Executing batch file and getting finished signal

[lukutis222]

Hello. I have batch file that is used for flashing external micrcocontrollers (ESP32). The batch file contents:

tools\esptool.exe -p COM33 -b 921600 --before default_reset --after hard_reset --chip esp32  write_flash --flash_mode dio --flash_size detect --flash_freq 80m 0x0 bins\flash_image.bin

As you can see from above, it is using esptool.exe that I have in the tools folder.

I need to execute this command batch file via QT and receive finished signal. I try 3 different methods:

METHOD 1:

void MainWindow::on_flash_button_clicked()
{
    QStringList arguments;
    QProcess process;
    connect(&process, &QProcess::errorOccurred, this, &MainWindow::processError);
    connect(&process, &QProcess::finished, this, &MainWindow::flash_finished);


    arguments << "/C"<<"start"<<"bats\\esp32_flash2.bat";
    process.setArguments(arguments);
    process.setProgram("cmd.exe");
    qDebug() << process.program() << " " << process.arguments();
    process.start();
  
}

When I click button to start my script, nothing happens and I get the following error:

"cmd.exe"   QList("/C", "start", "bats\\esp32_flash2.bat")
QProcess: Destroyed while process ("cmd.exe") is still running.
error enum val =  QProcess::Crashed 
flash finished 

METHOD 2: (identical to method 1 except using startDetached() instead of start()

//works but not getting flash finished signal
void MainWindow::on_flash2_button_clicked()
{
    QStringList arguments;
    QProcess process;

    connect(&process, &QProcess::errorOccurred, this, &MainWindow::processError);
    connect(&process, &QProcess::finished, this, &MainWindow::flash_finished);

    arguments << "/C"<<"start"<<"bats\\esp32_flash2.bat";
    process.setArguments(arguments);
    process.setProgram("cmd.exe");
    qDebug() << process.program() << " " << process.arguments();
    process.startDetached();
    
}

The above method opens a cmd instance and starts flashing the device. This works but I have 2 issues with this method:

1. It starts a cmd instance and I do not want user to see this. I want everything to happen "behind the curtains".
2. It does not generate a finished signal when the flashing is complete.

METHOD 3 (removed "start" from QProcess arguments)

void MainWindow::on_flash3_button_clicked()
{
    QStringList arguments;
    QProcess process;

    connect(&process, &QProcess::errorOccurred, this, &MainWindow::processError);
    connect(&process, &QProcess::finished, this, &MainWindow::flash_finished);

    arguments <<"/C"<<"bats\\esp32_flash2.bat";
    process.setArguments(arguments);
    process.setProgram("cmd.exe");
    qDebug() << process.program() << " " << process.arguments();
    process.startDetached();

}

This method works better than the method 2 because it does not start external cmd instance.
I have 1 issue with above method:

1. It does not generate QProcess::finished signal when the flashing is complete

Could someone help me understand how to call batch scripts from QT properly and how to make sure the QProcess::finished gets triggered at the end of batch file execution? I believe I need to use process.start() rather than startDetached() but I cannot get it to work using start(), I always get QProcess::Crashed error. Thanks in advance.

[ChrisW67]

Your QProcess instance needs to exist longer than the process it is managing.

By default, Windows start does not wait for its child process to complete.

In Method 1 QProcess:start() runs start, which returns ('finishes') quickly, and then the QProcess is destroyed while the command shell(s) involved are still running. This generates the overt warning.

Method 2 suffers the same problem except you moved the quick return into the Qt side.

Method 3 after the batch file process is launched detached the QProcess instance is destroyed and thus does not exist to send a finished() signal.

Make the a QProcess instance or a QProcess* used to track a heap-allocated QProcess a member variable.

[JonB]

@lukutis222
As @ChrisW67 has written. Note that to wait for finished you will want to use QProcess::start() and not startDetached(). Also do not try to run the Windows start command, that will prevent waiting for finished on the program it runs.

[lukutis222]

@ChrisW67
@JonB

Thanks both for responses. I managed to get it working by creating process on the heap as you have suggested.

void MainWindow::on_flash3_button_clicked()
{
    QStringList arguments;
    //QProcess* process;
    QProcess * process = new QProcess();
    connect(process, &QProcess::errorOccurred, this, &MainWindow::processError);
    connect(process, &QProcess::finished, this, &MainWindow::flash_finished);

    arguments <<"/C"<<"bats\\esp32_flash2.bat";
    process->setArguments(arguments);
    process->setProgram("cmd.exe");
    qDebug() << process->program() << " " << process->arguments();
    process->start();

}

Since I have a habit of never using heap (I always allocate everything on stack), I would like to know if there is anything else I need to be aware of when using this method. Do I need to manually free the memory once I receive the finished signal?

[JonB]

@lukutis222 said in Executing batch file and getting finished signal:

Do I need to manually free the memory once I receive the finished signal?

Yes!

If you don't like heap-allocating the QProcess you could make it a member variable of e.g. MainWindow? Then you wouldn't have to think about deleting it.