Using QIODevice with anonymous pipes

Sweepi

I want to use a QIODevice in order to read from a unnamed pipe if data is available.
I tried this with QFile:

m_pFile_Pipe = new QFile();
HANDLE hRead, hWrite;
connect(m_pFile_Pipe, SIGNAL(readyRead()), this, SLOT(OutputAvailable_QFile()));

SECURITY_ATTRIBUTES sa;
sa.nLength = sizeof(SECURITY_ATTRIBUTES);
sa.lpSecurityDescriptor = NULL;
sa.bInheritHandle = TRUE;

CreatePipe(&hRead, &hWrite, &sa, 0);
auto fhRead = _open_osfhandle((intptr_t)hRead, _O_RDONLY);
auto OpenResult = m_pFile_Pipe->open(fhRead, QIODevice::ReadOnly);

The "raw" pipe itself works, I can read data from it.
However, readyRead() is never signaled and during testing:

void TestPipe() {
    char acBuffer[128];
    DWORD NumBytesRead;
    
    auto NumBytes = m_pFile_Pipe->bytesAvailable();
    qDebug() << "NumBytes" << NumBytes;
    ReadFile(hRead, acBuffer, sizeof(acBuffer), &NumBytesRead, NULL);
    qDebug() << QString::fromUtf8(acBuffer);
    while (m_pFile_Pipe->canReadLine()) {
        auto out = m_pFile_Pipe->readLine(512);
        qDebug() << "Line: " << out;
    }
    auto out_all = m_pFile_Pipe->readAll();
    qDebug() << "Raw: " << out_all;
}

NumBytes was always 0, canReadLine() always returned false, and readAll() did not return.
ReadFile() could read the expected data.

Is there a QOIDevice to use for anonymous pipes?

kshegunov

@Sweepi said in Using QIODevice with anonymous pipes:

Is there a QOIDevice to use for anonymous pipes?

I don't believe so. Probably cleanest is to implement your own QIODevice. However there are questions about the code you posted.

For one what is OpenResult's value? I wouldn't expect the file to be opened multiple times successfully.
And secondly, in your test code you don't spin the event loop so I see no reason to expect any signals to be emitted from the QFile.
And lastly, QFile is a random access device, while a pipe is not, so weird behavior isn't unexpected.

Sweepi

@kshegunov

OpenResult was always true.
However, since you mentioned that QFile and the pipe handle is a mismatch, I guess this does not mean too much.

I stripped the code (in the starting post) down a little bit (since it contained unrelated "noise"), the event loop was working in my actual code, e.g. TestPipe() was called from a timer.

Probably cleanest is to implement your own QIODevice.

Can you point me to sth. similar where I can get ideas?
E.g. I have no concept of how I would know that there is data available but to "poll" the pipe, which seems wrong.

And lastly, QFile is a random access device, while a pipe is not, so weird behavior isn't unexpected.

Actually, I have a follow-up question for this one:
stderr etc. are also sequential devices, aren't they?
I am wondering, because I remember an Qt example to open stderr using QFile.

kshegunov

@Sweepi said in Using QIODevice with anonymous pipes:

Can you point me to sth. similar where I can get ideas?

Any of the socket classes, or QProcess's stream reading/writing should be sufficient to get you started. For example, take a look here https://code.woboq.org/qt5/qtbase/src/corelib/io/qprocess.cpp.html#_ZN15QProcessPrivate18tryReadFromChannelEPNS_7ChannelE

E.g. I have no concept of how I would know that there is data available but to "poll" the pipe, which seems wrong.

You should try with QSocketNotifier.

stderr etc. are also a sequential devices, aren't they?

Yes.

I am wondering, because I remember an Qt example to open stderr using QFile.

Maybe, I don't know which example you mean. I open the standard streams through QTextStream directly, like this:

QTextStream out(stdout);

However I haven't checked what's the underlying code. Maybe you could check that too, I'd be interested to know as well.

Sweepi

@kshegunov

Maybe, I don't know which example you mean.

Found it:
https://doc.qt.io/qt-5/qfile.html#open-1

But maybe this is different since stderr is opened with QIODevice::WriteOnly, so there is no random access possible anyway.

Thanks for all the hints, going to dig through this now!