QTcpSocket can't read all bytes

[wysota]

No, it's still wrong, you are discarding previous content of the buffer.
This is correct (by the way, there are numerous threads in this forum where I explain this particular situation, how come you didn't encounter any of the threads while using our search?):

QByteArray buffer; // global or member variable
void Cls::onSocketReadyRead() {
  buffer.append(socket->readAll());
  tryProcessData();
}
 
void Cls::tryProcessData() {
  forever {
    // for the sake of the example I'm assuming my record is always 24 bytes long
    if(buffer.size()<24) return;
    QByteArray record = buffer.left(24);
    processRecord(record);
  } 
}

QByteArray buffer; // global or member variable
void Cls::onSocketReadyRead() {
  buffer.append(socket->readAll());
  tryProcessData();
}

void Cls::tryProcessData() {
  forever {
    // for the sake of the example I'm assuming my record is always 24 bytes long
    if(buffer.size()<24) return;
    QByteArray record = buffer.left(24);
    processRecord(record);
  } 
}

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The above code has one small flaw introduced for purpose so that you need to understand completely what the code does before actually using it.

[Qiieha]

ok thank u!!
the return-statement after the processRecord(record) method is missing, isn't it?

[wysota]

No, that's not a problem. The forever loop wouldn't make sense then. Think why the loop is there.

[Qiieha]

the loop makes sure that no different method is executed until the record is not complete. A QEventLoop would be a different solution.

[wysota]

the loop makes sure that no different method is executed until the record is not complete.

No, that's not the reason. And no other method would be executed anyway since there is only one thread here. There is one line of code missing from my snippet. Analyze what the whole method does line by line and you'll quickly discover what's missing. But you need to understand why the forever loop is there in the first place, it's the heart of this code.

[Qiieha]

I don't know. Do you mean:

buffer.clear();

buffer.clear();

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but it works...

[wysota]

No, then bufferring wouldn't make any sense. What is the purpose of the 'forever' loop?

[Qiieha]

That's logic. The buffer has to b cleared after processing record.
forever loop is a infinite loop.

[wysota]

No, clearing the buffer after processing a record is wrong. What if you get more than one message in one chunk (one call to onReadyRead())? Clearing the buffer will discard everything, including the data you haven't processed.

[Qiieha]

can you explain the thing, please.
I want to understand it.

[wysota]

From the beginning: the fact that sender sends "AAA" and then it sends "BBB" doesn't mean that the receiver will receive "AAA" and then "BBB". He can receive "AAABBB" or "AA" and "ABBB" or "AAABB" and "B" or even "A", "A", "A", "B", "B", "B". If your record was three characters long and you received "AAABB" and "B" then after processing "AAABB" and extracting "AAA" from it, you would discard "BB" and when you are called again with the last "B", you would fall out of sync because of the missing two characters. You have two autonomous systems talking to each other and TCP does not know anything such as "record", it just transmits bytes as they flow in. If this explanation is not enough for you then I'm sorry but you'll have to read some book or paper on TCP.

[Qiieha]

Ok I understand how TCP works, but in my case the client sends a request to the server. After that the client waits for response from server.
All the bytes from server are written into the bytearray until the client notices that the response is complete.
If the response is completed the client process the response and serializes the data.

So I can be sure that two responses can't be mixed.