Question

I have an application which runs on a controlling hardware connected with different sensors. On loading the application, it checks the individual sensors one by one to see whether there is proper communication with the sensor according to predefined protocol or not.

Now, I have implemented the code for checking the individual sensor communication as a singleton thread and following is the run function, it used select system call and pipe for interprocess communication to signal the end of thread.

void SensorClass::run()
{
    mFdWind=mPort->GetFileDescriptor();
    fd_set readfs;
    int max_fd = (mFdWind > gPipeFdWind[0] ? mFdWind : gPipeFdWind[0]) + 1;
    int res;

    mFrameCorrect=false;
    qDebug("BEFORE WHILE"); 
    while(true)
    {
        qDebug("\n IN WHILE LOOP"); 
        usleep(50);
        FD_ZERO(&readfs);
        FD_SET(mFdWind,&readfs);
        FD_SET(gPipeFdWind[0],&readfs);

        res=select(max_fd,&readfs,NULL,NULL,NULL);
        if(res < 0)
            perror("Select Failed");
        else if(res == 0)
            puts("TIMEOUT");
        else
        {
            if(FD_ISSET(mFdWind,&readfs))
            {
                puts("*************** RECEIVED DATA ****************");
                mFrameCorrect=false;
                FlushBuf();
                //int n=mPort->ReadPort(mBuf,100);
                int n=mPort->ReadPort(mBuf,100);

                if(n>0)
                {
                    Count++;


                    QString str((const char*)mBuf);
                    //qDebug("\n %s",qPrintable(str));
                    //See if the Header of the frame is valid
                    if(IsHeaderValid(str))
                    {
                        if( (!IsCommaCountOk(str)) || (!IsChecksumOk(str,mBuf)) ||  (!CalculateCommaIndexes(str))  ) 
                        {

                            qDebug("\n  not ok");
                            mFrameCorrect=false;
                        } //if frame is incorrect
                        else 
                        {
                            qDebug("\n  OK");
                            mFrameCorrect=true;
                        }//if frame is correct(checksum etc are ok)
                    }//else if header is ok
                }//if n > 0
            }//if data received FD_ISSET
            if(FD_ISSET(gPipeFdWind[0],&readfs))
                break;
        }//end nested else res not <= 0
    }//infinite loop  
}

The above thread is run started from the main GUI thread. This runs fine. The problem is I have given an option to the user to retest the subsystem at will. For this I delete the singleton instance using

delete SensorClass::instance();

and then restart the singleton using

SensorClass::instace()->start();

The problem is this time the control comes out of while loop in run() function immedeately upon entering the while loop, my guess is the pipe read has again read from the write pipe which was written to the last time. I have tried to use the fflush() to clear out the I/O but no luck.
My question is

1. Am I thinking on the right track?

2. If yes then how do we clear out the pipes?

3. If not can anyone suggest why is the selective retest not working?

Thanks in advance..

Answer 1

fflush clears the output buffer. If you want to clear the input buffer, you’re going to need to read the data or seek to the end.

I’m not convinced the “Singleton” pattern is appropriate. There are other ways of ensuring at most one instance for each piece of hardware. What if you later want multiple threads, each working with a different sensor?