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

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Message-ID: <CAAZOWcKKvB=SoyCHOkqNbcTOR_BNmrSXUM8NUK0oLcP_C5c11A@mail.gmail.com>
Date: Fri, 30 Jan 2026 18:12:48 +0800
From: Cheng Li <im.lechain@...il.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Willy Tarreau <w@....eu>, Thomas Weißschuh <linux@...ssschuh.net>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tools/nolibc: add support zero pad (0) in printf

David Laight <david.laight.linux@...il.com> 于2026年1月30日周五 18:02写道：
>
> On Fri, 30 Jan 2026 16:37:35 +0800
> "licheng.li" <im.lechain@...il.com> wrote:
>
> > 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 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").
>
> I think it would be much better to change the contents of tmpbuf[]
> where it is filled with the converted number.
> You'd need to limit the number of zeros added (or the precision) to (say) 32
> and then set 'outbuf = tmpbuf + 32' so that the zeros and sign/0x can be
> added at the front.

Hi David,

You are absolutely right. Modifying the buffer generation logic
(tmpbuf) is indeed
the robust way to handle sign/prefix insertion combined with padding/precision.
My current approach of trying to handle it during the output phase is
too fragile
and misses edge cases (as I also realized regarding the negative sign issue).

Given the complexity you mentioned (like the subtle differences in
standard compliance)
and your plan to implement full field precision, I will drop this
"Zero Padding" patch entirely.
It is better implemented as part of a proper precision support overhaul.

I will focus on the "Left Alignment" patch. I am sending out v4 of
that series later,
which incorporates the code-swapping optimization you suggested earlier.

Thanks for the detailed feedback.

Best regards,
Cheng


> There is the (annoying) subtle difference between %#05x and %#.5x just
> waiting to catch the unwary.
>
>         David
>
> >
> > 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--;
> >                               }
>