Run two Lua-functions from Qt/C++-App parallel

Question

I have a Qt-Application written in C++. It's a generic Tool for testing hardware. The specific tests are defined in a lua-script.

In my script i have a function called 'RunTests()' which is called in a QThread in the Qt-Application. I put it into a QThread in order to prevent the script from freezing my application. Now there's another function in the lua script, called 'Interrupt()' which should be called sometimes by the Qt-Application during the tests. So now everytime I call this function 'Interrupt()' with lua_getglobal() my Qt-Application crashes.

How can I run those 2 Lua functions at the same time or how can I interrupt 'RunTests()' to call 'Interrupt()' and then move back to 'RunTests()'?

lhf · Accepted Answer · 2018-06-15 16:23:16Z

2

You cannot run concurrent scripts in the same Lua state.

You can run concurrent scripts in separate Lua states.

answered Jun 15, 2018 at 16:23

lhf

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2 Comments

Kuba hasn't forgotten Monica Over a year ago

Ding-a-ling ding ding! That'e exactly it. There are ways of safely sharing data between lua states, though, so the fact that there is a multitude of lua states doesn't mean that the scripts need to be completely isolated.

gpane Over a year ago

Thanks for the answers. So it means that I can load my script in two separate Lua states where one runs my RunTest() function and the other runs my so called 'interrupt' functions?

Ulrich Eckhardt · Accepted Answer · 2018-06-15 20:12:41Z

Your question boils down to "How can I interrupt a thread?". This is a common question and the answer is usually that you can't, but that there are other ways. The simple reason why you can't is that a thread might be in the middle of some operation that modifies global state, like a heap allocation. If you killed it in the middle, it would leave data structures in a temporarily invalid state and leave mutexes locked.

Two approaches come to mind that might work for you:

You could ask the thread to exit. If your thread regularly checked a flag (that could be a variable in a C++ function exported to Lua) and then reacted accordingly, you could set the flag and wait for the thread to finish.
Another common approach is that you don't use threads but processes instead. Other than threads, a process can be terminated and the OS guarantees proper cleanup. Downside is that processes don't share state, so you would have to set up some kind of IPC.
A third approach might also work, but it depends on the system you are on. On POSIX systems, you could use signals to trigger thread termination. However, for that, it must be properly supported by the code. If Lua doesn't support it, you are out of luck and on other OSs probably too.

gpane · Accepted Answer · 2018-06-19 08:55:32Z

0

I now solved the problem by using coroutines.

So when my two functions are:

function DataHandler(data)
    buffer = buffer .. data
    ...
end

function Tests()
    ...
end

I added the following lines of code:

co_DataHandler = coroutine.create(DataHandler)
co_Tests = coroutine.create(Tests)

With this I am able to run DataHandler() within the same QThread while Tests() is doing some other stuff.

answered Jun 19, 2018 at 8:55

gpane

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