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

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Message-ID: <CAK7LNARKxd-kr3pABzTC2+uGhEKbyLtXDCSoxn56P0go--bg1A@mail.gmail.com>
Date:   Tue, 1 Mar 2022 18:06:04 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     David Laight <David.Laight@...lab.com>
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

On Tue, Mar 1, 2022 at 11:28 AM David Laight <David.Laight@...lab.com> wrote:
>
> Someone send HTML mail – outlook is broken – only lets you top post :-(
>
>
>
> 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.





[ test code ]
#include <stdio.h>
int main(void)
{
        char buf[2049];
        int ret, i;

        for (i = 0; i < 2048; i++)
                buf[i] = 'a';

        buf[2048] = 0;

        ret = printf("%s", buf);
        fprintf(stderr, "printf() returned: %d\n", ret);

        ret = fclose(stdout);
        fprintf(stderr, "fclose() returned %d\n", ret);

        return 0;
}


I tested this on Debian buster.


I created a very small partition with 1K size,
then write data to that partition.




root@...ter:~# lsblk  /dev/vdb1
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
vdb1 254:17   0   1K  0 part
root@...ter:~# ./a.out  > /dev/vdb1
printf() returned: 2048
fclose() returned -1



The buffer size seems 4k
as far as I tested on Debian.





-- 
Best Regards
Masahiro Yamada