Don't use wasm global data sections when compiling with multithreading enabled so that pthread_create() will not reset the global data members. Instead use a separate .mem file or base64-embedded data in JS file for memory initialization. See WebAssembly/threads#62

Author: juj

emcc.py

@@ -1675,7 +1675,7 @@ def repl(m):
           if DEBUG:
             # Copy into temp dir as well, so can be run there too
             shared.safe_copy(memfile, os.path.join(shared.get_emscripten_temp_dir(), os.path.basename(memfile)))
-          if not shared.Settings.BINARYEN or 'asmjs' in shared.Settings.BINARYEN_METHOD or 'interpret-asm2wasm' in shared.Settings.BINARYEN_METHOD:
+          if not shared.Settings.BINARYEN or 'asmjs' in shared.Settings.BINARYEN_METHOD or 'interpret-asm2wasm' in shared.Settings.BINARYEN_METHOD or shared.Settings.USE_PTHREADS:
             return 'memoryInitializer = "%s";' % shared.JS.get_subresource_location(memfile, embed_memfile(options))
           else:
             return 'memoryInitializer = null;'
@@ -2267,9 +2267,12 @@ def do_binaryen(final, target, asm_target, options, memfile, wasm_binary_target,
     if options.opt_level > 0:
       cmd.append(shared.Building.opt_level_to_str(options.opt_level, options.shrink_level))
     # import mem init file if it exists, and if we will not be using asm.js as a binaryen method (as it needs the mem init file, of course)
+    # Note that importing (embedding) memory init file into the .wasm module as a data section is not compatible with multithreading in WebAssembly,
+    # because each thread would reinitialize the global data section at thread creation time, so only embed a data section to the generated
+    # .wasm file if not using multithreading
     mem_file_exists = options.memory_init_file and os.path.exists(memfile)
     ok_binaryen_method = 'asmjs' not in shared.Settings.BINARYEN_METHOD and 'interpret-asm2wasm' not in shared.Settings.BINARYEN_METHOD
-    import_mem_init = mem_file_exists and ok_binaryen_method
+    import_mem_init = mem_file_exists and ok_binaryen_method and not shared.Settings.USE_PTHREADS
     if import_mem_init:
       cmd += ['--mem-init=' + memfile]
       if not shared.Settings.RELOCATABLE:

tests/pthread/test_pthread_global_data_initialization.c

@@ -0,0 +1,36 @@
+#include <assert.h>
+#include <pthread.h>
+#include <emscripten.h>
+
+int globalData = 1;
+
+void *thread_main(void *arg)
+{
+  EM_ASM(Module.print('hello from pthread 1: ' + $0), globalData);
+  assert(globalData == 10);
+
+  globalData = 20;
+  EM_ASM(Module.print('hello from pthread 2: ' + $0), globalData);
+  assert(globalData == 20);
+	return 0;
+}
+
+int main()
+{
+  EM_ASM(Module.print('hello from main 1: ' + $0), globalData);
+  assert(globalData == 1);
+
+  globalData = 10;
+  EM_ASM(Module.print('hello from main 2: ' + $0), globalData);
+  assert(globalData == 10);
+
+	pthread_t thread;
+	pthread_create(&thread, NULL, thread_main, NULL);
+  pthread_join(thread, 0);
+
+  EM_ASM(Module.print('hello from main 3: ' + $0), globalData);
+  assert(globalData == 20);
+#ifdef REPORT_RESULT
+  REPORT_RESULT(globalData);
+#endif
+}

tests/test_browser.py

@@ -3362,6 +3362,12 @@ def test_pthread_call_async_on_main_thread(self):
     self.btest_wasm(path_from_root('tests', 'pthread', 'call_async_on_main_thread.c'), expected='7', args=['-O3', '-s', 'USE_PTHREADS=1', '-DPROXY_TO_PTHREAD=0', '--js-library', path_from_root('tests', 'pthread', 'call_async_on_main_thread.js')])
     self.btest_wasm(path_from_root('tests', 'pthread', 'call_async_on_main_thread.c'), expected='7', args=['-Oz', '-DPROXY_TO_PTHREAD=0', '--js-library', path_from_root('tests', 'pthread', 'call_async_on_main_thread.js')])
 
+  # Tests that spawning a new thread does not cause a reinitialization of the global data section of the application memory area.
+  def test_pthread_global_data_initialization(self):
+    for mem_init_mode in [[], ['--memory-init-file', '0'], ['--memory-init-file', '1'], ['--memory-init-file', '2']]:
+      for args in [[], ['-O3']]:
+        self.btest_wasm(path_from_root('tests', 'pthread', 'test_pthread_global_data_initialization.c'), expected='20', args=args+mem_init_mode+['-s', 'USE_PTHREADS=1', '-s', 'PROXY_TO_PTHREAD=1'])
+
   # test atomicrmw i64
   def test_atomicrmw_i64(self):
     Popen([PYTHON, EMCC, path_from_root('tests', 'atomicrmw_i64.ll'), '-s', 'USE_PTHREADS=1', '-s', 'IN_TEST_HARNESS=1', '-o', 'test.html']).communicate()