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Date: Mon, 25 Feb 2019 08:10:22 -0500
From: James Larrowe <larrowe.semaj11@...il.com>
To: musl@...ts.openwall.com
Subject: Re: fgets() doesn't call fsync() before getting input
Got it. Thank you.
On Fri, Feb 22, 2019 at 12:05 AM Markus Wichmann <nullplan@....net> wrote:
> On Thu, Feb 21, 2019 at 03:16:23PM -0500, James Larrowe wrote:
> > Sorry, this was a typo. I meant fflush(). However, it's still not called.
> > It's fixed in my program now, however I'm not sure what to do in this
> case.
> > Do I just call ffush() on stdin, stdout, and stderr or do I send a patch
> to
> > fgets()?
> >
>
> You call fflush(stdout). stderr is already unbuffered, so flushing it
> only makes sense if you used setvbuf() on it beforehand. Why would you,
> though? Being unbuffered is sort of the defining feature of stderr.
>
> Also fflush(stdin) is undefined behavior. fflush() is only defined for
> output streams. Since you are asking beginner's questions, you are
> likely to come accross the following idiom in C tutorials:
>
> cnt = scanf("Some format", some variables);
> fflush(stdin);
>
> Or maybe the other way round. The reason for this is that scanf()
> terminates at the newline character at the end of the line you inserted,
> but doesn't consume it. So the next scanf() will read it as the first
> thing. The format will likely not allow for whitespace at the start of
> input, and so the scanf() will fail.
>
> The solution is to either allow for the newline by starting the scanf
> format with a space, or to use:
>
> char buf[some appropriate line length];
> if (fgets(buf, sizeof buf, stdin))
> cnt = sscanf(buf, "Some format", some variables);
>
> Ciao,
> Markus
>
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