QProcess give FailedToStart after starting multiple times - c++

Question

I'm trying to use QProcess inside a thread to do some operations (read I2C connections).The update method is calling every 100 msec:
void TempsReader::update()
{
if (_currProcess == nullptr) {
_currProcess = new QProcess();
connect(_currProcess, &QProcess::errorOccurred, this, &TempsReader::onProcessError);
connect(_currProcess, SIGNAL(finished(int,QProcess::ExitStatus)),
this, SLOT(onProcessFinished()));
}
_currProcess->start("sh");
if (_currProcess->waitForStarted()) {
_currProcess->write("i2cdetect -y 1");
_currProcess->closeWriteChannel();
_currProcess->waitForFinished();
}
}
After some time, the process gives "FailedToStart" error and never starts again.
void TempsReader::onProcessError(QProcess::ProcessError error)
{
qDebug() << error;
_currProcess->close();
}
void TempsReader::onProcessFinished()
{
QString devs = _currProcess->readAll();
_currProcess->waitForFinished();
// doing some stuff with devs
_currProcess->close();
}
How can I fix this issue? Am I using QProcess in a wrong way? and how can I start the process again when it drops in error slot. Thanks in advance.
Update: QProcess::errorString() gives this: "Resource error (fork failure): Too many open files"
UPDATE: Finally I've found the issue and it was not related to QProcess itself. It was relating to I2C connection.

Answer 1

My guess is that you get the failure because all your update() calls share the same QProcess object.
What happens here is that when you call update(), you start the process. And 100ms later, you call it again without ensuring that the previous update() has finished to wait the end of the process.
The consequence is that you try to start an already started process and thus it fails.
For me, the easiest solution is to create one QProcess object for each update() call.
Something like:
void TempsReader::update()
{
QProcess * current_process = new QProcess;
connect(current_process, &QProcess::errorOccured, this, &TempReader::onProcessError);
connect(current_process, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), this, &TempReader::onProcessFinished());
current_process->start("sh"); // Your command
current_process->waitForStarted();
current_process->write("i2cdetect -y 1");
current_process->waitForFinished();
current_process->deleteLater();
}
Or without pointers:
void TempsReader::update()
{
QProcess current_process;
connect(&current_process, &QProcess::errorOccured, this, &TempReader::onProcessError);
connect(&current_process, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), this, &TempReader::onProcessFinished());
current_process.start("sh"); // Your command
current_process.waitForStarted();
current_process.write("i2cdetect -y 1");
current_process.waitForFinished();
}
As you did not show the calling part of the code (the thread creation, the 100ms loop, ...), this may not be the solution you need.
In this case, please let me know if it does not solve your issue so that I'll remove this answer.

Answer 2

Finally I've found the issue and it was not related to QProcess itself. It was relating to I2C connection. I was using this command in update: wiringPiI2CSetup(addr); and it opens a new device each time.