Re: fgets() doesn't call fsync() before getting input

[Rich Felker <dalias@libc.org>]

On Thu, Feb 21, 2019 at 03:16:23PM -0500, James Larrowe wrote:
> Sorry, this was a typo. I meant fflush(). However, it's still not called.
> It's fixed in my program now, however I'm not sure what to do in this case.
> Do I just call ffush() on stdin, stdout, and stderr or do I send a patch to
> fgets()?

If you want to ensure the output to stdout actually appears on the
terminal/output file before some point in your program, you need to
call fflush(stdout) at that point. In your case that would be between
the fputs and fgets calls.

Rich


> On Thu, Feb 21, 2019 at 12:08 PM Rich Felker <dalias@libc.org> wrote:
> 
> > 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:
> > >
> > >
> > https://code.foxkit.us/adelie/packages/blob/master/user/mac-fdisk/flush-stdout.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.
> >
> > Rich
> >