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


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.


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