Qprocess and avrdude

Question

i'm trying to create a simple QT program that allows me to launch avrdude without using command line operations.

I saw that with Qprocess it's easy to launch any kind of program and I tried succesfully with simple programs like Explorer Word and Others.

The problem is that when i try to open cmd.exe nothing happens, even if i try to pass a batch file containing all the information to launch correctly avrdude.

Here's the code

QProcess *process = new QProcess(this);
process->startDetached("cmd.exe",QStringList()<<"C:/avrdude/avr.bat"); 

Answer 1

I wrote a minimal sample application which shows how to start cmd with a command using QProcess::startDetached() (on button click):

// standard C++ header:
#include <iostream>

// Qt header:
#include <QApplication>
#include <QMainWindow>
#include <QPushButton>
#include <QProcess>

int main(int argc, char **argv)
{
  // main application
#undef qApp // undef macro qApp out of the way
  QApplication qApp(argc, argv);
  QMainWindow qWin;
  QPushButton qBtn(QString::fromLatin1("Start cmd"));
  QObject::connect(&qBtn, &QPushButton::clicked,
    [](bool) {
      bool ret = QProcess::startDetached(
#if 1 // let Windows search for cmd.exe in %PATH%
        QString::fromLatin1("cmd.exe"),
#else // define exact path of cmd.exe
        QString::fromLatin1("C:\\Windows\\system32\\cmd.exe"),
#endif // 1
        QStringList()
        << QString::fromLatin1("/K")
        << QString::fromLatin1("echo Hello"));
      std::cout << "QProcess::startDetached():"
        << (ret ? "OK." : "Failed!") << std::endl;
    });
  qWin.setCentralWidget(&qBtn);
  qWin.show();
  return qApp.exec();
}

The Qt project file is left as exercise. (Sorry, I used CMake for this.)

Please, note the #if 1. If 1 is replaced by 0 the alternative code with full path is used. (During chat session we examined special problems with starting the cmd.exe.) On my system, both alternatives did as well.

(My system: Windows 10, VS2013, Qt 5.7)

Answer 2

I too have been working on a Qt program where there are a couple calls to AVRDUDE. This is what worked for me. Here's the code I made for a read of the AVR device through AVRDUDE, followed by a couple of comments.

void MainWindow::call_AVRDUDE_read()    //AVR READ
{
    QProcess CommandPrompt;
    QStringList Arguments;
    QString COMPortUsed = (ui->COM_Port_Used->text());                    // get the COM port from the user off UI
    Arguments << "/C avrdude -c arduino -P "+ COMPortUsed +" -b 115200 -p ATmega328P -e  -U eeprom:r:fromEEPROM.bin:r";
    CommandPrompt.start("cmd",Arguments);
    CommandPrompt.waitForFinished();
}

Here's something else which may well influence things in your application. In my case, I am reading the AVR's EEPROM. There is another routine that writes the EEPROM, but it is essentially the same as above, but a different script is sent. In BOTH these cases, the AVRDUDE operation takes a few seconds to perform its task. When you use the QProcess::startDetached(), it has the disadvantage that control will return IMMEDIATELY after the AVRDUDE script is called through the QProcess. This can cause problems, if for instance you wanted to (as in my case) read the contents of the EEPROM and try to do so before the read actually completes.

An alternative to startDetached() you might consider trying is shown below. This will retain control until the process is finished, which may be pretty important to you. Use these two lines to replace the startDetached() call you are currently using.

CommandPrompt.start("cmd",Arguments);
CommandPrompt.waitForFinished();

This will wait for the AVRDUDE process to finish before control is returned.

The take away here though is that QProces::startDetached() may return prematurely in your application. Just beware of that.