tools/nolibc: implement strerror()

strerror() is commonly used.
For example in kselftest which currently needs to do an #ifdef NOLIBC to
handle the lack of strerror().

Keep it simple and reuse the output format of perror() for strerror().
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/include/nolibc/stdio.h

@@ -376,6 +376,16 @@ int setvbuf(FILE *stream __attribute__((unused)),
 	return 0;
 }
 
+static __attribute__((unused))
+const char *strerror(int errno)
+{
+	static char buf[18] = "errno=";
+
+	i64toa_r(errno, &buf[6]);
+
+	return buf;
+}
+
 /* make sure to include all global symbols */
 #include "nolibc.h"
 

tools/testing/selftests/nolibc/nolibc-test.c

@@ -1210,6 +1210,10 @@ int run_stdlib(int min, int max)
 		CASE_TEST(strtol_underflow);        EXPECT_STRTOX(1, strtol, "-0x8000000000000001", 16, LONG_MIN, -1, ERANGE); break;
 		CASE_TEST(strtoul_negative);        EXPECT_STRTOX(1, strtoul, "-0x1", 16, ULONG_MAX, 4, 0); break;
 		CASE_TEST(strtoul_overflow);        EXPECT_STRTOX(1, strtoul, "0x10000000000000000", 16, ULONG_MAX, -1, ERANGE); break;
+		CASE_TEST(strerror_success);        EXPECT_STREQ(is_nolibc, strerror(0), "errno=0"); break;
+		CASE_TEST(strerror_EINVAL);         EXPECT_STREQ(is_nolibc, strerror(EINVAL), "errno=22"); break;
+		CASE_TEST(strerror_int_max);        EXPECT_STREQ(is_nolibc, strerror(INT_MAX), "errno=2147483647"); break;
+		CASE_TEST(strerror_int_min);        EXPECT_STREQ(is_nolibc, strerror(INT_MIN), "errno=-2147483648"); break;
 
 		case __LINE__:
 			return ret; /* must be last */