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

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Message-ID: <20260128094224.11299-2-im.lechain@gmail.com>
Date: Wed, 28 Jan 2026 17:42:23 +0800
From: "licheng.li" <im.lechain@...il.com>
To: Willy Tarreau <w@....eu>
Cc: Thomas Weißschuh <linux@...ssschuh.net>,
	linux-kernel@...r.kernel.org,
	im.lechain@...il.com
Subject: [PATCH v2 1/2] tools/nolibc: support left alignment (-) and zero padding (0) in printf

From: Cheng Li <im.lechain@...il.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.

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@....eu>
Signed-off-by: Cheng Li <im.lechain@...il.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;
-- 
2.52.0