linux-kernel - Re: [PATCH 1/4] lib/sprintf: add scnprintf

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Message-ID: <aRHzJIFkgfHIilTl@pathway.suse.cz>
Date: Mon, 10 Nov 2025 15:13:56 +0100
From: Petr Mladek <pmladek@...e.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Junrui Luo <moonafterrain@...look.com>,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org, tiwai@...e.com,
	perex@...ex.cz, linux-sound@...r.kernel.org, mchehab@...nel.org,
	awalls@...metrocast.net, linux-media@...r.kernel.org,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH 1/4] lib/sprintf: add scnprintf_append() helper function

On Fri 2025-11-07 17:51:23, David Laight wrote:
> On Fri, 7 Nov 2025 13:52:27 +0100
> Petr Mladek <pmladek@...e.com> wrote:
> 
> > On Fri 2025-11-07 11:35:35, Andy Shevchenko wrote:
> > > On Fri, Nov 07, 2025 at 09:12:46AM +0000, David Laight wrote:  
> > > > On Thu, 6 Nov 2025 21:38:33 -0800
> > > > Andrew Morton <akpm@...ux-foundation.org> wrote:  
> > > > > On Fri,  7 Nov 2025 13:16:13 +0800 Junrui Luo <moonafterrain@...look.com> wrote:  
> > > 
> > > ...
> > >   
> > > > That is true for all the snprintf() functions.
> > > >   
> > > > > I wonder if we should instead implement a kasprintf() version of this
> > > > > which reallocs each time and then switch all the callers over to that.  
> > > > 
> > > > That adds the cost of a malloc, and I, like kasprintf() probably ends up
> > > > doing all the work of snprintf twice.
> > > > 
> > > > I'd be tempted to avoid the strlen() by passing in the offset.
> > > > So (say):
> > > > #define scnprintf_at(buf, len, off, ...) \
> > > > 	scnprintf((buf) + off, (len) - off, __VA_ARGS__)  
> > 
> > It does not handle correctly the situation when len < off.
> > Othersise, it looks good.
> 
> That shouldn't happen unless the calling code is really buggy.
> There is also a WARN_ON_ONCE() at the top of snprintf().

Fair enough.

BTW: I have found there exists a userspace library which implements
this idea, the funtion is called vsnoprintf(), see
https://arpa2.gitlab.io/arpa2common/group__snoprintf.html

I know that it is cryptic. But I like the name. The letters "no"
match the ordering of the parameters "size, offset".

In our case, it would be scnoprintf() ...

Best Regards,
Petr