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I was assigned to create a program that handles three WINAPI threads and reflects the workflow of those in progress bars. I decided to use Qt Widgets for these purposes. I create those threads in suspended state, using CREATE_SUSPENDED creation flag, then i resume it with ResumeThread function upon clicking the button. When I click it, the program crashes with an unhandled exception win32. Why could this happen?

My "create" button click slot

void MainWindow::on_pb_create_clicked()
{
    hThread[0] = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)&ThreadFunc1, NULL, CREATE_SUSPENDED, &thID[0]);
    hThread[1] = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)&ThreadFunc2, NULL, CREATE_SUSPENDED, &thID[1]);
    hThread[2] = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)&ThreadFunc3, NULL, CREATE_SUSPENDED, &thID[2]);
    threadsCreated = true;
}

My "Start" button click slot

void MainWindow::on_pb_start_clicked()
{
    if(threadsCreated)
    {
       for(int i = 0; i = 2; i++)
       {
           ResumeThread(hThread[i]);
       }
    }
    else
    {
        QMessageBox msg;
        msg.setWindowTitle("Error");
        msg.setText("Threads are not created");
        msg.exec();
    }
}

My thread function

DWORD WINAPI MainWindow::ThreadFunc1(LPVOID lpParams)
{
    for(int i = 0; i < 100; i++)
    {
        ui->progressBar->setValue(i);
        sleep(500);
    }
    return 0;
}

Other 2 thread functions are the same, only using different progress bar.

ventisk1ze
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    What does the debugger tell you about where the crash occurs? Also you have a typo in your `for` loop. – Jonathan Potter May 11 '20 at 11:48
  • Is `MainWindow::ThreadFunc1` a `static` function of `MainWindow`? – Algirdas Preidžius May 11 '20 at 11:50
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    You're accessing the same variable(s) from multiple threads without any synchronization (mutex etc.). That's undefined behaviour. – G.M. May 11 '20 at 11:55
  • @JonathanPotter Crash caused by segmentation fault occurs in the first thread function in the line, where progress bar value is set. Also, it was in the third function, until I fixed a typo. – ventisk1ze May 11 '20 at 12:05
  • If you have to explicitly cast a function pointer to `(LPTHREAD_START_ROUTINE)`, that's the root of the problem. Remove those casts in CreateThread and explicitly fix the function signature of the thread's entry point. – selbie May 11 '20 at 12:22
  • In fact, don't even call `CreateThread`. The [documentation](https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createthread) is already telling you not to. – IInspectable May 11 '20 at 14:50

2 Answers2

1

You are accessing the ui apparatus from a non-ui thread. This will result in a race condition. To do what you want safely you will need to post QEvents to to ui thread. Alternatively use QThread and the signals and slots can achieve the same thing.

doron
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1

It seems you are passing a member function pointer as the function that should be executed. Remember that you can't call a member function without an object instance that "performs" the function call, except for static member functions.

See this thread for a workaround: How do you use CreateThread for functions which are class members?

Bottom line is you call a static member function, pass it the object that should perform some member function call (your MainWindow instance in this case), then the instance performs the call.

Edit:

See the Microsoft Documentation for CreateThread under "Return value":

Note that CreateThread may succeed even if lpStartAddress points to data, code, or is not accessible. If the start address is invalid when the thread runs, an exception occurs, and the thread terminates. Thread termination due to a invalid start address is handled as an error exit for the thread's process.

https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createthread

MorningDewd
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