How should QLocalSocket/QDataStream be read to avoid deadlocks?

Question

How should QLocalSocket/QDataStream be read?

I have a program that communicates with another via named pipes using QLocalSocket and QDataStream. The recieveMessage() slot below is connected to the QLocalSocket's readyRead() signal.

void MySceneClient::receiveMessage()
{    
    qint32 msglength;
    (*m_stream) >> msglength;

    char* msgdata = new char[msglength];
    int read = 0;
    while (read < msglength) {
        read += m_stream->readRawData(&msgdata[read], msglength - read);
    }
    ...
}

I find that the application sometimes hangs on readRawData(). That is, it succesfully reads the 4 byte header, but then never returns from readRawData().

If I add...

if (m_socket->bytesAvailable() < 5)
    return;

...to the start of this function, the application works fine (with the short test message).

I am guessing then (the documentation is very sparse) that there is some sort of deadlock occurring, and that I must use the bytesAvailable() signal to gradually build up the buffer rather than blocking.

Why is this? And what is the correct approach to reading from QLocalSocket?

score 2 · Accepted Answer · answered Dec 30 '16 at 13:54

Your loop blocks the event loop, so you will never get data if all did not arrive pn first read, is what causes your problem I think.

Correct approach is to use signals and slots, readyRead-signal here, and just read the available data in your slot, and if there's not enough, buffer it and return, and read more when you get the next signal.

Be careful with this alternative approach: If you are absolutely sure all the data you expect is going to arrive promptly (perhaps not unreasonable with a local socket where you control both client and server), or if the whole thing is in a thread which doesn nothing else, then it may be ok to use waitForReadyRead method. But the event loop will remain blocked until data arrives, freezing GUI for example (if in GUI thread), and generally troublesome.

thanks, it didn't occur to me it was *my* loop blocking (even though I saw it spinning round in the debugger!) and not readRawData(). — sebf, Dec 30 '16 at 14:23

How should QLocalSocket/QDataStream be read to avoid deadlocks?

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