Add error handling for stderrno when errno is invalid

system/lib/libc/musl/src/errno/__strerror.h

@@ -3,7 +3,9 @@
  * with these messages, and then to define a lookup table translating
  * error codes to offsets of corresponding fields in the structure. */
 #if defined(__EMSCRIPTEN__)
-// This is intended to match the errno in llvm-libc.
+/* Error handling is introduced to match the behavior in llvm-libc.
+ * When invalid errno is specified, Unknown error: errno_code is emitted.
+ */
 E(0,            "Success")
 #else
 E(0,            "No error information")

system/lib/libc/musl/src/errno/strerror.c

@@ -34,15 +34,19 @@ char *__strerror_l(int e, locale_t loc)
 	if (e==EDQUOT) e=0;
 	else if (e==EDQUOT_ORIG) e=EDQUOT;
 #endif
-	if (e >= sizeof errmsgidx / sizeof *errmsgidx) e = 0;
-	s = (char *)&errmsgstr + errmsgidx[e];
 #ifdef __EMSCRIPTEN__
+	if (e < 0 || e >= sizeof errmsgidx / sizeof *errmsgidx || (e != 0 && !errmsgidx[e])) {
+		return "Unknown error";
+	}
+	s = (char *)&errmsgstr + errmsgidx[e];
 	// strerror is a (debug) dependency of many emscripten syscalls which mean it
 	// must be excluded from LTO, along with all of its dependencies.
 	// In order to limit the transitive dependencies we disable localization of
 	// rrno messages here.
 	return (char *)s;
 #else
+	if (e >= sizeof errmsgidx / sizeof *errmsgidx) e = 0;
+	s = (char *)&errmsgstr + errmsgidx[e];
 	return (char *)LCTRANS(s, LC_MESSAGES, loc);
 #endif
 }

test/test_other.py

@@ -13124,6 +13124,9 @@ def test_wasmfs_before_preload(self):
   def test_hello_world_above_2gb(self):
     self.do_run_in_out_file_test('hello_world.c', cflags=['-sGLOBAL_BASE=2GB', '-sINITIAL_MEMORY=3GB'])
 
+  def test_unistd_strerror(self):
+    self.do_run_in_out_file_test('unistd/strerror.c')
+
   def test_hello_function(self):
     # hello_function.cpp is referenced/used in the docs.  This test ensures that it
     # at least compiles.

test/unistd/strerror.c

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2025 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+int main() {
+    printf("errno: %s\n", strerror(-1));
+}

test/unistd/strerror.out

@@ -0,0 +1 @@
+errno: Unknown error