linux-kernel - Re: [PATCH 1/2] tools/nolibc: add support zero pad (0) in printf

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Message-ID: <20260131123209.1ed5df80@pumpkin>
Date: Sat, 31 Jan 2026 12:32:09 +0000
From: David Laight <david.laight.linux@...il.com>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: "licheng.li" <im.lechain@...il.com>, Willy Tarreau <w@....eu>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tools/nolibc: add support zero pad (0) in printf

On Sat, 31 Jan 2026 11:18:49 +0100
Thomas Weißschuh <linux@...ssschuh.net> wrote:

> Hey Cheng,
> 
> Jan 30, 2026 09:37:51 licheng.li <im.lechain@...il.com>:
> 
> > From: Cheng Li <im.lechain@...il.com>
> >
> > This patch correctly implements the '0' flag in __nolibc_printf() to
> > allow zero-padding for numeric and pointer outputs.  
> 
> Thanks for (all of) your patches.
> I am not sure when exactly I can take a proper look at them.
> As we are currently fairly late in the 6.20/7.0 development cycle I would like move your patches into the next one.
> We can still discuss the patches and you can send new revisions and patches,
> but they won't be picked up until in a few weeks.

Gives me time to re-write them :-)

There is still a bug in the 'align left' code as well.
snprintf(buf, 21, "%-25s", "abcd") outputs 20 spaces not "abcd" followed by 16.
Easiest fix is to move the truncation in the cb() function.

	David

> 
> > Thanks to David for pointing out the errors in the previous implementation.
> >
> > The logic ensures that the sign ('-') for negative numbers or the prefix
> > ('0x') for pointers is printed before the padding zeros, adhering to
> > standard printf behavior (e.g., producing "-0005" instead of "000-5").
> >
> > Examples of the corrected padding logic:
> > - ("%05d", -5)  -> "-0005"
> > - ("%05p", ptr)  -> "0x00..."
> >
> > Signed-off-by: Cheng Li <im.lechain@...il.com>
> > ---
> > tools/include/nolibc/stdio.h | 16 ++++++++++++++--
> > 1 file changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> > index f31b77f61d3b..8a4af259a31b 100644
> > --- a/tools/include/nolibc/stdio.h
> > +++ b/tools/include/nolibc/stdio.h
> > @@ -267,7 +267,7 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
> >             /* we're in an escape sequence, ofs == 1 */
> >             escape = 0;
> >
> > -           if (c == '-') {
> > +           if (c == '-' || c == '0') {
> >                 padc = c;
> >                 c = fmt[ofs++];
> >             }
> > @@ -364,9 +364,21 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
> >             if (n) {
> >                 w = len < n ? len : n;
> >                 n -= w;
> > +               if (padc == '0') {
> > +                   if (outstr[0] == '-') {
> > +                       if (cb(state, outstr, 1) != 0)
> > +                           return -1;
> > +                       outstr++;
> > +                   }
> > +                   if (outstr[0] == '0' && outstr[1] == 'x') {
> > +                       if (cb(state, outstr, 2) != 0)
> > +                           return -1;
> > +                       outstr += 2;
> > +                   }
> > +               }
> >                 while (width > w && padc != '-') {
> >                     written += 1;
> > -                   if (cb(state, " ", 1) != 0)
> > +                   if (cb(state, &padc, 1) != 0)
> >                         return -1;
> >                     width--;
> >                 }
> > --
> > 2.52.0  
>