Re: [PATCH/RFC 1/5] mingw: make fgetc raise SIGINT if apropriate

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Dec 4, 2012, 17:12 UTC
Message-ID: <alpine.DEB.1.00.1212041809150.31987@s15462909.onlinehome-server.info>
In-Reply-To: <CABPQNSaCV820zhJGdW++LMf2U7AeODSbfukix3fMfffmNex4YA@mail.gmail.com>

Hi kusma,

On Sat, 1 Dec 2012, Erik Faye-Lund wrote:

Show 15 quoted lines

> On Fri, Nov 30, 2012 at 6:58 PM, Johannes Schindelin
> <Johannes.Schindelin@gmx.de> wrote:
> > Hi,
> >
> > On Tue, 13 Nov 2012, Erik Faye-Lund wrote:
> >
> >> Set a control-handler to prevent the process from terminating, and
> >> simulate SIGINT so it can be handled by a signal-handler as usual.
> >
> > One thing you might want to mention is that the fgetc() handling is not
> > thread-safe, and intentionally so: if two threads read from the same
> > console, we are in trouble anyway.
> 
> I'm not entirely sure if I know what you mean. Do you suggest that two
> threads can race for setting the console ctrl-handler?

That was my idea, yes.

Show 6 quoted lines

> I don't think that's the case; "SetConsoleCtrlHandler(x, TRUE)" adds a
> console handler to the handler-chain, and SetConsoleCtrlHandler(x,
> FALSE) removes it. If two threads add handlers, it is my understanding
> that one of them will be run, only to report "no, no more ctrl-handling
> needed". Since both handlers block further ctrl-handling, I don't think
> there's a problem.

My idea was that the SetConsoleCtrlHandler(x, FALSE) could remove the handler prematurely iff another thread wanted to install the very same handler (but it was already installed by the first thread).

But as I said: for this to happen, *two* threads need to want to access the console for reading. In that case we'd be in bigger trouble than thread unsafety... We cannot read two passwords from the same console at the same time.

Ciao, Dscho

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