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Date: Thu, 21 Feb 2019 10:22:43 -0500
From: Rich Felker <>
Subject: Re: fgets() doesn't call fsync() before getting input

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.


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