Should you call Wait on a wxThreadHelper joinable thread that has stopped

[silver.moon]

Is it wrong to called Wait on joinable thread inside wxThreadHelper that has ended.

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GetThread()->Wait();

Doing so give this nice error dialog



But the docs of wxThread say -

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Regardless of whether it has terminated or not, you should call Wait() on a joinable thread to release its memory, as outlined in Types of wxThreads

[evstevemd]

Any reasona you are not using Detached Threads?

[silver.moon]

Any reasona you are not using Detached Threads?

Never thought about it. What are the pros and cons ?
There are situations when i need to manually stop threads, how would I do that with a Detached thread ?

[DenDev]

Where du you call GetThread()->Wait()? If you do this in the threads own context (in the Entry() function) it will fail (exception) or cause you program to hang since it will wait for itself to finish (indefinite loop). The Wait() function must always be called outside the threads context (by the thread that started it). If you call the Wait() function for a thread that has already finished it will cause problems, this should be avoided by checking "GetThread() != NULL && GetThread()->IsRunning()". The thread helper is used for a single background task, it cannot be started multiple times and the attempt will restart the task in a "dirty" way.

Using a joinable thread is an advantage if the task it does has to be synchronized with a visual interface. This could be a dialog that copies a file, downloads a file or something like that, and displays the status in a progress bar. In every situation where the thread(s) does not need to have a constant, visual representation (but still can have) detached threads are better. This could be a chat server where each detached thread manages a chatting client or an FTP server where each thread serves one connected FTP client. These invisible "servers" are usually called daemons. In such scenarios there would be no need for the server to display anything (waste of energy). If an admin starts the daemons GUI, the status of the detached threads can still be polled into the visual interface.

So; Joinable threads are only good for single background tasks with constant, visual feedback. Detached threads are good for everything

[silver.moon]

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If you call the Wait() function for a thread that has already finished it will cause problems

The why does the docs say (http://docs.wxwidgets.org/trunk/classwx_thread.html)

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Regardless of whether it has terminated or not, you should call Wait() on a joinable thread to release its memory, as outlined in Types of wxThreads

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 The thread helper is used for a single background task, it cannot be started multiple times and the attempt will restart the task in a "dirty" way.

I am not starting it multiple times. Just reusing it "after it is done". I guess there is nothing wrong with that. Once the thread is finished, it can be started again by pressing the start button.

So should CreateThread be called again ? or the existing thread object that was created in the previous call to CreateThread be used ?

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So; Joinable threads are only good for single background tasks with constant, visual feedback. Detached threads are good for everything

So how do you stop a detached thread "on-demand", when the user presses the "Stop" button.
The thread might already have finished and then calling IsRunning or Delete will cause problems ?

[DenDev]

1) You should not call the Wait() function if the thread has already been released. Before you call the Wait() function you must check that the wait operation is valid or you might get into trouble.

2) You can of course reuse the thread helper as many times as you require. But you cannot start it (with CreateThread) while it is already running with causing a dirty termination of the thread already running.

3) You must keep a reference to the thread in a variable. The variable should be set to NULL when the thread finishes using an event to signal that the thread has finished. Please view the sample on this page http://docs.wxwidgets.org/trunk/classwx_thread.html the MyFrame::OnClose method shows how to do this.

[silver.moon]

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1) You should not call the Wait() function if the thread has already been released. Before you call the Wait() function you must check that the wait operation is valid or you might get into trouble.

Incase of JOINABLE threads inside wxThreadHelper "its released" only in wxThreadHelper destructor. So as long as the wxThreadHelper object is alive calling Wait should be always valid.

But inspite of that I get the error dialog shown in the picture above. That is the whole story.

[DenDev]

Incase of JOINABLE threads inside wxThreadHelper "its released" only in wxThreadHelper destructor. So as long as the wxThreadHelper object is alive calling Wait should be always valid.

No, the thread is only valid after a successful call to CreateThread(). wxThreadHelper is used to manage the lifecycle of the thread you create with CreateThread(), you should only Wait() when you need to halt program execution until the thread has finished. It is hard to figure out what is wrong with your code since you do not provide it but the error is either a result of calling Wait() from within the thread's context or a result of calling Wait() for at thread that cannot be waited for (not started, not created etc.).

[silver.moon]

@DenDev

Yes, you are right.
I was actually calling Wait, after already calling Delete, which was causing the error dialog.