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Message-Id: <20260206191121.3602-9-david.laight.linux@gmail.com>
Date: Fri, 6 Feb 2026 19:11:18 +0000
From: david.laight.linux@...il.com
To: Willy Tarreau <w@....eu>,
Thomas Weißschuh <linux@...ssschuh.net>,
linux-kernel@...r.kernel.org,
Cheng Li <lechain@...il.com>
Cc: David Laight <david.laight.linux@...il.com>
Subject: [PATCH v2 next 08/11] tools/nolibc/printf: Add support for zero padding and field precision
From: David Laight <david.laight.linux@...il.com>
Include support for variable field widths (eg "%*.*d") and the special
processing required for zero especially when the precision is 0.
Zero padding is limited to 30 zero characters.
This is wider than the largest numeric field so shouldn't be a problem.
When processing "%#01x" subtracting the number of 'sign' characters
from the field width generates a negative precision.
Fix by making all the variables 'signed int'.
This also makes the code smaller.
The space padding can then optimised as well (saves another 60 bytes of
code).
All the standard printf formats are now supported except octal
and floating point.
Signed-off-by: David Laight <david.laight.linux@...il.com>
---
Changes for v2:
- These changes were previously in patch 9.
However you need to apply the old patch 10 to get anything like
the same source. The files then more of less match apart from 'c'
being renamed 'ch' and the 'magic' #defines.
tools/include/nolibc/stdio.h | 103 +++++++++++++++++++++++++++--------
1 file changed, 80 insertions(+), 23 deletions(-)
diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index 032416e900ee..d85607942784 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -241,13 +241,12 @@ char *fgets(char *s, int size, FILE *stream)
/* printf(). Supports most of the normal integer and string formats.
- * - %[#-+ ][width][{l,t,z,ll,L,j,q}]{d,i,u,c,x,X,p,s,m,%}
+ * - %[#0-+ ][width|*[.precision|*]][{l,t,z,ll,L,j,q}]{d,i,u,c,x,X,p,s,m,%}
* - %% generates a single %
* - %m outputs strerror(errno).
* - # only affects %x and prepends 0x to non-zero values.
* - %o (octal) isn't supported.
* - %X outputs a..f the same as %x.
- * - No support for zero padding, precision or variable widths.
* - No support for wide characters.
* - invalid formats are copied to the output buffer.
*/
@@ -282,9 +281,8 @@ static __attribute__((unused, format(printf, 3, 0)))
int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list args)
{
char ch;
- unsigned int written, width;
+ int len, written, width, precision;
unsigned int flags, ch_flag;
- size_t len;
char tmpbuf[32 + 24];
const char *outstr;
@@ -314,12 +312,24 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
flags |= ch_flag;
}
- /* width */
- while (ch >= '0' && ch <= '9') {
- width *= 10;
- width += ch - '0';
-
- ch = *fmt++;
+ /* Width and precision */
+ for (;; ch = *fmt++) {
+ if (ch == '*') {
+ precision = va_arg(args, unsigned int);
+ ch = *fmt++;
+ } else {
+ for (precision = 0; ch >= '0' && ch <= '9'; ch = *fmt++)
+ precision = precision * 10 + (ch - '0');
+ }
+ if (_NOLIBC_PF_FLAGS_CONTAIN(flags, '.'))
+ break;
+ width = precision;
+ if (ch != '.') {
+ /* Default precision for strings */
+ precision = INT_MAX;
+ break;
+ }
+ flags |= _NOLIBC_PF_FLAG('.');
}
/* Length modifier.
@@ -388,12 +398,13 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
/* "%s" - character string. */
if (!v) {
outstr = "(null)";
- len = 6;
+ /* Match glibc, nothing output if precision too small */
+ len = precision >= 6 ? 6 : 0;
goto do_output;
}
outstr = (void *)v;
do_strnlen_output:
- len = strnlen(outstr, INT_MAX);
+ len = strnlen(outstr, precision);
goto do_output;
}
@@ -410,19 +421,64 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
}
}
- /* 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);
+ if (v == 0) {
+ /* There are special rules for zero. */
+ if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'p')) {
+ /* "%p" match glibc, precision is ignored */
+ outstr = "(nil)";
+ len = 5;
+ goto do_output;
+ }
+ if (!precision) {
+ /* Explicit %nn.0d, no digits output */
+ len = 0;
+ goto prepend_sign;
+ }
+ /* All formats (including "%#x") just output "0". */
+ *out = '0';
+ len = 1;
} else {
- /* Base 16 */
- if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'p', '#' - 1)) {
- /* "%p" and "%#x" need "0x" prepending. */
- sign = 'x' | '0' << 8;
+ /* 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 = 'x' | '0' << 8;
+ }
+ len = u64toh_r(v, out);
+ }
+ }
+
+ /* Add zero padding */
+ if (_NOLIBC_PF_FLAGS_CONTAIN(flags, '0', '.')) {
+ if (!_NOLIBC_PF_FLAGS_CONTAIN(flags, '.')) {
+ if (_NOLIBC_PF_FLAGS_CONTAIN(flags, '-'))
+ /* Left justify overrides zero pad */
+ goto prepend_sign;
+ /* eg "%05d", Zero pad to field width less sign */
+ precision = width;
+ if (sign) {
+ precision--;
+ if (sign >= 256)
+ precision--;
+ }
+ }
+ if (precision > 30)
+ /* Don't run off the start of tmpbuf[] */
+ precision = 30;
+ for (; len < precision; len++) {
+ /* Stop gcc generating horrid code and memset().
+ * This is OPTIMIZER_HIDE_VAR() from compiler.h.
+ */
+ __asm__ volatile("" : "=r"(len) : "0"(len));
+ *--out = '0';
}
- len = u64toh_r(v, out);
}
+prepend_sign:
/* Add 0, 1 or 2 ("0x") sign characters left of any zero padding */
for (; sign; sign >>= 8) {
len++;
@@ -465,11 +521,12 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
__asm__ volatile("" : "=r"(len) : "0"(len));
/* Output 'left pad', 'value' then 'right pad'. */
+ width -= len;
flags = _NOLIBC_PF_FLAGS_CONTAIN(flags, '-');
if (flags && cb(state, outstr, len) != 0)
return -1;
- while (width > len) {
- unsigned int pad_len = ((width - len - 1) & 15) + 1;
+ while (width > 0) {
+ int pad_len = ((width - 1) & 15) + 1;
width -= pad_len;
written += pad_len;
if (cb(state, " ", pad_len) != 0)
--
2.39.5
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