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Date: Mon, 25 Oct 2021 20:13:20 +0200
From: Lorenzo Beretta <>
To: Rich Felker <>
Subject: Re: Re: request: please detect reads from stdin with unflushed

I was thinking something like a warning on stderr fwiw, but I guess you're
right, I didn't consider multithreading.
Thank you for the detailed reply.

Il giorno lun 25 ott 2021 alle ore 19:25 Rich Felker <> ha

> On Mon, Oct 25, 2021 at 03:17:13PM +0200, Lorenzo Beretta wrote:
> > > Suggestion: fix those broken programs!
> >
> > I know, I know... the problem is that they work with glibc and they fail
> > silently with musl, and on top of that I've just discovered that at least
> > netbsd and openbsd do the same as glibc, ie a broken program like
> > #include <stdio.h>
> > int main(){
> >         printf("not flushed: ");
> >         char line[123];
> >         return fgets(line,sizeof line,stdin) ? 0 : 123;
> > }
> >
> > happens to work!!!
> >
> > What I'm asking is that musl (while technically correct!) helps
> > __detecting__ those programs, possibly as an option ("#ifdef
> >
> > PS
> > I'm not subscribed to this mailing list, sorry for not mentioning it the
> > first time
> There's nothing detectable here because there's nothing wrong with the
> program; the bug is in the programmer's *expectation* that the output
> be visible.
> It's possible to implement the behavior the programmer here desired,
> the optional flushing of line-buffered output streams before reading
> input. This would not help detect the bug in expectaions though; it
> would just help mask it. The reason this behavior is not present in
> musl is because it does not scale with significant numbers of stdio
> streams open, and can even produce deadlock conditions in
> multithreaded programs where there is no semantic deadlock but the
> additional flushing produces an extraneous operation on a stream in a
> way that causes deadlock.
> If you hit a program with an issue like this, it should be fairly easy
> to fix by adding fflush(stdout) or fflush(0) immediately before the
> relevant input operations.
> Rich

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