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Date: Thu, 21 Feb 2019 12:07:48 -0500
From: Rich Felker <>
Subject: Re: fgets() doesn't call fsync() before getting input

On Thu, Feb 21, 2019 at 10:31:36AM -0600, A. Wilcox wrote:
> On 02/21/19 09:22, Rich Felker wrote:
> > On Thu, Feb 21, 2019 at 10:09:03AM -0500, James Larrowe wrote:
> >> I'm writing a program that prints a dialogue to the screen and then asks
> >> for input. In musl, the dialogue does not show before fgets() is called,
> >> however in glibc it does. That causes a blank prompt and also some
> >> confusion. Attached is a minimal example and a log.
> > 
> > This difference is intentional. The specification allows but does not
> > require that attempting to read from a line-buffered input stream
> > causes all line-buffered output streams to be flushed. This behavior
> > was somewhat convenient for old-style input-prompt idioms, but it
> > doesn't scale with large numbers of files open and deadlocks with some
> > multi-threaded usage. The portable solution here for applications is
> > to fflush (not fsync) the particular stream you want flushed.
> > 
> > Rich
> FWIW, the only package we've come across where this is a problem is
> mac-fdisk (which hasn't been updated since 1997 - yes, 22 years ago).
> We have a patch:

I think it's more of an issue for the early examples in C books and
tutorials, which invariably but inexplicably use a 1970s-era "prompt
for input" model rather than argv[] or something that would be a lot
more familiar (and amenable to testing) to modern readers.


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