QProcess not working.

[SPlatten]

I am using Qt 5.9.2 with MSVC2015 64bit. I am trying to execute mysqldump using QProcess. I've tried several different things, below is the latest code, none of these work and in the Event View I see:

Log Name:      Application
Source:        MariaDB
Date:          26/07/2021 08:18:28
Event ID:      100
Task Category: None
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      LHR1-EUD-L20381
Description:
Aborted connection 84 to db: 'training' user: 'trainer' host: 'localhost' (Got an error reading communication packets)
 
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="MariaDB" />
    <EventID Qualifiers="49152">100</EventID>
    <Version>0</Version>
    <Level>3</Level>
    <Task>0</Task>
    <Opcode>0</Opcode>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2021-07-26T07:18:28.6591139Z" />
    <EventRecordID>149973</EventRecordID>
    <Correlation />
    <Execution ProcessID="0" ThreadID="0" />
    <Channel>Application</Channel>
    <Computer>LHR1-EUD-L20381</Computer>
    <Security />
  </System>
  <EventData>
    <Data>Aborted connection 84 to db: 'training' user: 'trainer' host: 'localhost' (Got an error reading communication packets)
 
</Data>
  </EventData>
</Event>

My code:

process* pobjProcess(new process(this));
            if ( pobjProcess != nullptr )
            {
                QString strApp(DataSets::mscszMariaDBdumpApp);
                QStringList slstArguments;
                slstArguments << ("\"" + strDBinstFolder + strApp + "\"")
                              << (QString(DataSets::mscszOptionHost) + Trainer::strHost())
                              << (QString(DataSets::mscszOptionUser) + Trainer::strUser())
                              << (QString(DataSets::mscszOptionPassword) + Trainer::strPassword())
                              << (Trainer::strDatabase())
                              << (QString(">") + strFilename);
                strApp = "";
                foreach( QString strArg, slstArguments )
                {
                    if ( strApp.isEmpty() != true )
                    {
                        strApp += " ";
                    }
                    strApp += strArg;
                }
qdbg() << strApp;
                pobjProcess->setWorkingDirectory(strDBinstFolder);
                pobjProcess->setArguments(slstArguments);
                pobjProcess->setProgram(strApp);
                pobjProcess->start();
                QObject::connect(pobjProcess, &QProcess::stateChanged,
                    [pobjProcess](QProcess::ProcessState newState)
                    {
                        const qint64 cint64PID(pobjProcess->processId());
                        if ( newState == QProcess::NotRunning )
                        {
 
                        }
                        else if ( newState == QProcess::Running && cint64PID > 0 )
                        {
 
                        }
qdbg() << "PID: " << cint64PID << ", newState: " << newState;
                    }
                );
            }

In the console I am seeing:

PID: 4968, newState:  QProcess::ProcessState(NotRunning)

The actual command line is:

"C:/Program Files (x86)/MariaDB 10.5/bin/mysqldump" --host=localhost --user=trainer --password=CobhamAC training >C:/Users/simon.platten/Documents/TrainerS1D5.sql

If I copy the above and paste into a command prompt, it works. But not from the application.

[JonB]

@SPlatten said in QProcess not working.:

@JonB , where are you getting process and exec from?

It's an example of QProcess::exec(). Actually that's static. Exactly the same principle if you want to use pobjProcess->start();. It's the argument passing I'm trying to tell you about.

if I copy the command line and arguments to a command prompt it works,

Yes, as I said this will (only) work from cmd.

                          << QString(">")

                          << strFilename;

As I said, this won't work for mysqldump run as an executable. It will only work via cmd /c .... Up to you whether you believe me :)

[J.Hilk]

@SPlatten said in QProcess not working.:

pobjProcess->setArguments(slstArguments);
pobjProcess->setProgram(strApp);

hi,

you do realise, that at this point strApp also has the arguments appended to it?

it should be only the program, and the arguments as string list separately

[KroMignon]

@SPlatten said in QProcess not working.:

If I copy the above and paste into a command prompt, it works. But not from the application.

I have 2 remarks:

1. I am always surprise about your coding style, it seems that you always have to overload every Qt base class. If I have right understood your code, process is a sub-class of QProcess.
2. It looks to me as the application is also part of the arguments, why?

[jsulm]

@SPlatten Why don't you connect a slot to https://doc.qt.io/qt-5/qprocess.html#errorOccurred and check what https://doc.qt.io/qt-5/qiodevice.html#errorString returns?

I would split "<< (QString(">") + strFilename);" into two parameters.

process* pobjProcess(new process(this));
if ( pobjProcess != nullptr )

Why do you check pobjProcess pointer after alloocating memory? This is useless as in case of out of memory your app will be terminated anyway.

[J.Hilk]

@jsulm said in QProcess not working.:

This is useless as in case of out of memory your app will be terminated anyway.

not necessarily
I see 2 cases where one would do such a thing

• if the OP compiles with -fno-exceptions than it will be indeed a null pointer returned instead of an exception
• If the OP overloaded new and explicitly returns nullptr if some condition is met/not met

[JonB]

@SPlatten said in QProcess not working.:

"C:/Program Files (x86)/MariaDB 10.5/bin/mysqldump" --host=localhost --user=trainer --password=CobhamAC training >C:/Users/simon.platten/Documents/TrainerS1D5.sql

You have chosen to use a DOS/Windows > redirection symbol on your command line. This cannot be executed directly, i.e. executing mysqldump and passing >... to it.

If you do it this way you must execute via

QStringList args { "/c", "\"C:\\Program Files (x86)\\MariaDB 10.5\\bin\\mysqldump\" --host=localhost --user=trainer --password=CobhamAC training >C:\\Users\\simon.platten\\Documents\\TrainerS1D5.sql"  };
process->exec("cmd", args);

Or handle the output redirection yourself in the caller, or via an "output-to" argument to mysqldump if it supports that.

[SPlatten]

@J-Hilk , yes, it didn't originally, I add those later just to see if it made any difference.

[SPlatten]

@JonB , where are you getting process and exec from?

[SPlatten]

I've changed the code back to:

process* pobjProcess(new process(this));
            if ( pobjProcess != nullptr )
            {
                QString strApp(DataSets::mscszMariaDBdumpApp);
                QStringList slstArguments;
                slstArguments << ("\"" + strDBinstFolder + strApp + "\"")
                              << (QString(DataSets::mscszOptionHost) + Trainer::strHost())
                              << (QString(DataSets::mscszOptionUser) + Trainer::strUser())
                              << (QString(DataSets::mscszOptionPassword) + Trainer::strPassword())
                              << (Trainer::strDatabase())
                              << QString(">")
                              << strFilename;
qdbg() << slstArguments;
qdbg() << strApp;
                pobjProcess->setWorkingDirectory(strDBinstFolder);
                pobjProcess->setArguments(slstArguments);
                pobjProcess->setProgram(strApp);
                pobjProcess->start();
                QObject::connect(pobjProcess, &QProcess::errorOccurred,
                     [this](QProcess::ProcessError err)
                    {
qdbg() << "Error: " << err;
                    }
                );
                QObject::connect(pobjProcess, &QProcess::stateChanged,
                    [pobjProcess](QProcess::ProcessState newState)
                    {
                        const qint64 cint64PID(pobjProcess->processId());
                        if ( newState == QProcess::NotRunning )
                        {
 
                        }
                        else if ( newState == QProcess::Running && cint64PID > 0 )
                        {
 
                        }
qdbg() << "PID: " << cint64PID << ", newState: " << newState;
                    }
                );
            }

Still doesn't work and I'm not seeing anything errorOccurred

[KroMignon]

@SPlatten said in QProcess not working.:

yes, it didn't originally, I add those later just to see if it made any difference.

I don't really know what is the purpose of this, I guess you have to get SQL statements from you DB.

My way to achieve it would be:

• start mysqldump with QProcess
• wait until QProcess / mysqldump finished
• read QProcess output (cf. QProcess::readAll()) ==> no need of redirection at all

And last remark:
you have to do connection before starting process, to be sure not to loose state changes ;)

[SPlatten]

@KroMignon I don't follow you, if I copy the command line and arguments to a command prompt it works, the arguments are everything it needs for mysqldump to connect and export the database.

[JonB]

@SPlatten said in QProcess not working.:

@JonB , where are you getting process and exec from?

It's an example of QProcess::exec(). Actually that's static. Exactly the same principle if you want to use pobjProcess->start();. It's the argument passing I'm trying to tell you about.

if I copy the command line and arguments to a command prompt it works,

Yes, as I said this will (only) work from cmd.

                          << QString(">")

                          << strFilename;

As I said, this won't work for mysqldump run as an executable. It will only work via cmd /c .... Up to you whether you believe me :)

[KroMignon]

@SPlatten said in QProcess not working.:

I don't follow you, if I copy the command line and arguments to a command prompt it works, the arguments are everything it needs for mysqldump to connect and export the database.

With QProcess, you don't start the process from a shell / command prompt, that is the difference here.
Try this:

auto proc = new QProcess;
QObject::connect(proc, &QProcess::stateChanged,
                    [proc](QProcess::ProcessState newState)
{
    qDebug() << "new state is " <<  newState;
    if(newState == QProcess::NotRunning)
    {
        QString output(proc->readAll());
        qDebug() << "Output is" << output;
        proc->delateLater();
    }
});
proc->setWorkingDirectory(strDBinstFolder);
proc->start(strApp,slstArguments );

EDIT: do NOT add redirection in slstArguments !!!

EDIT BIS:

QStringList slstArguments;
slstArguments << (QString(DataSets::mscszOptionHost) + Trainer::strHost())
                              << (QString(DataSets::mscszOptionUser) + Trainer::strUser())
                              << (QString(DataSets::mscszOptionPassword) + Trainer::strPassword())
                              << (Trainer::strDatabase());

==> I suppose that DataSets::mscszOptionHost contains --host=

[SPlatten]

@JonB , not in Qt 5.9.2 it ins't, execute ?

[JonB]

@SPlatten
int QProcess::execute(const QString &program, const QStringList &arguments) [static]. So it's execute. It was introduced into some Qt. I don't know about 5.9.2.

As I said, forget I ever wrote exec or execute. The same argument processing applies to QProcess::start. You keep passing > argument, and I've explained this won't work.

[SPlatten]

@KroMignon , I've tried this:

QString strApp(DataSets::mscszMariaDBdumpApp);
QStringList slstArguments;
slstArguments << ("\"" + strDBinstFolder + strApp + "\"")
              << (QString(DataSets::mscszOptionHost) + Trainer::strHost())
              << (QString(DataSets::mscszOptionUser) + Trainer::strUser())
              << (QString(DataSets::mscszOptionPassword) + Trainer::strPassword())
              << (Trainer::strDatabase())
              << QString(">")
              << strFilename;
pobjProcess->setWorkingDirectory(strDBinstFolder);
pobjProcess->start(strApp, slstArguments);
QObject::connect(pobjProcess, &QProcess::errorOccurred,
    [this](QProcess::ProcessError err) {
qDebug() << "Error: " << err;
});
QObject::connect(pobjProcess, &QProcess::stateChanged,
    [pobjProcess](QProcess::ProcessState newState)  {
        const qint64 cint64PID(pobjProcess->processId());
        if ( newState == QProcess::NotRunning )  {
        }  else if ( newState == QProcess::Running && cint64PID > 0 )  {
        }
qDebug() << "PID: " << cint64PID << ", newState: " << newState;
});

[jsulm]

@SPlatten Did you notice "EDIT: do NOT add redirection in slstArguments !!!" in @KroMignon post?
Why do you need redirection to a file if you can simply read the stdout from the process as @KroMignon already suggested?

[KroMignon]

@SPlatten This don't make sense to me:

• Why do you add application into argument list?
• Why to you redirect it to a file and not directly read it for QProcess stream?

[SPlatten]

@jsulm The purpose of this is to provide an export / backup facility of the database to an SQL file.

[jsulm]

@SPlatten You can still write the stdout from the process to a file...