fflush(NULL) empties stdin

fflush(NULL) empties stdin

[Adam Dinwoodie]

Hello,

I seem to have found a bug in Cygwin's fflush implementation, where
fflush(NULL) is unexpectedly flushing stdin.

Working through https://cygwin.com/snapshots/ with the STC below, this
behaviour does not appear in the 2017-03-08 snapshot, and 2017-03-10 is
the first snapshot in which this does appear, with thanks to Ramsay
Jones on the Git mailing list for initially pointing me at the change
coming in some time between 2.7.0 and 2.8.0.

With thanks to Jeff King on the Git mailing list, here's a simple test
case:

    $ cat a.c
    #include <stdio.h>
    
    int main(void)
    {
            char buf[256];
            while (fgets(buf, sizeof(buf), stdin)) {
                    fprintf(stdout, "got: %s", buf);
                    fflush(NULL);
            }
            return 0;
    }
    
    $ gcc a.c
    
    $ seq 10 | ./a.exe
    got: 1

Compare this to the expected output, which I see on my handy CentOS 6
box:

    $ seq 10 | ./a.out
    got: 1
    got: 2
    got: 3
    got: 4
    got: 5
    got: 6
    got: 7
    got: 8
    got: 9
    got: 10

By my reading of Cygwin's fflush(3p), the stdin stream should be
unaffected by a fflush(NULL), as it is neither an output stream, an
update stream, nor a file capable of seeking.

I originally noticed this behaviour due to it causing a failure in one
of the Git test scripts.

Cheers,

Adam

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Re: fflush(NULL) empties stdin

[Ken Brown]

On 8/26/2017 5:52 PM, Adam Dinwoodie wrote:
> Hello,
> 
> I seem to have found a bug in Cygwin's fflush implementation, where
> fflush(NULL) is unexpectedly flushing stdin.

This has already been fixed.  Have you tried the latest snapshot?

Ken


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Re: fflush(NULL) empties stdin

[Adam Dinwoodie]

On Sat, Aug 26, 2017 at 07:57:21PM -0400, Ken Brown wrote:
> On 8/26/2017 5:52 PM, Adam Dinwoodie wrote:
> >Hello,
> >
> >I seem to have found a bug in Cygwin's fflush implementation, where
> >fflush(NULL) is unexpectedly flushing stdin.
> 
> This has already been fixed.  Have you tried the latest snapshot?

Ah, so it has!  I'd tried the latest release, but not the latest
snapshot.

Thank you, and apologies for the noise!

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