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

[Petr Mladek]

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@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > On Fri,  7 Nov 2025 13:16:13 +0800 Junrui Luo <moonafterrain@xxxxxxxxxxx> 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.

> > 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