Re: [PATCH v4 next 19/23] tools/nolibc/printf: Special case 0 and add support for %#x

[Willy Tarreau <w@1wt.eu>]

On Mon, Mar 02, 2026 at 10:18:11AM +0000, david.laight.linux@gmail.com wrote:
> From: David Laight <david.laight.linux@gmail.com>
> 
> The output for %#x is almost the same as that for %p, both output in
> hexadecimal with a leading "0x".
> However for zero %#x should just output "0" (the same as decimal and ocal).
> For %p match glibc and output "(nil)" rather than "0x0" or "0".
> 
> Add tests for "%#x", "% d", "%+d" and passing NULL to "%p".
> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>

Acked-by: Willy Tarreau <w@1wt.eu>
Willy

> ---
> 
> v4: Split into its' own patch.
> 
>  tools/include/nolibc/stdio.h                 | 38 ++++++++++++++------
>  tools/testing/selftests/nolibc/nolibc-test.c |  3 ++
>  2 files changed, 30 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index 5d75e8530ce2..ef8e7069d5cf 100644
> --- a/tools/include/nolibc/stdio.h
> +++ b/tools/include/nolibc/stdio.h
> @@ -296,7 +296,7 @@ int fseek(FILE *stream, long offset, int whence)
>   *  - %% generates a single %
>   *  - %m outputs strerror(errno).
>   *  - %X outputs a..f the same as %x.
> - *  - The modifiers [#-0] are currently ignored.
> + *  - The modifiers [-0] are currently ignored.
>   *  - No support for precision or variable widths.
>   *  - No support for floating point or wide characters.
>   *  - Invalid formats are copied to the output buffer.
> @@ -410,8 +410,11 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
>  		 * so that 'X' can be allowed through.
>  		 * 'X' gets treated and 'x' because _NOLIBC_PF_FLAG() returns the same
>  		 * value for both.
> +		 *
> +		 * We need to check for "%p" or "%#x" later, merging here gives better code.
> +		 * But '#' collides with 'c' so shift right.
>  		 */
> -		ch_flag = _NOLIBC_PF_FLAG(ch);
> +		ch_flag = _NOLIBC_PF_FLAG(ch) | (flags & _NOLIBC_PF_FLAG('#')) >> 1;
>  		if (((ch >= 'a' && ch <= 'z') || ch == 'X') &&
>  		    _NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'c', 'd', 'i', 'u', 'x', 'p', 's')) {
>  			/* 'long' is needed for pointer/string conversions and ltz lengths.
> @@ -470,17 +473,30 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
>  			 */
>  			out = outbuf + 2;
>  
> -			/* Convert the number to ascii in the required base. */
> -			if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'd', 'i', 'u')) {
> -				/* Base 10 */
> -				len = u64toa_r(v, out);
> -			} else {
> -				/* Base 16 */
> +			if (v == 0) {
> +				/* There are special rules for zero. */
>  				if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'p')) {
> -					/* "%p" needs "0x" prepending. */
> -					sign_prefix = 'x' | '0' << 8;
> +					/* "%p" match glibc, precision is ignored */
> +					outstr = "(nil)";
> +					len = 5;
> +					goto do_output;
> +				}
> +				/* All other formats (including "%#x") just output "0". */
> +				out[0] = '0';
> +				len = 1;
> +			} else {
> +				/* Convert the number to ascii in the required base. */
> +				if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'd', 'i', 'u')) {
> +					/* Base 10 */
> +					len = u64toa_r(v, out);
> +				} else {
> +					/* Base 16 */
> +					if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'p', '#' - 1)) {
> +						/* "%p" and "%#x" need "0x" prepending. */
> +						sign_prefix = 'x' | '0' << 8;
> +					}
> +					len = u64toh_r(v, out);
>  				}
> -				len = u64toh_r(v, out);
>  			}
>  
>  			/* Add the 0, 1 or 2 ("0x") sign/prefix characters at the front. */
> diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> index a140c54e4d72..4267c69ec37c 100644
> --- a/tools/testing/selftests/nolibc/nolibc-test.c
> +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> @@ -1825,6 +1825,7 @@ static int run_printf(int min, int max)
>  		CASE_TEST(string);       EXPECT_VFPRINTF(1, "foo", "%s", "foo"); break;
>  		CASE_TEST(number);       EXPECT_VFPRINTF(1, "1234", "%d", 1234); break;
>  		CASE_TEST(negnumber);    EXPECT_VFPRINTF(1, "-1234", "%d", -1234); break;
> +		CASE_TEST(num_sign);     EXPECT_VFPRINTF(1, "| 1|+2|+3|+4|5|", "|% d|%+d|% +d|%+ d|%#d|", 1, 2, 3, 4, 5); break;
>  		CASE_TEST(unsigned);     EXPECT_VFPRINTF(1, "12345", "%u", 12345); break;
>  		CASE_TEST(signed_max);   EXPECT_VFPRINTF(1, "2147483647", "%i", ~0u >> 1); break;
>  		CASE_TEST(signed_min);   EXPECT_VFPRINTF(1, "-2147483648", "%i", (~0u >> 1) + 1); break;
> @@ -1832,7 +1833,9 @@ static int run_printf(int min, int max)
>  		CASE_TEST(char);         EXPECT_VFPRINTF(1, "c", "%c", 'c'); break;
>  		CASE_TEST(hex_nolibc);   EXPECT_VFPRINTF(is_nolibc, "|f|d|", "|%x|%X|", 0xf, 0xd); break;
>  		CASE_TEST(hex_libc);     EXPECT_VFPRINTF(!is_nolibc, "|f|D|", "|%x|%X|", 0xf, 0xd); break;
> +		CASE_TEST(hex_alt);      EXPECT_VFPRINTF(1, "|0x1|  0x2|    0|", "|%#x|%#5x|%#5x|", 1, 2, 0); break;
>  		CASE_TEST(pointer);      EXPECT_VFPRINTF(1, "0x1", "%p", (void *) 0x1); break;
> +		CASE_TEST(pointer_NULL); EXPECT_VFPRINTF(1, "(nil)", "%p", (void *)0); break;
>  		CASE_TEST(percent);      EXPECT_VFPRINTF(1, "a%d42%69%", "a%%d%d%%%d%%", 42, 69); break;
>  		CASE_TEST(perc_qual);    EXPECT_VFPRINTF(1, "a%d2", "a%-14l%d%d", 2); break;
>  		CASE_TEST(invalid);      EXPECT_VFPRINTF(1, "a%12yx3%y42%P", "a%12yx%d%y%d%P", 3, 42); break;
> -- 
> 2.39.5