coding!