Re: [PATCH v2 1/2] tools/nolibc: support left alignment (-) and zero padding (0) in printf

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

On Wed, 28 Jan 2026 17:42:23 +0800
"licheng.li" <im.lechain@gmail.com> wrote:

> From: Cheng Li <im.lechain@gmail.com>
> 
> Currently, __nolibc_printf() in nolibc parses the width field but always
> pads with spaces on the left. It ignores the '-' flag (left alignment)
> and treats leading zeros in the width as part of the number parsing
> (resulting in space padding instead of zero padding).
> 
> This patch implements support for:
> 1. The '-' flag: forces left alignment by padding spaces on the right.
> 2. The '0' flag: forces zero padding on the left instead of spaces.

Does that work properly?
I think it is padding zeros the wrong side of the sign (or "0x").
Try testing with a negative number or a pointer.

You might need to limit the width for numeric fields to (say) 32,
make the on-stack buffer 64 bytes and convert into the middle.
Then add pad zeros to the front before adding any "-" or "0x".

It then gets very close to supporting precisions as well as widths.
It might be worth changing the 'pad' loop to:
	while (width > w) {
		unsigned int pad_len = ((width - w - 1) & 0xf) + 1;
		width -= pad_len;
		written += pad_len;
		if (cb(state, "                ", pad_len) != 0)
			return -1;
	}
while the code is marginally larger writing the pad in blocks of 16
will be much faster.
The linker should merge all the strings of spaces.

I looked at the (existing) version last night.
If the 'l' flag code is moved to after the width is calculated it all
becomes a lot simpler.
Set 'outstr = fmt' at the top of the loop and the code can use *fmt++
and do a 'fast scan' of the format string for a '%' or '\0' then
output the chars from the format string at the bottom.
I didn't size the change, but getting rid of ofs and escape must help.

Might post it later.

	David


> 
> The implementation reuses the padding character logic to handle both
> cases.
> 
> Logic behavior:
>  - "%5d"  -> "   12" (unchanged)
>  - "%-5d" -> "12   " (new: left align)
>  - "%05d" -> "00012" (new: zero pad)
>  - "%-05d"-> "12   " (new: left align overrides zero pad)
> 
> The code is optimized to keep the binary size impact minimal.
> Measuring the nolibc-test binary on x86_64:
> 
>   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.
> 
> Suggested-by: Willy Tarreau <w@1wt.eu>
> Signed-off-by: Cheng Li <im.lechain@gmail.com>
> ---
> v2 changes:
>  - Adopted optimization suggestions from Willy Tarreau:
>    - Incremented 'written' counter at the start of loops.
>    - Reordered loop checks to optimize compiler register usage.
>  - Updated commit message to explicitly mention zero-padding ('0') support.
> ---
>  tools/include/nolibc/stdio.h | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index 1f16dab2ac88..df8a96e39986 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)
> -						return -1;
> +				while (width > w && padc != '-') {
>  					written += 1;
> +					if (cb(state, &padc, 1) != 0)
> +						return -1;
> +					width--;
>  				}
>  				if (cb(state, outstr, w) != 0)
>  					return -1;
> +				while (width-- > w) {
> +					written += 1;
> +					if (cb(state, " ", 1) != 0)
> +						return -1;
> +				}
>  			}
>  
>  			written += len;