linux-kernel - RE: [PATCH v2] fixdep: use fflush() and ferror() to ensure successful write to files

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Date:   Tue, 1 Mar 2022 09:48:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Masahiro Yamada' <masahiroy@...nel.org>
CC:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Michal Marek" <michal.lkml@...kovi.net>
Subject: RE: [PATCH v2] fixdep: use fflush() and ferror() to ensure successful
 write to files

From: Masahiro Yamada
> Sent: 01 March 2022 09:06
> 
> On Tue, Mar 1, 2022 at 11:28 AM David Laight <David.Laight@...lab.com> wrote:
> >
> > The return value from fprintf() is normally the number of bytes written to
> > the internal buffer (8k in glibc?)
> >
> > Only if the buffer is full and an actual write() is done do you get any indication of an error.
> >
> > So you can use the error return from fprintf() to terminate a loop – but it usually
> > just isn’t worth the effort.
> >
> > The error status returned by ferror() is ‘sticky’, so you need only check once.
> >
> > But you need to check before fclose().
> >
> > Since fclose() has to write out the buffer – that write can also fail.
> >
> > I’m not sure whether fclose() returns and error in that case, but adding fflush()
> > makes the coding easier.
> 
> 
> I just checked this.
> 
> fclose() returns -1 if it fails to flush the buffer.

But you still need to check ferror() before it.
So you might as well do fflush(); if (ferror()); fclose();

Then there is only one error exit path.

	David

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