[WASM_WORKERS] Don't define pthread function in WASM_WORKERS builds (#26336)

Using the pthread API with WASM_WORKERS simply will not work.

This also means function-local-static C++ initializes will now fail to
link with WASM_WORKERS, at least until we fix #26277. Note that prior to
this change this was already broken but would fail at runtime.

See #26314

Author: sbc100

ChangeLog.md

@@ -20,6 +20,10 @@ See docs/process.md for more on how version tagging works.
 
 5.0.3 (in development)
 ----------------------
+- When building with `-sWASM_WORKERS` emscripten will no longer include pthread
+  API stub functions.  These stub functions where never designed to work under
+  Wasm Workers, so its safer to error at link time if pthread APIs are used
+  in Wasm Worker-based programs. (#26336)
 
 5.0.2 - 02/25/26
 ----------------

system/lib/libc/emscripten_yield_stub.c

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2026 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 <features.h>
+
+static void dummy(double now) {
+}
+
+weak_alias(dummy, _emscripten_check_timers);
+
+void _emscripten_yield(double now) {
+  _emscripten_check_timers(now);
+}

system/lib/pthread/library_pthread_stub.c

@@ -17,11 +17,14 @@
 #include <emscripten/threading.h>
 #include <emscripten/emscripten.h>
 
+#ifdef __EMSCRIPTEN_WASM_WORKERS__
+#error "This file contains stubs that should not be included in wasm workers builds."
+#endif
+
 bool emscripten_has_threading_support() { return false; }
 
 int emscripten_num_logical_cores() { return 1; }
 
-#ifndef __EMSCRIPTEN_WASM_WORKERS__
 // These low level primites are defined in both pthreads and wasm workers
 // builds.
 
@@ -55,7 +58,6 @@ void __unlock(void* ptr) {}
 void __acquire_ptc() {}
 
 void __release_ptc() {}
-#endif
 
 void emscripten_main_thread_process_queued_calls() {
   // nop
@@ -65,16 +67,6 @@ void emscripten_current_thread_process_queued_calls() {
   // nop
 }
 
-static void dummy(double now)
-{
-}
-
-weak_alias(dummy, _emscripten_check_timers);
-
-void _emscripten_yield(double now) {
-  _emscripten_check_timers(now);
-}
-
 int pthread_mutex_init(
   pthread_mutex_t* __restrict mutex, const pthread_mutexattr_t* __restrict attr) {
   return 0;

test/test_other.py

@@ -13519,6 +13519,16 @@ def test_shared_memory_preprocessor_flags(self):
   def test_wasm_worker_preprocessor_flags(self):
     self.run_process([EMCC, '-c', test_file('wasm_worker/wasm_worker_preprocessor_flags.c'), '-sWASM_WORKERS'])
 
+  @also_with_minimal_runtime
+  def test_wasm_worker_pthread_api_usage(self):
+    self.assert_fail([EMCC, test_file('wasm_worker/wasm_worker_pthread_api_usage.c'), '-sWASM_WORKERS'], 'undefined symbol: pthread_mutex_lock')
+
+  @also_with_minimal_runtime
+  def test_wasm_worker_cxx_init(self):
+    # For now, C++ init guards do not work with Wasm Workers.
+    # See https://github.com/emscripten-core/emscripten/issues/26277
+    self.assert_fail([EMCC, test_file('wasm_worker/wasm_worker_cxx_init.cpp'), '-sWASM_WORKERS'], 'undefined symbol: pthread_cond_wait')
+
   @parameterized({
     # we will warn here since -O2 runs the optimizer and -g enables DWARF
     'O2_g': (True, ['-O2', '-g']),

test/wasm_worker/wasm_worker_cxx_init.cpp

@@ -0,0 +1,28 @@
+#include <stdio.h>
+
+struct Foo {
+  Foo() {
+    printf("create Foo\n");
+  }
+
+  ~Foo() {
+    printf("destroy Foo\n");
+  }
+
+  void baz() {
+    printf("method called\n");
+  }
+};
+
+
+void bar() {
+  static Foo foo;
+  foo.baz();
+}
+
+int main() {
+  printf("main\n");
+  bar();
+  printf("done\n");
+  return 0;
+}

test/wasm_worker/wasm_worker_pthread_api_usage.c

@@ -0,0 +1,7 @@
+#include <pthread.h>
+
+int main() {
+  // This program should fail to link with WASM_WORKERS.
+  pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;
+  pthread_mutex_lock(&my_mutex);
+}

tools/system_libs.py

@@ -1256,13 +1256,10 @@ def get_files(self):
           'thrd_sleep.c',
           'thrd_yield.c',
         ])
+
       libc_files += files_in_path(
-        path='system/lib/pthread',
-        filenames=[
-          'library_pthread_stub.c',
-          'pthread_self_stub.c',
-          'proxying_stub.c',
-        ])
+        path='system/lib/libc',
+        filenames=['emscripten_yield_stub.c'])
 
       if self.is_ww:
         libc_files += files_in_path(
@@ -1272,6 +1269,18 @@ def get_files(self):
             '__wait.c',
             'lock_ptc.c',
           ])
+      else:
+        # Include stub version of thread functions when building
+        # in single theaded mode.
+        # Note: We do *not* include these stubs in the Wasm Workers build since it would
+        # never be safe to call these from a Wasm Worker.
+        libc_files += files_in_path(
+          path='system/lib/pthread',
+          filenames=[
+            'library_pthread_stub.c',
+            'pthread_self_stub.c',
+            'proxying_stub.c',
+          ])
 
     if self.is_mt or self.is_ww:
       # Low level thread primitives available in both pthreads and wasm workers builds.