Re: libstdc++/9533: Regression: Can't read from tty with ifstream

[Paolo Carlini <pcarlini at unitus dot it>]

Pétur Runólfsson wrote:

> >    Patching: indeed, reverting this hunk of 6746 commit:
> >    
> >    +
> >    +	    // Set input to nonblocking for fifos.
> >    +	    if (__mode & ios_base::in)
> >    +	      fcntl(this->fd(), F_SETFL, O_NONBLOCK);
> >    +
> >    
> >    Fixes the problem without regressing on 6746.
> >    
> Interesting. I would have thought that removing this would
> cause the call to underflow() in basic_filebuf::open() to
> block, causing the test case to wait for input *before*
> printing out the prompt (this seems to be the reason that
> non-blocking input is specified).
Yes, you are right. My bad.
I didn't notice that since was using a further reduced :( testcase.
Also, we got a regression elsewhere by simply reverting that hunk (not 
on 6746): see
the libstdc++ list.

> So far, this is what I think is going on:
> * non-blocking input is specified so underflow() doesn't
>  block (which is wrong, underflow() should block).
> * underflow() must not block because it is called from
>  open() which must not block.
> * open() calls underflow() so that in_avail() will return
>  non-zero after open() (libstdc++/6746).
Agreed.

> However, I don't see how calling underflow() from open()
> fixes libstdc++/6746.
I concur, but in fact it does.

> Unless I am missing something,
> readsome will return up to BUFSIZ characters, and then
> return 0 until the end of time.
>  
Paolo.