linux-kernel - Re: [PATCH 1/2] tools/nolibc: support left-aligned printing in printf

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Message-ID: <aXm_EPmtxuGlIHpu@1wt.eu>
Date: Wed, 28 Jan 2026 08:47:28 +0100
From: Willy Tarreau <w@....eu>
To: "licheng.li" <im.lechain@...il.com>
Cc: Thomas Weißschuh <linux@...ssschuh.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tools/nolibc: support left-aligned printing in printf

On Mon, Jan 26, 2026 at 11:10:37AM +0800, licheng.li wrote:
> From: Cheng Li <im.lechain@...il.com>
> 
> Currently, __nolibc_printf() in nolibc does not support the '-' flag
> for left alignment. This limits the ability to format tabular output
> nicely.
> 
> This patch adds support for the '-' flag. The implementation is kept
> minimal to minimize the binary size impact.
> 
> Measuring the nolibc-test binary on x86_64 shows a small increase in
> the text section:
> 
>   text    data     bss     dec     hex filename
>  43552     248     112   43912    ab88 nolibc-test (before)
>  43677     248     112   44037    ac05 nolibc-test (after)
> 
> The net increase is 125 bytes.
> 
> Signed-off-by: Cheng Li <im.lechain@...il.com>
> ---
>  tools/include/nolibc/stdio.h | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index 1f16dab2ac88..b90b59237bda 100644
> --- a/tools/include/nolibc/stdio.h
> +++ b/tools/include/nolibc/stdio.h
> @@ -250,7 +250,7 @@ typedef int (*__nolibc_printf_cb)(intptr_t state, const char *buf, size_t size);
>  static __attribute__((unused, format(printf, 4, 0)))
>  int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char *fmt, va_list args)
>  {
> -	char escape, lpref, c;
> +	char escape, lpref, padc, c;
>  	unsigned long long v;
>  	unsigned int written, width;
>  	size_t len, ofs, w;
> @@ -261,11 +261,17 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
>  	while (1) {
>  		c = fmt[ofs++];
>  		width = 0;
> +		padc = ' ';
>  
>  		if (escape) {
>  			/* we're in an escape sequence, ofs == 1 */
>  			escape = 0;
>  
> +			if (c == '-' || c == '0') {
> +				padc = c;
> +				c = fmt[ofs++];
> +			}
> +
>  			/* width */
>  			while (c >= '0' && c <= '9') {
>  				width *= 10;
> @@ -358,13 +364,19 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
>  			if (n) {
>  				w = len < n ? len : n;
>  				n -= w;
> -				while (width-- > w) {
> -					if (cb(state, " ", 1) != 0)
> +				while (padc != '-' && width > w) {
> +					if (cb(state, &padc, 1) != 0)
>  						return -1;
>  					written += 1;
> +					width--;
>  				}
>  				if (cb(state, outstr, w) != 0)
>  					return -1;
> +				while (width-- > w) {
> +					if (cb(state, " ", 1) != 0)
> +						return -1;
> +					written += 1;
> +				}
>  			}
>  
>  			written += len;

Thank you, this looks good to me. Since you have paid particular attention
to the size increase, I noticed a few tricks which help a little bit here:

- increment written at the beginning of the loop instead of the end
  helps the compiler make the code more compact.

- testing width > w before padc != '-' also helps by benefitting from
  the result of the last iteration's increment:

  	while (width > w && padc != '-') {
  		written += 1;
  		if (cb(state, &padc, 1) != 0)
  			return -1;
  		width--;
  	}

        ...

	while (width-- > w) {
		written += 1;
		if (cb(state, " ", 1) != 0)
			return -1;
	}

  $ printf '#include "stdio.h"\nvoid *p = printf;\n' |  gcc -Os -xc -c -o printf.o -
  $ nm -oa --size printf*o|grep 'printf$'
  printf-before.o:000000000000035f t printf
  printf-licheng.o:00000000000003b5 t printf
  printf.o:00000000000003a3 t printf

=> that's 19 less bytes, or almost 10% smaller ;-)

I also tried changing the 3 blocks for a single loop checking width and w
to call cb() once and adjust written/w once but it gave the exact same
size for less efficient code so that was not worth it.

Anyway, just to say that if you're interested in trying the minor
adjustments above, you're welcome, otherwise I'm fine with this patch.

Thanks!
Willy