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Qprocess exiting with exit code 255 when trying to run python script

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  • AdithyaA Adithya

    I am trying to create a Qprocess to run a python script and it is throwing error with exit code 255 .
    Code :

    int main(int argc, char *argv[])
    {
    QApplication a(argc, argv);

        // Path to the directory containing your Python script and other files
        QString workingDirectory = QDir::currentPath() + "/Tools/script";
        qDebug() << "Path = " <<workingDirectory;
    
        // Path to the Python script you want to execute
        QString pythonScript = QDir::currentPath() + "/Tools/script/main.py";
    
        // Arguments to pass to the Python script
        QStringList arguments;
        arguments << pythonScript << "1" << "2";
    
        // Create a QProcess instance
        QProcess process;
    
        // Set the working directory
        process.setWorkingDirectory(workingDirectory);
    
        // Set the command to execute
        process.start("python", arguments);
    
        if (!process.waitForStarted()) {
             qDebug() << "Failed to start Python process";
             //return 1;
         }
    
        // Wait for the process to finish (optional)
        process.waitForFinished();
    
        // Get the exit code of the process
        int exitCode = process.exitCode();
    
        // Handle the exit code if needed
        if (exitCode != 0) {
            qDebug() << "Python script execution failed with exit code:" << exitCode;
            //return 1;
        }
        return a.exec();
    
    }
    

    main.py

    if name == 'main':
    print("Hey there")

    error:
    Failed to start Python process
    Python script execution failed with exit code: 255

    Thanks for the help in advance

    J.HilkJ Offline
    J.HilkJ Offline
    J.Hilk
    Moderators
    wrote on last edited by
    #6

    @Adithya

    Exit code 255 generally indicates that the program couldn't start. This can happen due to various reasons.

    in this case, I think its a typical case of a wrong path.

    currentPath() should be the location of your generated executable, not your source files.
    Check that QDir::currentPath() + "/Tools/script/main.py" is actually a valid file and the correct file.


    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


    Q: What's that?
    A: It's blue light.
    Q: What does it do?
    A: It turns blue.

    AdithyaA 1 Reply Last reply
    1
    • jsulmJ jsulm

      @Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

      Can you please answer my first question?

      "Python interpreter is in the path /usr/bin/python3.exe" - in your code you're using "python" not python3 - is there a "python" executable?

      AdithyaA Offline
      AdithyaA Offline
      Adithya
      wrote on last edited by
      #7

      @jsulm I dont have python.exe , but I have python3.exe .I changed process.start("python", arguments); to process.start("python3", arguments); aswell .I was not getting the log from py file but there was no 255 exit code . But -2 when I used QProcess::execute().

      jsulmJ 1 Reply Last reply
      0
      • J.HilkJ J.Hilk

        @Adithya

        Exit code 255 generally indicates that the program couldn't start. This can happen due to various reasons.

        in this case, I think its a typical case of a wrong path.

        currentPath() should be the location of your generated executable, not your source files.
        Check that QDir::currentPath() + "/Tools/script/main.py" is actually a valid file and the correct file.

        AdithyaA Offline
        AdithyaA Offline
        Adithya
        wrote on last edited by
        #8

        Hi @J-Hilk , This is not the issue path. the py file preset in the specified path

        J.HilkJ 1 Reply Last reply
        0
        • AdithyaA Adithya

          @jsulm I dont have python.exe , but I have python3.exe .I changed process.start("python", arguments); to process.start("python3", arguments); aswell .I was not getting the log from py file but there was no 255 exit code . But -2 when I used QProcess::execute().

          jsulmJ Offline
          jsulmJ Offline
          jsulm
          Lifetime Qt Champion
          wrote on last edited by
          #9

          @Adithya I will ask for the third time (I will stop here if you do not answer): What does https://doc.qt.io/qt-6/qprocess.html#error return?

          https://forum.qt.io/topic/113070/qt-code-of-conduct

          AdithyaA 1 Reply Last reply
          0
          • jsulmJ jsulm

            @Adithya I will ask for the third time (I will stop here if you do not answer): What does https://doc.qt.io/qt-6/qprocess.html#error return?

            AdithyaA Offline
            AdithyaA Offline
            Adithya
            wrote on last edited by
            #10

            @jsulm I am getting QProcess::error as "ERROR: QProcess::UnknownError"

            jsulmJ 1 Reply Last reply
            0
            • AdithyaA Adithya

              @jsulm I am getting QProcess::error as "ERROR: QProcess::UnknownError"

              jsulmJ Offline
              jsulmJ Offline
              jsulm
              Lifetime Qt Champion
              wrote on last edited by
              #11

              @Adithya Then maybe your Python script exits with -2?
              You can also connect a slot to https://doc.qt.io/qt-6/qprocess.html#stateChanged and log state changes.
              And you can also connect slots to https://doc.qt.io/qt-6/qprocess.html#readyReadStandardError and https://doc.qt.io/qt-6/qprocess.html#readyReadStandardOutput and print out stderr/stdout from your process there.

              https://forum.qt.io/topic/113070/qt-code-of-conduct

              AdithyaA 1 Reply Last reply
              1
              • AdithyaA Adithya

                Hi @J-Hilk , This is not the issue path. the py file preset in the specified path

                J.HilkJ Offline
                J.HilkJ Offline
                J.Hilk
                Moderators
                wrote on last edited by
                #12

                @Adithya did you really check, or did you just type this answer ?

                If I had a € for every time I heard "ThE PaTh Is CoRrEcT!!111eleven" and it turns out it's not, because currentPath() or applicationDirPath() etc. did not actually point to the assumed directory, I would have a surprising amount of money :D.

                qDebug() << (QDir::currentPath() + "/Tools/script/main.py");
                

                Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                Q: What's that?
                A: It's blue light.
                Q: What does it do?
                A: It turns blue.

                AdithyaA 1 Reply Last reply
                0
                • J.HilkJ J.Hilk

                  @Adithya did you really check, or did you just type this answer ?

                  If I had a € for every time I heard "ThE PaTh Is CoRrEcT!!111eleven" and it turns out it's not, because currentPath() or applicationDirPath() etc. did not actually point to the assumed directory, I would have a surprising amount of money :D.

                  qDebug() << (QDir::currentPath() + "/Tools/script/main.py");
                  
                  AdithyaA Offline
                  AdithyaA Offline
                  Adithya
                  wrote on last edited by
                  #13

                  @J-Hilk // Path to the Python script you want to execute
                  QString pythonScript = QDir::currentPath() + "/Tools/Extractor_OD/main_od_for_rci.py";
                  qDebug() << "Script Path = " <<pythonScript;

                  Log : Script Path = "/home/adithya/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD/main_od_for_rci.py"

                  adithya@adithya:~/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD$ pwd
                  /home/adithya/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD
                  adithya@adithya:~/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD$ ls
                  copy_script RCI_Front_LUT.h RCI_Rear_LUT.h src
                  main_od_for_rci.py RCI_Left_LUT.h RCI_Right_LUT.h

                  I reallt checked multiple time .The oath is not the issue . You are not gettiing the € this time :p

                  1 Reply Last reply
                  0
                  • jsulmJ jsulm

                    @Adithya Then maybe your Python script exits with -2?
                    You can also connect a slot to https://doc.qt.io/qt-6/qprocess.html#stateChanged and log state changes.
                    And you can also connect slots to https://doc.qt.io/qt-6/qprocess.html#readyReadStandardError and https://doc.qt.io/qt-6/qprocess.html#readyReadStandardOutput and print out stderr/stdout from your process there.

                    AdithyaA Offline
                    AdithyaA Offline
                    Adithya
                    wrote on last edited by Adithya
                    #14

                    @jsulm I tried using stateChanged something like this
                    QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
                    qDebug() << "Process = "<< newState;
                    });
                    Log :Process = QProcess::Starting
                    ERROR: QProcess::UnknownError
                    Process = QProcess::NotRunning
                    Failed to start Python process
                    Python script execution failed with exit code: 255

                    jsulmJ 1 Reply Last reply
                    0
                    • AdithyaA Adithya

                      @jsulm I tried using stateChanged something like this
                      QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
                      qDebug() << "Process = "<< newState;
                      });
                      Log :Process = QProcess::Starting
                      ERROR: QProcess::UnknownError
                      Process = QProcess::NotRunning
                      Failed to start Python process
                      Python script execution failed with exit code: 255

                      jsulmJ Offline
                      jsulmJ Offline
                      jsulm
                      Lifetime Qt Champion
                      wrote on last edited by
                      #15

                      @Adithya Please share your current code, so we can see what exactly you're trying now.
                      Also try with an absolute path to the Python executable to make sure that is not the issue.

                      https://forum.qt.io/topic/113070/qt-code-of-conduct

                      AdithyaA 1 Reply Last reply
                      0
                      • AdithyaA Adithya

                        @jsulm Hi , the start() exit code is 255 and I am not using execute() . I am trying this in ubuntu 22 qt c++ .Python interpreter is in the path /usr/bin/python3.exe

                        echo $PATH
                        /opt/ros/noetic/bin:/home/adithya/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

                        So ideally python should take from PATH

                        JonBJ Offline
                        JonBJ Offline
                        JonB
                        wrote on last edited by JonB
                        #16

                        @Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

                        I am trying this in ubuntu 22 qt c++ .Python interpreter is in the path /usr/bin/python3.exe

                        You are under Ubuntu/Linux and the executable you want to run ends in .exe, right? Even though that is for Windows? So you can type /usr/bin/python3.exe in a terminal and that executes fine, right?

                        That aside. Since the Ubuntu executable is indeed python3 why are you trying to start a process named python?

                        #include <QCoreApplication>
                        #include <QDebug>
                        #include <QProcess>
                        
                        int main(int argc, char *argv[])
                        {
                            QCoreApplication a(argc, argv);
                        
                            QProcess process;
                            process.start("python", QStringList());
                            QObject::connect(&process, &QProcess::errorOccurred, &process, [&process]() { qDebug() << process.errorString();  } );
                            if (!process.waitForStarted()) {
                                 qDebug() << "Failed to start Python process";
                                 return 1;
                             }
                            process.waitForFinished();
                            int exitCode = process.exitCode();
                            if (exitCode != 0) {
                                qDebug() << "Python script execution failed with exit code:" << exitCode;
                                return 1;
                            }
                        
                            return a.exec();
                        }
                        

                        shows me

                        "execvp: No such file or directory"
                        Failed to start Python process
                        

                        So execvp() on python complains No such file or directory under Ubuntu 22.04, as I would expect.

                        Changing to process.start("python3", ...); produces no such error and runs the python3 interpreter....

                        1 Reply Last reply
                        3
                        • jsulmJ jsulm

                          @Adithya Please share your current code, so we can see what exactly you're trying now.
                          Also try with an absolute path to the Python executable to make sure that is not the issue.

                          AdithyaA Offline
                          AdithyaA Offline
                          Adithya
                          wrote on last edited by
                          #17

                          @jsulm
                          //trying it in main funtion itself for now , have commented everything else.
                          main.cpp
                          int main(int argc, char *argv[])
                          {
                          QApplication a(argc, argv);

                          // Path to the Python interpreter
                          QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe
                          
                          // Path to the directory containing your Python script and other files
                          QString workingDirectory = QDir::currentPath() + "/Tools/Extractor_OD";
                          qDebug() << "Path = " <<workingDirectory;
                          
                          // Path to the Python script you want to execute
                          QString pythonScript = QDir::currentPath() + "/Tools/Extractor_OD/main_od_for_rci.py";
                          qDebug() << "Script Path = " <<pythonScript;
                          
                          // Arguments to pass to the Python script
                          QStringList arguments;
                          arguments << pythonScript << "1" << "2";
                          
                          QString command = pythonScript + " 1 2";
                          
                          // Create a QProcess instance
                          QProcess process;
                          
                          QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
                                     qDebug() << "Process = "<< newState;
                             });
                          
                          // Set the working directory
                          //process.setWorkingDirectory(workingDirectory);
                          
                          // Set the command to execute
                          process.start(pythonInterpreter, arguments);
                          
                          QProcess::ProcessError error = process.error();
                          qDebug() << "ERROR: " << error;
                          
                          if (!process.waitForStarted()) {
                               qDebug() << "Failed to start Python process";
                               //return 1;
                           }
                          
                          // Wait for the process to finish (optional)
                          process.waitForFinished();
                          
                          // Get the exit code of the process
                          int exitCode = process.exitCode();
                          
                          // Handle the exit code if needed
                          if (exitCode != 0) {
                              qDebug() << "Python script execution failed with exit code:" << exitCode;
                              //return 1;
                          }
                          return a.exec();
                          

                          }

                          //Have commented out everything just trying to log for now
                          main_od_for_rci.py
                          if name == 'main':
                          print("Hey there")

                          jsulmJ JonBJ 2 Replies Last reply
                          0
                          • AdithyaA Adithya

                            @jsulm
                            //trying it in main funtion itself for now , have commented everything else.
                            main.cpp
                            int main(int argc, char *argv[])
                            {
                            QApplication a(argc, argv);

                            // Path to the Python interpreter
                            QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe
                            
                            // Path to the directory containing your Python script and other files
                            QString workingDirectory = QDir::currentPath() + "/Tools/Extractor_OD";
                            qDebug() << "Path = " <<workingDirectory;
                            
                            // Path to the Python script you want to execute
                            QString pythonScript = QDir::currentPath() + "/Tools/Extractor_OD/main_od_for_rci.py";
                            qDebug() << "Script Path = " <<pythonScript;
                            
                            // Arguments to pass to the Python script
                            QStringList arguments;
                            arguments << pythonScript << "1" << "2";
                            
                            QString command = pythonScript + " 1 2";
                            
                            // Create a QProcess instance
                            QProcess process;
                            
                            QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
                                       qDebug() << "Process = "<< newState;
                               });
                            
                            // Set the working directory
                            //process.setWorkingDirectory(workingDirectory);
                            
                            // Set the command to execute
                            process.start(pythonInterpreter, arguments);
                            
                            QProcess::ProcessError error = process.error();
                            qDebug() << "ERROR: " << error;
                            
                            if (!process.waitForStarted()) {
                                 qDebug() << "Failed to start Python process";
                                 //return 1;
                             }
                            
                            // Wait for the process to finish (optional)
                            process.waitForFinished();
                            
                            // Get the exit code of the process
                            int exitCode = process.exitCode();
                            
                            // Handle the exit code if needed
                            if (exitCode != 0) {
                                qDebug() << "Python script execution failed with exit code:" << exitCode;
                                //return 1;
                            }
                            return a.exec();
                            

                            }

                            //Have commented out everything just trying to log for now
                            main_od_for_rci.py
                            if name == 'main':
                            print("Hey there")

                            jsulmJ Offline
                            jsulmJ Offline
                            jsulm
                            Lifetime Qt Champion
                            wrote on last edited by
                            #18

                            @Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

                            /usr/bin/python3.exe

                            This is for sure wrong.
                            You're on Linux, not Windows...

                            https://forum.qt.io/topic/113070/qt-code-of-conduct

                            1 Reply Last reply
                            0
                            • AdithyaA Adithya

                              @jsulm
                              //trying it in main funtion itself for now , have commented everything else.
                              main.cpp
                              int main(int argc, char *argv[])
                              {
                              QApplication a(argc, argv);

                              // Path to the Python interpreter
                              QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe
                              
                              // Path to the directory containing your Python script and other files
                              QString workingDirectory = QDir::currentPath() + "/Tools/Extractor_OD";
                              qDebug() << "Path = " <<workingDirectory;
                              
                              // Path to the Python script you want to execute
                              QString pythonScript = QDir::currentPath() + "/Tools/Extractor_OD/main_od_for_rci.py";
                              qDebug() << "Script Path = " <<pythonScript;
                              
                              // Arguments to pass to the Python script
                              QStringList arguments;
                              arguments << pythonScript << "1" << "2";
                              
                              QString command = pythonScript + " 1 2";
                              
                              // Create a QProcess instance
                              QProcess process;
                              
                              QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
                                         qDebug() << "Process = "<< newState;
                                 });
                              
                              // Set the working directory
                              //process.setWorkingDirectory(workingDirectory);
                              
                              // Set the command to execute
                              process.start(pythonInterpreter, arguments);
                              
                              QProcess::ProcessError error = process.error();
                              qDebug() << "ERROR: " << error;
                              
                              if (!process.waitForStarted()) {
                                   qDebug() << "Failed to start Python process";
                                   //return 1;
                               }
                              
                              // Wait for the process to finish (optional)
                              process.waitForFinished();
                              
                              // Get the exit code of the process
                              int exitCode = process.exitCode();
                              
                              // Handle the exit code if needed
                              if (exitCode != 0) {
                                  qDebug() << "Python script execution failed with exit code:" << exitCode;
                                  //return 1;
                              }
                              return a.exec();
                              

                              }

                              //Have commented out everything just trying to log for now
                              main_od_for_rci.py
                              if name == 'main':
                              print("Hey there")

                              JonBJ Offline
                              JonBJ Offline
                              JonB
                              wrote on last edited by JonB
                              #19

                              @Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

                              QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe

                              You read my answer where it said .exe is for Windows not Linux, didn't you? What happened when you tried my:

                              So you can type /usr/bin/python3.exe in a terminal and that executes fine, right?

                              In a terminal please copy & paste:

                              ls -l /usr/bin/python3.exe
                              

                              and paste the output here.

                              AdithyaA 1 Reply Last reply
                              2
                              • JonBJ JonB

                                @Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

                                QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe

                                You read my answer where it said .exe is for Windows not Linux, didn't you? What happened when you tried my:

                                So you can type /usr/bin/python3.exe in a terminal and that executes fine, right?

                                In a terminal please copy & paste:

                                ls -l /usr/bin/python3.exe
                                

                                and paste the output here.

                                AdithyaA Offline
                                AdithyaA Offline
                                Adithya
                                wrote on last edited by Adithya
                                #20

                                @JonB @jsulm using python3 actually worked .But not sure whether the script is being executed or not
                                I changed .py file something like this (running in infinite loop)

                                if name == 'main':
                                while True:
                                print("Hey there")

                                The process is still running
                                Log :
                                Process = QProcess::Starting
                                ERROR: QProcess::UnknownError
                                Process = QProcess::Running

                                But any idea why I am not getting the log .

                                jsulmJ 1 Reply Last reply
                                0
                                • AdithyaA Adithya

                                  @JonB @jsulm using python3 actually worked .But not sure whether the script is being executed or not
                                  I changed .py file something like this (running in infinite loop)

                                  if name == 'main':
                                  while True:
                                  print("Hey there")

                                  The process is still running
                                  Log :
                                  Process = QProcess::Starting
                                  ERROR: QProcess::UnknownError
                                  Process = QProcess::Running

                                  But any idea why I am not getting the log .

                                  jsulmJ Offline
                                  jsulmJ Offline
                                  jsulm
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #21

                                  @Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

                                  running in infinite loop

                                  Please do NOT do such infinite loops in an event driven frameworks like Qt! There is really no need for that and it blocks Qt event loop.
                                  "I am not getting the log" - please explain how you're getting this log and what do you actually mean exactly by "log". If you mean the output of your script then I already suggested to read its stderr/stdout.

                                  https://forum.qt.io/topic/113070/qt-code-of-conduct

                                  1 Reply Last reply
                                  2
                                  • AdithyaA Offline
                                    AdithyaA Offline
                                    Adithya
                                    wrote on last edited by
                                    #22

                                    Thanks guys it worked. python3 and readyReadStandardOutput(showed the log ,was expecting it to work like qDebig() and cout .My bad) . Sorry for bothering with wrong misconception.

                                    1 Reply Last reply
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                                    • AdithyaA Adithya has marked this topic as solved on

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