linux-kernel - Re: [PATCH] printk.c: removed unnecessary code

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Date:   Wed, 30 Nov 2016 10:14:28 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Sebastian Duda <sebastian.duda@....de>
Cc:     linux-kernel@...cs.fau.de,
        Tobias Baumeister <tobias.baumeister@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Borislav Petkov <bp@...e.de>, Tejun Heo <tj@...nel.org>,
        Thierry Reding <treding@...dia.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] printk.c: removed unnecessary code

[Resending with the full CC list as my email client has clobbered it in
the previous attempt for some reason]

On Tue 29-11-16 16:19:01, Sebastian Duda wrote:
> snprintf((char *) ?, 0, ...); always returns Zero and doesn't change the data.
> Thus the execution of
> 	snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
> has no effect on program.
> The substitution with 0 increases the readability of the code.

Are you sure this is correct. As per vsnprintf documentation:
"
 * The return value is the number of characters which would
 * be generated for the given input, excluding the trailing
 * '\0', as per ISO C99.
"

this should just work as 35dac27cedd1 ("printk: fix incorrect length
from print_time() when seconds > 99999") intended.

I haven't checked the implementation though so I might be wrong here.

> 
> Signed-off-by: Sebastian Duda <sebastian.duda@....de>
> Signed-off-by: Tobias Baumeister <tobias.baumeister@....de>
> ---
>  kernel/printk/printk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 5028f4f..fe3fec1 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1186,7 +1186,7 @@ static size_t print_time(u64 ts, char *buf)
>  	rem_nsec = do_div(ts, 1000000000);
>  
>  	if (!buf)
> -		return snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
> +		return 0;
>  
>  	return sprintf(buf, "[%5lu.%06lu] ",
>  		       (unsigned long)ts, rem_nsec / 1000);
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs