CODEWITHSUNDEEP
qtserial-port
Shnd
Shnd

Reputation: 2050

why QSerialPort read() method doesn't work as expected?

I'm going to write 5 bytes of data to a COM port and expect to receive 11 with the last byte of 0xFF to receive. I store the receiving bytes in pantilt_value variable.

This is a code which works: (I receive the 11 bytes as expected with the last byte of 0xFF)

_port->write(data_pantilt, 5);
QByteArray pantilt_value = _port->readAll();
while (_port->waitForReadyRead(100))
    pantilt_value.append(_port->readAll());

This is a code which doesn't work: (I receive just a first 2 bytes)

_port->write(data_pantilt, 5);
char d;
QByteArray pantilt_value;
while (_port->waitForReadyRead(100)){
   _port->read(&d,1);
   pantilt_value.append(d);
   if (d == 0xff)
       break;
}

Can you explain why the second code doesn't work.

Upvotes: 0

Views: 2131

Answers (2)

Shnd
Shnd

Reputation: 2050

It seems that, The first time you call waitForReadyRead() to check if there is a data on serial port to read, it returns true, but you should read the whole data on the serial port after you call that method because if you fail to read the whole data after calling waitForReadyRead(), next time you call that method, it returns false, although there is still some data left to read from the port.

So my second code should be changed to something like this:

_port->write(data_pantilt, 5);
char d;
QByteArray pantilt_value;
while(_port->waitForReadyRead(100)){
    while (_port->bytesAvailable() > 0){
       _port->read(&d,1);
       pantilt_value.append(d);
       if (d == 0xff)
           break;
    }
}

Upvotes: 1

thuga
thuga

Reputation: 12931

waitForReadyRead() returns true only when there is new data available. In your second case, the while loop has time to execute twice before all data is received. Use QSerialPort::bytesAvailable to see how much data there is to be read.

Upvotes: 1

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