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Message-Id: <20260203103000.20206-10-david.laight.linux@gmail.com>
Date: Tue, 3 Feb 2026 10:29:57 +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 next 09/12] tools/nolibc/printf: support precision and zero padding
From: David Laight <david.laight.linux@...il.com>
Support '*' for both length and precision.
Output "(nil)" for NULL pointers (matches glibc).
Output "0" not "0x0" for printf("%#x", 0).
Output no digits for printf("%.0d", 0).
Signed-off-by: David Laight <david.laight.linux@...il.com>
---
tools/include/nolibc/stdio.h | 114 +++++++++++++++++++++++++++--------
1 file changed, 89 insertions(+), 25 deletions(-)
diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index d8b6184ff0f3..bad528921590 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -240,10 +240,15 @@ char *fgets(char *s, int size, FILE *stream)
}
-/* simple printf(). It supports the following formats:
- * - %[-+ ][width][{l,t,z,ll,L,j,q}]{d,i,u,c,x,X,p,s,m}
- * - %%
- * - invalid formats are copied to the output buffer
+/* printf(). Supports most of the normal integer and string formats.
+ * - %[#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 wide characters.
+ * - invalid formats are copied to the output buffer.
*/
typedef int (*__nolibc_printf_cb)(void *state, const char *buf, size_t size);
@@ -252,7 +257,7 @@ static __attribute__((unused, format(printf, 3, 0)))
int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list args)
{
char c;
- int len, written, width;
+ int len, written, width, precision;
unsigned int flags;
char tmpbuf[32 + 24];
const char *outstr;
@@ -277,17 +282,30 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
/* Flag characters */
for (; c >= 0x20 && c <= 0x3f; c = *fmt++) {
- if ((__PF_FLAG(c) & (__PF_FLAG('-') | __PF_FLAG(' ') | __PF_FLAG('+'))) == 0)
+ if ((__PF_FLAG(c) & (__PF_FLAG('-') | __PF_FLAG(' ') | __PF_FLAG('+') |
+ __PF_FLAG('#') | __PF_FLAG('0'))) == 0)
break;
flags |= __PF_FLAG(c);
}
- /* width */
- while (c >= '0' && c <= '9') {
- width *= 10;
- width += c - '0';
-
- c = *fmt++;
+ /* width and precision */
+ for (;; c = *fmt++) {
+ if (c == '*') {
+ precision = va_arg(args, unsigned int);
+ c = *fmt++;
+ } else {
+ for (precision = 0; c >= '0' && c <= '9'; c = *fmt++)
+ precision = precision * 10 + (c - '0');
+ }
+ if (flags & __PF_FLAG('.'))
+ break;
+ width = precision;
+ if (c != '.') {
+ /* Default precision for strings */
+ precision = INT_MAX;
+ break;
+ }
+ flags |= __PF_FLAG('.');
}
/* Length modifiers are lower case except 'L' which is the same a 'q' */
@@ -308,7 +326,7 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
if (!((c >= 'a' && c <= 'z') || (c == 'X' && (c = 'x'))))
goto bad_conversion_specifier;
- /* Conversion specifiers */
+ /* Numeric and pointer conversion specifiers */
if (__PF_FLAG(c) & (__PF_FLAG('c') | __PF_FLAG('d') | __PF_FLAG('i') | __PF_FLAG('u') |
__PF_FLAG('x') | __PF_FLAG('p') | __PF_FLAG('s'))) {
unsigned long long v;
@@ -338,12 +356,13 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
case 's':
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;
case 'd':
case 'i':
@@ -356,17 +375,62 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
} else if (flags & __PF_FLAG(' ')) {
sign = ' ';
}
- __nolibc_fallthrough;
- case 'u':
- len = u64toa_r(v, out);
- break;
- case 'p':
- sign = 'x' | '0' << 8;
- __nolibc_fallthrough;
- default: /* 'x' and 'p' above */
- len = u64toh_r(v, out);
- break;
+ c = 'u';
+ }
+
+ if (v == 0) {
+ /* There are special rules for zero. */
+ if (c == '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;
+ }
+ /* "#x" should output "0" not "0x0" */
+ *out = '0';
+ len = 1;
+ } else {
+ if (c == 'u') {
+ len = u64toa_r(v, out);
+ } else {
+ len = u64toh_r(v, out);
+ if (c == 'p' || (flags & __PF_FLAG('#')))
+ sign = 'x' | '0' << 8;
+ }
}
+
+ /* Add zero padding */
+ if (flags & (__PF_FLAG('0') | __PF_FLAG('.'))) {
+ if (!(flags & __PF_FLAG('.'))) {
+ if (flags & __PF_FLAG('-'))
+ /* Left justify overrides zero pad */
+ goto prepend_sign;
+ /* 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';
+ }
+ }
+
+prepend_sign:
for (; sign; sign >>= 8) {
len++;
*--out = sign;
--
2.39.5
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