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tools/nolibc/printf: Add support for zero padding and field precision

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Includes support for variable field widths (eg "%*.*d").

Zero padding is limited to 31 zero characters.
This is wider than the largest numeric field so shouldn't be a problem.

All the standard printf formats are now supported except octal
and floating point.

Add tests for new features

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260308113742.12649-16-david.laight.linux@gmail.com
[Thomas: fixup testcases for musl libc]
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
This commit is contained in:
David Laight
2026-03-08 11:37:40 +00:00
committed by Thomas Weißschuh
parent b5f3f59cf4
commit d3d3f64f8e
2 changed files with 81 additions and 18 deletions
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80
tools/include/nolibc/stdio.h
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@@ -292,12 +292,10 @@ int fseek(FILE *stream, long offset, int whence)
/* printf(). Supports most of the normal integer and string formats.
* - %[#-+ 0][width][{l,t,z,ll,L,j,q}]{c,d,i,u,x,X,p,s,m,%}
* - %[#0-+ ][width|*[.precision|*}][{l,t,z,ll,L,j,q}]{c,d,i,u,x,X,p,s,m,%}
* - %% generates a single %
* - %m outputs strerror(errno).
* - %X outputs a..f the same as %x.
* - 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.
*
@@ -343,9 +341,9 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
char ch;
unsigned long long v;
long long signed_v;
int written, width, len;
int written, width, precision, len;
unsigned int flags, ch_flag;
char outbuf[2 + 22 + 1];
char outbuf[2 + 31 + 22 + 1];
char *out;
const char *outstr;
unsigned int sign_prefix;
@@ -378,12 +376,24 @@ 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.
@@ -444,8 +454,12 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
if (ch == 's') {
/* "%s" - character string. */
outstr = (const char *)(uintptr_t)v;
if (!outstr)
if (!outstr) {
outstr = "(null)";
/* Match glibc, nothing output if precision too small */
len = precision >= 6 ? 6 : 0;
goto do_output;
}
goto do_strlen_output;
}
@@ -468,11 +482,11 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
}
/* The value is converted offset into the buffer so that
* the sign/prefix can be added in front.
* 31 zero pad characters and the sign/prefix can be added in front.
* The longest digit string is 22 + 1 for octal conversions, the
* space is reserved even though octal isn't currently supported.
*/
out = outbuf + 2;
out = outbuf + 2 + 31;
if (v == 0) {
/* There are special rules for zero. */
@@ -482,6 +496,11 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
len = 5;
goto do_output;
}
if (!precision) {
/* Explicit %nn.0d, no digits output */
len = 0;
goto prepend_sign;
}
/* All other formats (including "%#x") just output "0". */
out[0] = '0';
len = 1;
@@ -500,6 +519,35 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
}
}
/* 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.
* Note that precision can end up negative so all
* the variables have to be 'signed int'.
*/
precision = width;
if (sign_prefix) {
precision--;
if (sign_prefix >= 256)
precision--;
}
}
if (precision > 31)
/* Don't run off the start of outbuf[], arbitrary limit
* longer than the longest number field. */
precision = 31;
for (; len < precision; len++) {
/* Stop gcc generating horrid code and memset(). */
_NOLIBC_OPTIMIZER_HIDE_VAR(len);
*--out = '0';
}
}
prepend_sign:
/* Add the 0, 1 or 2 ("0x") sign/prefix characters at the front. */
for (; sign_prefix; sign_prefix >>= 8) {
/* Force gcc to increment len inside the loop. */
@@ -534,8 +582,8 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
goto do_output;
do_strlen_output:
/* Open coded strlen() (slightly smaller). */
for (len = 0;; len++)
/* Open coded strnlen() (slightly smaller). */
for (len = 0; len < precision; len++)
if (!outstr[len])
break;