Return ENOTSUP from pthread_create when threads are not available.

See WebAssembly/wasi-libc#716

Author: sbc100

ChangeLog.md

@@ -20,6 +20,8 @@ See docs/process.md for more on how version tagging works.
 
 4.0.24 (in development)
 -----------------------
+- Calling pthread_create in a single-threaded build will now return ENOTSUP
+  rather then EAGAIN.  (#26105)
 - compiler-rt and libunwind were updated to LLVM 21.1.8. (#26036 and #26045)
 - A new `-sEXECUTABLE` setting was added which adds a #! line to the resulting
   JavaScript and makes it executable.  This setting defaults to true when the

system/lib/pthread/library_pthread_stub.c

@@ -92,7 +92,11 @@ int pthread_barrier_destroy(pthread_barrier_t* mutex) { return 0; }
 int pthread_barrier_wait(pthread_barrier_t* mutex) { return 0; }
 
 int __pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg) {
-  return EAGAIN;
+  // ENOTSUP, while not mentioned in the pthread_create docs, does better
+  // describe the situation.
+  // See https://github.com/WebAssembly/wasi-libc/pull/716 for discussion
+  // on this error code vs, for example, EAGAIN.
+  return ENOTSUP;
 }
 
 weak_alias(__pthread_create, emscripten_builtin_pthread_create);

test/other/test_pthread_stub.c

@@ -22,12 +22,12 @@ void* start_pthread(void* arg) {
 }
 
 #define CHECK(X, EXPECTED) rtn = X; if (rtn != EXPECTED) printf(#X " returned %s\n", strerror(rtn)); assert(rtn == EXPECTED)
-#define CHECK_FAIL(X) CHECK(X, EAGAIN)
+#define CHECK_FAIL(X) CHECK(X, ENOTSUP)
 #define CHECK_SUCCESS(X) CHECK(X, 0)
 
 #define CHECK_C11(X, expected) rtn = X; if (rtn != expected) printf(#X " returned %d\n", rtn); assert(rtn == expected)
 #define CHECK_C11_SUCCESS(X) CHECK_C11(X, thrd_success)
-#define CHECK_C11_FAIL(X) CHECK_C11(X, thrd_nomem)
+#define CHECK_C11_FAIL(X) CHECK_C11(X, thrd_error)
 
 void test_c11_threads() {
   printf("test_c11_threads\n");

test/test_other.py

@@ -15095,7 +15095,7 @@ def test_libcxx_errors(self):
 
     # Since we are building without -pthread the thread constructor will fail,
     # and in debug mode at least we expect to see the error message from libc++
-    expected = 'system_error was thrown in -fno-exceptions mode with error 6 and message "thread constructor failed"'
+    expected = 'system_error was thrown in -fno-exceptions mode with error 138 and message "thread constructor failed"'
     self.do_runf('main.cpp', expected, assert_returncode=NON_ZERO)
 
   def test_parsetools_make_removed_fs_assert(self):