linux-kernel - [PATCH 02/15] tools/nolibc: add %m printf format

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Message-Id: <20250423-nolibc-misc-v1-2-a925bf40297b@linutronix.de>
Date: Wed, 23 Apr 2025 17:01:32 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Willy Tarreau <w@....eu>, 
 Thomas Weißschuh <linux@...ssschuh.net>, 
 Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH 02/15] tools/nolibc: add %m printf format

The %m format can be used to format the current errno.
It is non-standard but supported by other commonly used libcs like glibc and
musl, so applications do rely on them.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 tools/include/nolibc/stdio.h                 |  7 +++++++
 tools/testing/selftests/nolibc/nolibc-test.c | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index fb0417477759ee6c9663e84807c1d1067e735dec..02426682c34bcd82d9556036c6c3e73a6a5a3b4d 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -17,6 +17,8 @@
 #include "string.h"
 #include "compiler.h"
 
+static const char *strerror(int errnum);
+
 #ifndef EOF
 #define EOF (-1)
 #endif
@@ -289,6 +291,11 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
 				if (!outstr)
 					outstr="(null)";
 			}
+#ifndef NOLIBC_IGNORE_ERRNO
+			else if (c == 'm') {
+				outstr = strerror(errno);
+			}
+#endif /* NOLIBC_IGNORE_ERRNO */
 			else if (c == '%') {
 				/* queue it verbatim */
 				continue;
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index 3e15a25ccddf5135db1f59bce1eefdff2ffe57f6..b7440a667db6b541a2548bdf5182bee0277100ed 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -1393,6 +1393,23 @@ static int test_scanf(void)
 	return 0;
 }
 
+int test_strerror(void)
+{
+	char buf[100];
+	ssize_t ret;
+
+	memset(buf, 'A', sizeof(buf));
+
+	errno = EINVAL;
+	ret = snprintf(buf, sizeof(buf), "%m");
+	if (is_nolibc) {
+		if (ret < 6 || memcmp(buf, "errno=", 6))
+			return 1;
+	}
+
+	return 0;
+}
+
 static int run_printf(int min, int max)
 {
 	int test;
@@ -1421,6 +1438,7 @@ static int run_printf(int min, int max)
 		CASE_TEST(number_width); EXPECT_VFPRINTF(10, "         1", "%10d", 1); break;
 		CASE_TEST(width_trunc);  EXPECT_VFPRINTF(25, "                    ", "%25d", 1); break;
 		CASE_TEST(scanf);        EXPECT_ZR(1, test_scanf()); break;
+		CASE_TEST(strerror);     EXPECT_ZR(1, test_strerror()); break;
 		case __LINE__:
 			return ret; /* must be last */
 		/* note: do not set any defaults so as to permit holes above */

-- 
2.49.0