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Date: Thu, 21 Feb 2019 10:31:36 -0600
From: "A. Wilcox" <>
Subject: Re: fgets() doesn't call fsync() before getting input

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:


A. Wilcox (awilfox)
Project Lead, Adélie Linux

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