emscripten-core · sbc100 · Mar 18, 2026 · Mar 18, 2026
diff --git a/src/struct_info_generated.json b/src/struct_info_generated.json
@@ -1036,11 +1036,11 @@
             "p_proto": 8
         },
         "pthread": {
-            "__size__": 132,
-            "profilerBlock": 112,
-            "stack": 52,
-            "stack_size": 56,
-            "waiting_async": 128
+            "__size__": 124,
+            "profilerBlock": 104,
+            "stack": 48,
+            "stack_size": 52,
+            "waiting_async": 120
         },
         "pthread_attr_t": {
             "__size__": 44,

diff --git a/src/struct_info_generated_wasm64.json b/src/struct_info_generated_wasm64.json
@@ -1036,11 +1036,11 @@
             "p_proto": 16
         },
         "pthread": {
-            "__size__": 232,
-            "profilerBlock": 200,
-            "stack": 88,
-            "stack_size": 96,
-            "waiting_async": 228
+            "__size__": 216,
+            "profilerBlock": 184,
+            "stack": 80,
+            "stack_size": 88,
+            "waiting_async": 212
         },
         "pthread_attr_t": {
             "__size__": 88,

diff --git a/system/lib/libc/musl/src/internal/locale_impl.h b/system/lib/libc/musl/src/internal/locale_impl.h
@@ -45,9 +45,17 @@ hidden char *__gettextdomain(void);
 #define C_LOCALE ((locale_t)&__c_locale)
 #define UTF8_LOCALE ((locale_t)&__c_dot_utf8_locale)
 
+#ifdef __EMSCRIPTEN__
+extern _Thread_local locale_t __tls_locale;
+
+#define CURRENT_LOCALE (__tls_locale)
+
+#define CURRENT_UTF8 (!!__tls_locale->cat[LC_CTYPE])
+#else
 #define CURRENT_LOCALE (__pthread_self()->locale)
 
 #define CURRENT_UTF8 (!!__pthread_self()->locale->cat[LC_CTYPE])
+#endif
 
 #undef MB_CUR_MAX
 #define MB_CUR_MAX (CURRENT_UTF8 ? 4 : 1)

diff --git a/system/lib/libc/musl/src/internal/pthread_impl.h b/system/lib/libc/musl/src/internal/pthread_impl.h
@@ -39,7 +39,10 @@ struct pthread {
 
 	/* Part 2 -- implementation details, non-ABI. */
 	int tid;
+#ifndef __EMSCRIPTEN__
+	// Emscripten uses C11 _Thread_local instead for errno
 	int errno_val;
+#endif
 	volatile int detach_state;
 	volatile int cancel;
 	volatile unsigned char canceldisable, cancelasync;
@@ -60,7 +63,10 @@ struct pthread {
 	} robust_list;
 	int h_errno_val;
 	volatile int timer_id;
+#ifndef __EMSCRIPTEN__
+	// Emscripten uses C11 _Thread_local instead for locale
 	locale_t locale;
+#endif
 	volatile int killlock[1];
 	char *dlerror_buf;
 	void *stdio_locks;

diff --git a/system/lib/libc/musl/src/locale/uselocale.c b/system/lib/libc/musl/src/locale/uselocale.c
@@ -2,13 +2,24 @@
 #include "pthread_impl.h"
 #include "libc.h"
 
+#ifdef __EMSCRIPTEN__
+_Thread_local locale_t __tls_locale = &libc.global_locale;
+#endif
+
 locale_t __uselocale(locale_t new)
 {
+#ifdef __EMSCRIPTEN__
+	locale_t old = __tls_locale;
+	locale_t global = &libc.global_locale;
+
+	if (new) __tls_locale = new == LC_GLOBAL_LOCALE ? global : new;
+#else
 	pthread_t self = __pthread_self();
 	locale_t old = self->locale;
 	locale_t global = &libc.global_locale;
 
 	if (new) self->locale = new == LC_GLOBAL_LOCALE ? global : new;
+#endif
 
 	return old == global ? LC_GLOBAL_LOCALE : old;
 }

diff --git a/system/lib/pthread/library_pthread.c b/system/lib/pthread/library_pthread.c
@@ -147,7 +147,6 @@ void _emscripten_init_main_thread(void) {
   // Main thread ID is always 1.  It can't be 0 because musl assumes
   // tid is always non-zero.
   __main_pthread.tid = getpid();
-  __main_pthread.locale = &libc.global_locale;
   // pthread struct prev and next should initially point to itself (see __init_tp),
   // this is used by pthread_key_delete for deleting thread-specific data.
   __main_pthread.next = __main_pthread.prev = &__main_pthread;

diff --git a/system/lib/pthread/pthread_create.c b/system/lib/pthread/pthread_create.c
@@ -182,7 +182,6 @@ int __pthread_create(pthread_t* restrict res,
   // pthread struct robust_list head should point to itself.
   new->robust_list.head = &new->robust_list.head;
 
-  new->locale = &libc.global_locale;
   if (attr._a_detach) {
     new->detach_state = DT_DETACHED;
   } else {

diff --git a/system/lib/pthread/pthread_self_stub.c b/system/lib/pthread/pthread_self_stub.c
@@ -30,7 +30,6 @@ extern int __stack_low;
 
 __attribute__((constructor))
 static void init_pthread_self(void) {
-  __main_pthread.locale = &libc.global_locale;
   __main_pthread.tid = getpid();
   __main_pthread.stack = &__stack_high;
   __main_pthread.stack_size = ((size_t)&__stack_high) - ((size_t)&__stack_low);

diff --git a/test/codesize/test_codesize_cxx_ctors1.json b/test/codesize/test_codesize_cxx_ctors1.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 19214,
   "a.out.js.gz": 7981,
-  "a.out.nodebug.wasm": 132658,
-  "a.out.nodebug.wasm.gz": 49938,
-  "total": 151872,
-  "total_gz": 57919,
+  "a.out.nodebug.wasm": 132638,
+  "a.out.nodebug.wasm.gz": 49927,
+  "total": 151852,
+  "total_gz": 57908,
   "sent": [
     "__cxa_throw",
     "_abort_js",

diff --git a/test/codesize/test_codesize_cxx_ctors2.json b/test/codesize/test_codesize_cxx_ctors2.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 19191,
   "a.out.js.gz": 7966,
-  "a.out.nodebug.wasm": 132082,
-  "a.out.nodebug.wasm.gz": 49598,
-  "total": 151273,
-  "total_gz": 57564,
+  "a.out.nodebug.wasm": 132064,
+  "a.out.nodebug.wasm.gz": 49586,
+  "total": 151255,
+  "total_gz": 57552,
   "sent": [
     "__cxa_throw",
     "_abort_js",

diff --git a/test/codesize/test_codesize_cxx_except.json b/test/codesize/test_codesize_cxx_except.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 22875,
   "a.out.js.gz": 8956,
-  "a.out.nodebug.wasm": 172547,
-  "a.out.nodebug.wasm.gz": 57453,
-  "total": 195422,
-  "total_gz": 66409,
+  "a.out.nodebug.wasm": 172540,
+  "a.out.nodebug.wasm.gz": 57452,
+  "total": 195415,
+  "total_gz": 66408,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",

diff --git a/test/codesize/test_codesize_cxx_except_wasm.json b/test/codesize/test_codesize_cxx_except_wasm.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 19025,
   "a.out.js.gz": 7908,
-  "a.out.nodebug.wasm": 147940,
-  "a.out.nodebug.wasm.gz": 55311,
-  "total": 166965,
-  "total_gz": 63219,
+  "a.out.nodebug.wasm": 147922,
+  "a.out.nodebug.wasm.gz": 55312,
+  "total": 166947,
+  "total_gz": 63220,
   "sent": [
     "_abort_js",
     "_tzset_js",

diff --git a/test/codesize/test_codesize_cxx_except_wasm_legacy.json b/test/codesize/test_codesize_cxx_except_wasm_legacy.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 19099,
   "a.out.js.gz": 7930,
-  "a.out.nodebug.wasm": 145746,
-  "a.out.nodebug.wasm.gz": 54944,
-  "total": 164845,
-  "total_gz": 62874,
+  "a.out.nodebug.wasm": 145729,
+  "a.out.nodebug.wasm.gz": 54945,
+  "total": 164828,
+  "total_gz": 62875,
   "sent": [
     "_abort_js",
     "_tzset_js",

diff --git a/test/codesize/test_codesize_cxx_lto.json b/test/codesize/test_codesize_cxx_lto.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 18562,
   "a.out.js.gz": 7668,
-  "a.out.nodebug.wasm": 101965,
-  "a.out.nodebug.wasm.gz": 39470,
-  "total": 120527,
-  "total_gz": 47138,
+  "a.out.nodebug.wasm": 101956,
+  "a.out.nodebug.wasm.gz": 39461,
+  "total": 120518,
+  "total_gz": 47129,
   "sent": [
     "a (emscripten_resize_heap)",
     "b (_setitimer_js)",

diff --git a/test/codesize/test_codesize_cxx_mangle.json b/test/codesize/test_codesize_cxx_mangle.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 22925,
   "a.out.js.gz": 8978,
-  "a.out.nodebug.wasm": 238988,
-  "a.out.nodebug.wasm.gz": 79856,
-  "total": 261913,
-  "total_gz": 88834,
+  "a.out.nodebug.wasm": 238981,
+  "a.out.nodebug.wasm.gz": 79855,
+  "total": 261906,
+  "total_gz": 88833,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",

diff --git a/test/codesize/test_codesize_cxx_noexcept.json b/test/codesize/test_codesize_cxx_noexcept.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 19214,
   "a.out.js.gz": 7981,
-  "a.out.nodebug.wasm": 134678,
-  "a.out.nodebug.wasm.gz": 50780,
-  "total": 153892,
-  "total_gz": 58761,
+  "a.out.nodebug.wasm": 134661,
+  "a.out.nodebug.wasm.gz": 50777,
+  "total": 153875,
+  "total_gz": 58758,
   "sent": [
     "__cxa_throw",
     "_abort_js",

diff --git a/test/codesize/test_codesize_cxx_wasmfs.json b/test/codesize/test_codesize_cxx_wasmfs.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 7053,
   "a.out.js.gz": 3325,
-  "a.out.nodebug.wasm": 172754,
-  "a.out.nodebug.wasm.gz": 63338,
-  "total": 179807,
-  "total_gz": 66663,
+  "a.out.nodebug.wasm": 172736,
+  "a.out.nodebug.wasm.gz": 63329,
+  "total": 179789,
+  "total_gz": 66654,
   "sent": [
     "__cxa_throw",
     "_abort_js",

diff --git a/test/codesize/test_codesize_hello_O0.json b/test/codesize/test_codesize_hello_O0.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 24271,
-  "a.out.js.gz": 8729,
-  "a.out.nodebug.wasm": 14973,
-  "a.out.nodebug.wasm.gz": 7397,
-  "total": 39244,
-  "total_gz": 16126,
+  "a.out.js.gz": 8724,
+  "a.out.nodebug.wasm": 14850,
+  "a.out.nodebug.wasm.gz": 7311,
+  "total": 39121,
+  "total_gz": 16035,
   "sent": [
     "fd_write"
   ],
@@ -33,7 +33,6 @@
     "$__emscripten_stdout_seek",
     "$__errno_location",
     "$__fwritex",
-    "$__get_tp",
     "$__lock",
     "$__lshrti3",
     "$__memcpy",
@@ -43,7 +42,6 @@
     "$__original_main",
     "$__stdio_write",
     "$__strerror_l",
-    "$__syscall_getpid",
     "$__towrite",
     "$__trunctfdf2",
     "$__unlock",
@@ -65,8 +63,6 @@
     "$fmt_x",
     "$frexp",
     "$getint",
-    "$getpid",
-    "$init_pthread_self",
     "$main",
     "$memchr",
     "$out",

diff --git a/test/codesize/test_codesize_hello_dylink.json b/test/codesize/test_codesize_hello_dylink.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 26256,
   "a.out.js.gz": 11230,
-  "a.out.nodebug.wasm": 17675,
-  "a.out.nodebug.wasm.gz": 8932,
-  "total": 43931,
-  "total_gz": 20162,
+  "a.out.nodebug.wasm": 17670,
+  "a.out.nodebug.wasm.gz": 8926,
+  "total": 43926,
+  "total_gz": 20156,
   "sent": [
     "__syscall_stat64",
     "emscripten_resize_heap",

diff --git a/test/codesize/test_codesize_hello_dylink_all.json b/test/codesize/test_codesize_hello_dylink_all.json
@@ -1,7 +1,7 @@
 {
   "a.out.js": 244367,
-  "a.out.nodebug.wasm": 577300,
-  "total": 821667,
+  "a.out.nodebug.wasm": 577323,
+  "total": 821690,
   "sent": [
     "IMG_Init",
     "IMG_Load",
@@ -1908,6 +1908,7 @@
     "__sysv_signal",
     "__threwValue",
     "__timezone",
+    "__tls_locale",
     "__tolower_l",
     "__toupper_l",
     "__towctrans_l",

diff --git a/test/codesize/test_codesize_minimal_pthreads.json b/test/codesize/test_codesize_minimal_pthreads.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 7884,
-  "a.out.js.gz": 3854,
-  "a.out.nodebug.wasm": 19726,
-  "a.out.nodebug.wasm.gz": 9135,
-  "total": 27610,
-  "total_gz": 12989,
+  "a.out.js.gz": 3855,
+  "a.out.nodebug.wasm": 19707,
+  "a.out.nodebug.wasm.gz": 9122,
+  "total": 27591,
+  "total_gz": 12977,
   "sent": [
     "a (memory)",
     "b (emscripten_get_now)",

diff --git a/test/codesize/test_codesize_minimal_pthreads_memgrowth.json b/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
@@ -1,10 +1,10 @@
 {
   "a.out.js": 8306,
-  "a.out.js.gz": 4059,
-  "a.out.nodebug.wasm": 19727,
-  "a.out.nodebug.wasm.gz": 9137,
-  "total": 28033,
-  "total_gz": 13196,
+  "a.out.js.gz": 4060,
+  "a.out.nodebug.wasm": 19708,
+  "a.out.nodebug.wasm.gz": 9123,
+  "total": 28014,
+  "total_gz": 13183,
   "sent": [
     "a (memory)",
     "b (emscripten_get_now)",

diff --git a/test/codesize/test_minimal_runtime_code_size_hello_embind_val.json b/test/codesize/test_minimal_runtime_code_size_hello_embind_val.json
@@ -3,8 +3,8 @@
   "a.html.gz": 371,
   "a.js": 5353,
   "a.js.gz": 2524,
-  "a.wasm": 5806,
-  "a.wasm.gz": 2737,
-  "total": 11707,
-  "total_gz": 5632
+  "a.wasm": 5797,
+  "a.wasm.gz": 2732,
+  "total": 11698,
+  "total_gz": 5627
 }
diff --git a/test/codesize/test_minimal_runtime_code_size_random_printf_wasm.json b/test/codesize/test_minimal_runtime_code_size_random_printf_wasm.json
@@ -1,4 +1,4 @@
 {
-  "a.html": 10873,
-  "a.html.gz": 5681
+  "a.html": 10815,
+  "a.html.gz": 5649
 }
diff --git a/test/codesize/test_minimal_runtime_code_size_random_printf_wasm2js.json b/test/codesize/test_minimal_runtime_code_size_random_printf_wasm2js.json
@@ -1,4 +1,4 @@
 {
-  "a.html": 17175,
-  "a.html.gz": 7509
+  "a.html": 17114,
+  "a.html.gz": 7478
 }