Re: [PATCH 1/4] lib/sprintf: add scnprintf_append() helper function

[David Laight <david.laight.linux@gmail.com>]

On Fri, 7 Nov 2025 13:52:27 +0100
Petr Mladek <pmladek@suse.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@linux-foundation.org> wrote:  
> > > > On Fri,  7 Nov 2025 13:16:13 +0800 Junrui Luo <moonafterrain@outlook.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().

	David

> 
> > > Then you can chain calls, eg:
> > > 	off = scnprintf(buf, sizeof buf, ....);
> > > 	off += scnprintf_at(buf, sizeof buf, off, ....);  
> > 
> > I like this suggestion. Also note, that the original implementation works directly
> > on static buffers.  
> 
> I would prefer this as well. IMHO, it encourages people to write a better code.
> 
> Best Regards,
> Petr