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Date: Mon, 25 Oct 2021 13:25:57 -0400
From: Rich Felker <>
To: Lorenzo Beretta <>
Subject: Re: Re: request: please detect reads from stdin with unflushed

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.


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