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/codesize/test_codesize_cxx_except.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 23325,
   "a.out.js.gz": 9128,
-  "a.out.nodebug.wasm": 171265,
-  "a.out.nodebug.wasm.gz": 57337,
-  "total": 194590,
-  "total_gz": 66465,
+  "a.out.nodebug.wasm": 171311,
+  "a.out.nodebug.wasm.gz": 57348,
+  "total": 194636,
+  "total_gz": 66476,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",

test/codesize/test_codesize_cxx_except_wasm.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 19486,
   "a.out.js.gz": 8077,
-  "a.out.nodebug.wasm": 144620,
-  "a.out.nodebug.wasm.gz": 54891,
-  "total": 164106,
-  "total_gz": 62968,
+  "a.out.nodebug.wasm": 144667,
+  "a.out.nodebug.wasm.gz": 54907,
+  "total": 164153,
+  "total_gz": 62984,
   "sent": [
     "_abort_js",
     "_tzset_js",

test/codesize/test_codesize_cxx_except_wasm_legacy.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 19555,
   "a.out.js.gz": 8099,
-  "a.out.nodebug.wasm": 142209,
-  "a.out.nodebug.wasm.gz": 54350,
-  "total": 161764,
-  "total_gz": 62449,
+  "a.out.nodebug.wasm": 142256,
+  "a.out.nodebug.wasm.gz": 54366,
+  "total": 161811,
+  "total_gz": 62465,
   "sent": [
     "_abort_js",
     "_tzset_js",

test/codesize/test_codesize_cxx_mangle.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 23375,
   "a.out.js.gz": 9148,
-  "a.out.nodebug.wasm": 235299,
-  "a.out.nodebug.wasm.gz": 78928,
-  "total": 258674,
-  "total_gz": 88076,
+  "a.out.nodebug.wasm": 235343,
+  "a.out.nodebug.wasm.gz": 78952,
+  "total": 258718,
+  "total_gz": 88100,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",

test/codesize/test_codesize_hello_O0.json

@@ -1,10 +1,10 @@
 {
   "a.out.js": 24194,
   "a.out.js.gz": 8694,
-  "a.out.nodebug.wasm": 15117,
-  "a.out.nodebug.wasm.gz": 7456,
-  "total": 39311,
-  "total_gz": 16150,
+  "a.out.nodebug.wasm": 15168,
+  "a.out.nodebug.wasm.gz": 7488,
+  "total": 39362,
+  "total_gz": 16182,
   "sent": [
     "fd_write"
   ],

test/codesize/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
   "a.out.js": 245483,
-  "a.out.nodebug.wasm": 574056,
-  "total": 819539,
+  "a.out.nodebug.wasm": 573907,
+  "total": 819390,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/codesize/test_unoptimized_code_size.json

@@ -1,16 +1,16 @@
 {
   "hello_world.js": 56928,
   "hello_world.js.gz": 17700,
-  "hello_world.wasm": 15117,
-  "hello_world.wasm.gz": 7456,
+  "hello_world.wasm": 15168,
+  "hello_world.wasm.gz": 7488,
   "no_asserts.js": 26632,
   "no_asserts.js.gz": 8884,
   "no_asserts.wasm": 12217,
   "no_asserts.wasm.gz": 6015,
   "strict.js": 54943,
   "strict.js.gz": 17045,
-  "strict.wasm": 15117,
-  "strict.wasm.gz": 7451,
-  "total": 180954,
-  "total_gz": 64551
+  "strict.wasm": 15168,
+  "strict.wasm.gz": 7482,
+  "total": 181056,
+  "total_gz": 64614
 }

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