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Message-ID: <CAD+Cw=cVbgEUz_VETZL9-rOP+aaDxGqW0CGAbKYi+cbpq9GwhA@mail.gmail.com>
Date: Wed, 10 Feb 2016 18:08:47 -0500
From: Max Ruttenberg <mruttenberg@...technology.com>
To: musl@...ts.openwall.com
Subject: Re: Enforcing expected ordering of operations on stdout,
stdin, and stderr
>
> fflush(stdout);
> This is more of a basic C thing than a libc ml thing. You should
> consider picking up a copy of The C Programming Language by Kernighan
> and Ritchie. It will explain all of this.
I know about fflush, thanks.
Consider this program:
int main()
{
char buff[2];
puts("enter a character");
buff[0] = getchar();
buff[1] = '\0';
puts(buff);
return 0;
}
If I compile that on linux-amd64, with or without musl, I will see "enter a
character" printed to my console and then be prompted for a character, as
opposed to the other way around. I don't know if this is formally
guaranteed by the C standard, but somehow that order seems to be
maintained.
But if I grep the source code in musl/src/stdio for "fflush" I don't see a
bunch of calls to fflush. I see a call to it in fclose and freopen... but
that's neither surprising nor helpful. If I do the same in
musl/src/internal I also don't get anything. I've even tried just greping
for "flush."
And yet somehow the order is maintained within those calls to puts and
getchar. So what I'm asking is: how? What part of the internal musl source
even attempts to enforce that ordering? I know the calling application can
do it with calls to fflush, but somehow that doesn't seem to be necessary
short of a signal interrupting the expected flow of execution. Am I just
getting lucky 100% of the time or is there some source in the stdio library
that's enforcing this?
On Wed, Feb 10, 2016 at 5:05 PM, Joakim Sindholt <opensource@...sha.com>
wrote:
> On Wed, 2016-02-10 at 16:49 -0500, Max Ruttenberg wrote:
> > All,
> >
> >
> > I guess my question is more easily asked through an example. If I have
> > code that makes a call to puts and then a call to getchar, what
> > mechanism enforces that stdout gets flushed before blocking for stdin?
> > Is there a such a mechanism? My gut says yes but I haven't been able
> > to pinpoint it.
> >
> >
> > Thanks,
> > Max
>
> fflush(stdout);
>
> This is more of a basic C thing than a libc ml thing. You should
> consider picking up a copy of The C Programming Language by Kernighan
> and Ritchie. It will explain all of this.
>
>
--
Max Ruttenberg,
Member of the Technical Staff
Emu *Technology*
1400 E Angela Blvd, Unit 101
South Bend, IN 46617
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