[Qt-interest] Why Bus error clientSocket->waitForConnected()?

[Thiago Macieira]

Em Domingo 2. Maio 2010, às 17.56.35, Kustaa Nyholm escreveu:
> >That's why canReadLine() exists. You shouldn't call readLine() unless you
> >can read a line.
> 
> I know, I know, my point was that this is not a good API design, two calls
> when one would do. And different from say C standard getline() or Java
> readline, thus likely to trap many people.
> 
> >It does read a line.
> 
> Only if a line is available when I make the call.
> 
> >What it doesn't do is *block* until more bytes are
> >available. Only functions called "waitFor" block.
> 
> My point was that it was bad design decision to make this non blocking,

No, that was a good design. All of the API is non-blocking and that's a good 
design.

> or a bad decision to call it readline when many other APIs/libraries and
> common user expectation is that readline will block.

Maybe the bad decision was to return anything if full a line can't be read but 
the device isn't at the end yet.

Unfortunately, by the time we made QIODevice properly understand "can't read 
because it's EOF" and "can't read because there is no more data but the 
socket/process can still receive more", the readLine behaviour was set.

Maybe we can still fix the behaviour.

> Further, why would anyone want to have it that way?
> 
> What purpose does it server?

Sometimes devices and files end without a newline. So readLine() returns an 
unfinished line if EOF is found first.

> And the API is even worse in the details, the obvious solution to block
> until line is available or the connection is closed:
> 
>         while( !socket->canReadLine() ) ;
>         line=socket->readLine();

As I said before, if you want to block, you have to call a "waitFor" function. 
There's none above, so it doesn't block. it simply busy-loops forever.

> Is wrong, as the API documentation points out:
> 
> "Note that if the peer closes the connection unexpectedly, this function
> returns FALSE. This means that loops such as this won't work:"
> 
> So I think Qt has got this small detail wrong.

Or you did.

canReadLine() will return true for as long as there's a line to be read. If 
the device, process or socket is closed without a newline at the end, 
canReadLine() will return false for that last chunk of data. But readLine() 
will return it.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 190 bytes
Desc: This is a digitally signed message part.
Url : http://lists.qt-project.org/pipermail/qt-interest-old/attachments/20100502/cb96b1d0/attachment.bin