Understanding lua_pushcclosure: How to Pass Upvalues Correctly in Lua C API

Описание: This guide explains the common mistake made when using `lua_pushcclosure` to create closures in the Lua C API, and provides a clear solution to ensure that parameters are correctly passed to your callback functions.
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This video is based on the question https://stackoverflow.com/q/74054921/ asked by the user 'Lua_beg' ( https://stackoverflow.com/u/20220704/ ) and on the answer https://stackoverflow.com/a/74055200/ provided by the user 'user253751' ( https://stackoverflow.com/u/106104/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding lua_pushcclosure: How to Pass Upvalues Correctly in Lua C API

When working with the Lua C API, many developers encounter challenges with lua_pushcclosure. A particular issue arises when trying to pass parameters (upvalues) to callback functions. In this post, we will explore why lua_pushcclosure may not seem to work as expected and provide a detailed explanation on how to effectively use it to pass parameters.

The Problem: Why are Upvalues Not Available?

In your case, you've defined a callback function, my_callback, and are attempting to use lua_pushcclosure to set it up with two parameters (param1 and param2). However, when you try to access these parameters within my_callback, they appear empty.

The Code Breakdown

Let’s break down the code to see how the Lua stack is utilized:

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

The key takeaway here is that after pushing the closure with the upvalues, a separate cfunction without upvalues is implemented as a parameter to lua_pcall. Thus, the closure you initially created—which carries param1 and param2—is not being used when you call Set_Callback.

The Solution: Correct Usage of Closures

To successfully pass upvalues to your callback function, you need to ensure that the closure you created remains on top of the stack when it comes time to call lua_pcall. Here’s how you can do this:

Push the Strings: First, push your parameters onto the stack.

Create your Closure: Use lua_pushcclosure to create the closure that wraps around your callback function.

Prepare the Callback: When getting the field for Set_Callback, directly pass the closure with upvalues and not a different C function.

Revised Code Example

Here’s the corrected version of your code:

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

Explanation of Changes

Pushing Closure Again: By using lua_pushvalue(L, -2), the closure remains at the top of the stack when calling lua_pcall, ensuring that the parameters are available to my_callback.

Avoiding Extra cfunction: Instead of pushing an unrelated C function onto the stack, we reused the closure with the upvalues, allowing your parameters to be accessible when my_callback is invoked.

Conclusion

Understanding how closures and upvalues work in the Lua C API can be challenging, but with a careful management of the stack, you can ensure that your callback functions receive the necessary parameters. By following the corrected approach provided, you should now be able to pass values successfully to your function using lua_pushcclosure. Happy coding!