Important: Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

QProcess C++ syntax - passing program and arguments



  • I am trying to utilize QProcess - replacing

    system ("hcitool scan ");

    with

    myProcess->start(program, arguments);

    while start is defined as

    start(QString, QStringList)

    Please help me to transfer original "system call" CORRECTLY to
    QProcess -> start.

    I am hopelessly lost.

    Cheers

    [Edit aha_1980: Fixed QProgress to QProcess]


  • Qt Champions 2019

    @AnneRanch said in QProcess C++ syntax - passing program and arguments:

    QProcess process;
    process.start("hciconfig", QStringList() << "-a");
    
    QString stdout = process.readAllStandardOutput();
    //   ui->plainTextEdit->appendPlainText(stdout);
    QString stderr = process.readAllStandardError();
    

    QProcess is asynchron! Read the docs!



  • @AnneRanch said in QProgress C++ syntax - passing program and arguments:

    system ("hcitool scan ");

    I guess you will be able to answer the following questions yourself from previous snippet>

    1. What's the program name?
    2. What's the argument(s)?

    so you will be able to fill in the proper values to call QProgress then



  • Here is my implementation,
    system("hciconfig -a"); works as expected

    process.start((QString) "system" , (QStringList) "hciconfig -a " );

    validates
    if(stdout.isEmpty())

    as empty

        std::cerr<< "String is empty " << std::endl;
    

    #ifdef TRACE
    std::cerr <<"TRACE \nSTART QProcess \nfunction "<< FUNCTION << " \nfile " << FILE<< " \nline # "<< LINE<< std::endl;
    #endif
    system("hciconfig -a");
    //#ifdef BYPASS
    QProcess process;
    process.start((QString) "system" , (QStringList) "hciconfig -a " );
    process.waitForFinished(-1); // will wait forever until finished

    QString stdout = process.readAllStandardOutput();
    

    // ui->plainTextEdit->appendPlainText(stdout);
    QString stderr = process.readAllStandardError();
    if(stdout.isEmpty())
    std::cerr<< "String is empty " << std::endl;
    ui->plainTextEdit->setPlainText("TEST"); // (QString) stderr);
    ui->plainTextEdit->appendPlainText((QString) stderr);
    ui->plainTextEdit->appendPlainText((QString) stdout);

    process.close();
    

    #ifdef TRACE
    std::cerr <<"TRACE \nEND finished QProcess \nfunction "<< FUNCTION << " \nfile " << FILE<< " \nline # "<< LINE<< std::endl;
    #endif



  • @AnneRanch said in QProgress C++ syntax - passing program and arguments:

    while start is defined as
    start(QString, QStringList)

    That's correct.

    So what do you expect from doing

    process.start((QString) "system" , (QStringList) "hciconfig -a " );

    ?
    (hciconfig is your command, not system)

    Try (this is in terms of "how to start a QProcess" the correct way - But I don't know if your command is correct):

    // declare + define your programm string (name of program)
    QString cmd = "hciconfig"; 
    
    // declare a StringList
    QStringList args; 
    
    // Append argument "-a" to stringList
    args << "-a";
    
    // start process
    process.start(cmd, args);
    

    Edit:
    C-style casts (e.g. (QString) test) are not safe to use in C++. Better use the QString - constructors to create your string or the way, I've used above.
    process.start(QString("hciconfig"), QStringList(QString("-a")));



  • Here is my test implementation

       ui->setupUi(this);
        system ("hciconfig -a");
        // declare + define your programm string (name of program)
        qDebug ("declare + define your programm string (name of program ");\
        
        QProcess process;
        QString cmd = "hciconfig";
        // declare a StringList
        QStringList args;
        // Append argument "-a" to stringList
        args << "-a";
          // start process
        process.start(cmd, args);
        //   QProcess process;
        //   process.start((QString) "system" , (QStringList)  "hciconfig -a " );
        //    process.waitForFinished(-1); // will wait forever until finished
         QString stdout = process.readAllStandardOutput();
        //   ui->plainTextEdit->appendPlainText(stdout);
        QString stderr = process.readAllStandardError();
        if(stdout.isEmpty())
        {
            std::cerr<< "stdout String is empty " << std::endl;
            std::cout<< "stdout String is empty " << std::endl;
        }
        if(stderr.isEmpty())
        {
            std::cerr<< "stderr String is empty " << std::endl;
            std::cout<< "stderr String is empty " << std::endl;
        }
         std::cout << " Actual process output "<< stdout.toStdString()<< std::endl;
        std::cout << " Actual process output "<< stderr.toStdString()<< std::endl;
           stdout = "TEST DUMMY stdout output ";
        std::cout << " Actual process output "<< stdout.toStdString()<< std::endl;
        std::cout << " Actual process output "<< stderr.toStdString()<< std::endl;
         //    ui->plainTextEdit->setPlainText("TEST"); // (QString) stderr);
        //    ui->plainTextEdit->appendPlainText((QString) stderr);
        //    ui->plainTextEdit->appendPlainText((QString) stdout);
        
        process.close();
        qDebug ("declare + define your programm string (name of program ");
        exit(53);
        
    

    And here is the concole output

    Starting /media/z/DEV_COPY_LABEL/Qt/QT/qtconnectivity/examples/bluetooth/build-CAT_BT-Desktop-Debug/btscanner...
    hci0:	Type: BR/EDR  Bus: USB
    	BD Address: 00:15:83:15:A2:CB  ACL MTU: 672:4  SCO MTU: 48:1
    	UP RUNNING PSCAN ISCAN 
    	RX bytes:5516 acl:0 sco:0 events:84 errors:0
    	TX bytes:790 acl:0 sco:0 commands:77 errors:0
    	Features: 0xff 0x3e 0x85 0x38 0x18 0x18 0x00 0x00
    	Packet type: DM1 DM3 DM5 DH1 DH3 DH5 HV1 HV2 HV3 
    	Link policy: RSWITCH HOLD SNIFF 
    	Link mode: SLAVE ACCEPT 
    	Name: 'z-desktop'
    	Class: 0x1c0104
    	Service Classes: Rendering, Capturing, Object Transfer
    	Device Class: Computer, Desktop workstation
    	HCI Version: 2.0 (0x3)  Revision: 0xc5c
    	LMP Version: 2.0 (0x3)  Subversion: 0xc5c
    	Manufacturer: Cambridge Silicon Radio (10)
    
    stdout String is empty 
    stderr String is empty 
     Actual process output 
     Actual process output 
     Actual process output TEST DUMMY stdout output 
     Actual process output 
    declare + define your programm string (name of program 
    stdout String is empty 
    stderr String is empty 
    declare + define your programm string (name of program 
    /media/z/DEV_COPY_LABEL/Qt/QT/qtconnectivity/examples/bluetooth/build-CAT_BT-Desktop-Debug/btscanner exited with code 53
    

    I believe I am missing instructing the "process" to run "system" function with "hci.." as parameter.
    My syntax MUST be wrong and I really do not know how to fix it.

    Cheers .



  • @AnneRanch

    hciconfig -a did exactly what it is supposed to do

    hciconfig prints name and basic information about all the Bluetooth devices installed in the system. If hciX is given but no command is given, it prints basic information on device hciX only. Basic information is interface type, BD address, ACL MTU, SCO MTU, flags (up, init, running, raw, page scan enabled, inquiry scan enabled, inquiry, authentication enabled, encryption enabled).

    (https://linux.die.net/man/8/hciconfig)

    You need some more arguments / parameters. hciconfig -a just prints the whole (-a) configuration.

    But:
    Why you want to deal with BlueZ directly, when you can use the Qt interface, which will use BlueZ for QBluetooth (since you are on Linux)?



  • You need some more arguments / parameters. hciconfig -a just prints the whole (-a) configuration.

    That is NOT the issue , I am using hciconfig -a as an example.
    The issue is how to correctly implement " system " call and retrieve the response for further processing .

    I have a reason to deal direct with blueZ , not related to this post.

    After looking at the attached link, I have a sneaky suspicion that same apply for redirecting "system calls" using QProgress. QProgress is probably implemented by some of the methods discussed there.

    I am learning the hard way that SOME of the "Qtx" wrappers of low level code are really not helping, especially when the implementation is not explained and one feels like it is all high level magic.

    In this case I really only want to pass "hci..." output to file for further processing

    In crudest pseudocode
    hci command output redirect to file
    by whatever method.

    And I did picked QProgress and having an issue with passing the parameters to it.

    THAT is the problem - the parameter passing syntax of ANY external instructions / parameters / arguments to QProgress

    https://stackoverflow.com/questions/2655374/how-to-redirect-the-output-of-a-system-call-to-inside-the-program-in-c-c



  • @AnneRanch
    If all you want to do is execute a single, simple command, you can indeed do that via

    QProcess process;
    process.start("hciconfig", QStringList() << "-a");
    

    and then arrange to send the bytes received to some redirection file you create from your Qt program via QProcess::setStandardOutputFile(). That is probably the simplest in this case.

    However, if you want to do "hci command output redirect to file" where the redirection is done for you, like system("hciconfig -a > file") would do for you, you need the shell to interpret and act on a symbol like >. That is actually what the C/C++ system() call does. So, e.g if you are under Linux, you would need:

    // next line for Linux
    process.start("/bin/sh", QStringList() << "-c" << "hciconfig -a > file");
    
    // or, next line for Windows
    process.start("cmd", QStringList() << "/c" << "hciconfig -a > file");
    

    This is the true equivalent of what system(string) does. Note in this case the command you want executed is passed as a single argument to /bin/sh -c "command-line", with suitable quoting as necessary for the shell. Among other things, you need to follow this route if you wish to execute a command containing >, < or | redirection symbols, as well as other situations.



  • @JonB said in QProcess C++ syntax - passing program and arguments:

    @AnneRanch
    If all you want to do is execute a single, simple command, you can indeed do that via

    QProcess process;
    process.start("hciconfig", QStringList() << "-a");
    

    and then arrange to send the bytes received to some redirection file you create from your Qt program via QProcess::setStandardOutputFile(). That is probably the simplest in this case.

    However, if you want to do "hci command output redirect to file" where the redirection is done for you, like system("hciconfig -a > file") would do for you, you need the shell to interpret and act on a symbol like >. That is actually what the C/C++ system() call does. So, e.g if you are under Linux, you would need:

    // next line for Linux
    process.start("/bin/sh", QStringList() << "-c" << "hciconfig -a > file");
    
    // or, next line for Windows
    process.start("cmd", QStringList() << "/c" << "hciconfig -a > file");
    

    This is the true equivalent of what system(string) does. Note in this case the command you want executed is passed as a single argument to /bin/sh -c "command-line", with suitable quoting as necessary for the shell. Among other things, you need to follow this route if you wish to execute a command containing >, < or | redirection symbols, as well as other situations.

    Sorry about mixing process and progress.

    I am trying to VERIFY process.star(....) using process.read...

        QProcess process;
        process.start((QString) "system" , (QStringList)  "hciconfig -a " );
        process.waitForFinished(-1); // will wait forever until finished
    
        **QString stdout = process.readAllStandardOutput();
     //   ui->plainTextEdit->appendPlainText(stdout);
        QString stderr = process.readAllStandardError();**
    

    And both stdout an stderr are empty.

    I figured my usage of start is wrong or my read.. is wrong .
    Perhaps I need another way to verify this process.

    Cheers



  • @AnneRanch

    process.start((QString) "system" , (QStringList)  "hciconfig -a " );
    

    You usage of system as the command to execute is mistaken.

    I showed you what will work (guessing you are Linux):

    QProcess process;
    process.start("hciconfig", QStringList() << "-a");
    // or
    process.start("/bin/sh", QStringList() << "-c" << "hciconfig -a > file");
    

    The second case above is (approximately, but very close) what the C/C++ library function system() does.



  • @JonB said in QProcess C++ syntax - passing program and arguments:

    process.start("hciconfig", QStringList() << "-a");

    Code :

        qDebug ("START NEW process");
        QProcess process;
        process.start("hciconfig", QStringList() << "-a");
        
        QString stdout = process.readAllStandardOutput();
        //   ui->plainTextEdit->appendPlainText(stdout);
        QString stderr = process.readAllStandardError();
        if(stdout.isEmpty())
        {
            std::cerr<< "stdout String is empty " << std::endl;
            std::cout<< "stdout String is empty " << std::endl;
        }
        if(stderr.isEmpty())
        {
            std::cerr<< "stderr String is empty " << std::endl;
            std::cout<< "stderr String is empty " << std::endl;
        }
        
        std::cout << " Actual process output "<< stdout.toStdString()<< std::endl;
        std::cout << " Actual process output "<< stderr.toStdString()<< std::endl;
        
        stdout = "TEST DUMMY stdout output ";
        
        std::cout << " Actual process output "<< stdout.toStdString()<< std::endl;
        std::cout << " Actual process output "<< stderr.toStdString()<< std::endl;
        
        
        
        
        
        //    ui->plainTextEdit->setPlainText("TEST"); // (QString) stderr);
        //    ui->plainTextEdit->appendPlainText((QString) stderr);
        //    ui->plainTextEdit->appendPlainText((QString) stdout);
        
        process.close();
        qDebug( "END  NEW process");
    

    Result

    Starting /media/z/DEV_COPY_LABEL/Qt/QT/qtconnectivity/examples/bluetooth/build-CAT_BT-Desktop-Debug/btscanner...
    START NEW process
    **stdout String is empty 
    stderr String is empty** 
     Actual process output 
     Actual process output 
     Actual process output TEST DUMMY stdout output 
     Actual process output 
    TRACE 
    START Orignal constructor 
    function  DeviceDiscoveryDialog 
    file ../CAT_BT_892020/device.cpp 
    line # 91
    stdout String is empty 
    stderr String is empty 
    END  NEW process
    

  • Qt Champions 2019

    @AnneRanch said in QProcess C++ syntax - passing program and arguments:

    QProcess process;
    process.start("hciconfig", QStringList() << "-a");
    
    QString stdout = process.readAllStandardOutput();
    //   ui->plainTextEdit->appendPlainText(stdout);
    QString stderr = process.readAllStandardError();
    

    QProcess is asynchron! Read the docs!



  • @AnneRanch
    At one point earlier you had code:

    process.waitForFinished(-1); // will wait forever until finished
    

    Although this is not the best way to do things, from where you are now as a one-liner: put this line immediately after process.start("hciconfig", QStringList() << "-a");, so as to get the output from the command successfully.



  • OK, finally an answer matching the question - WHAT is the correct syntax.

    process.start("hciconfig", QStringList() << "-a");

    I am not comfortable using the << operator , hence this also works:
    process.start("hciconfig", QStringList("-a"));

    Unfortunately QProcess , and others, won't work replacing this
    system call

    system("hcitool scan ");

    with
    start("hcitool" , QSstringList (" scan"));

    It does work as command only - "hcitool scan" outputs
    "Scanning..." immediately and
    bluetooth device info AFTER it is physically detected - few seconds later

    BUT this issue is now solved , thanks.


Log in to reply