How to free a through istream blocked thread.

Question

Hello

i have created two classes. One for input reading (through an istream object) and parsing and the other one for processing the output of the parser.
There is one instance of each of those.
I have the parser running in a loop calling istream::get() and then creating commands for the second object based upon the input. These commands are then put on a queue which the second object processes in a separate thread.
Now it is quite obvious that I eventually need to be able to send a "Quit" command. Here the problem arises though: The "Quit" command needs to end the parsing loop as well but I can't find a way to signal the parser that it should quit because it is caught within istream::get().
I would need a way to wake it from that method, but I cannot find any...
I have thought of writing some sort of "termination sequence" to the istream object (which in this case is cin) by creating an ostream object from istream::rdbuf(). But that doesn't work - The badbit is set after the attempt to write to the buffer.
In another question at StackOverflow I saw the asio class of the Boost library mentioned, but I'd rather not depend on third party libraries.
Is there a way to wake the thread from istream::get() - i.e. is there a way to write to the istream buffer (maybe assuming it actually is cin) from within the program?
Another approach would be to kill the thread which I could find acceptable as well since there is no cleanup needed in that specific place. But how can this be done? (I'm relying on a POSIX thread implementation)

Answer 1

Any chance this is implemented in .NET? - if so take a look at the Reactive Framework. It provides a very elegant way of handling streams and especially cancelling them on the fly. - On top of this, you get a very extensible library of Linq extension for all sorts of stuff, like Buffering, Memoization, Zip ect..

We use it a lot for transforming (and parsing), modelling of streamed data.

Jeff from the Reative team has a couble of nice blogs about Streaming and Reative here:

Answer 2

You will have to depend on something other than the standard iostream classes, because they don't provide select()-style behaviour.

Also, killing the thread is impossible with POSIX (and utterly broken in Windows). You can issue a cancellation request via pthread_cancel(), but in your case, it may be stuck in an un-cancellable system call. Of particular interest to you, read() may or may not be cancellable, depending on the environment. At least one environment says that a cancellation point may occur in read(), though admittedly it is a Windows POSIX layer. Also, Mac OS X, as recently as Leopard 10.5.1, had a broken read() implementation with respect to cancellability.

Once past this hurdle, you also have to consider the uneasy relationship between C++ destructors and pthread_cancel. Not all environments guarantee that destructors will be called, so you have to be extremely cautions when using pthread_cancel in C++ code.

In short, for interruptible I/O, use low-level I/O and select(): one fd for I/O, a second fd (created by pipe()) for signalling. Or, if you're brave, use AIO, but you're probably better off using a high level interface such as Boost.Asio.