QProcess C++ syntax - passing program and arguments

AnneRanch

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]

Christian Ehrlicher

@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!

Pablo J. Rogina

@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>

What's the program name?
What's the argument(s)?

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

AnneRanch

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

Pl45m4

@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")));

AnneRanch

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 .

Pl45m4

@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)?

AnneRanch

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

JonB

@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.

AnneRanch

@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

JonB

@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.

AnneRanch

@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

Christian Ehrlicher

@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!

JonB

@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.

AnneRanch

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.

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