[VCSTARTUP] Implement atexit and _onexit

These must use a local table and cannot use the one from ucrtbase.
The table is initialized with a .CRT section initializer.

sdk/lib/vcruntime/CMakeLists.txt

@@ -35,6 +35,8 @@ list(APPEND VCRT_STARTUP_SOURCES
     __acrt_initialize_stub.cpp
     __scrt_uninitialize_crt.cpp
     __vcrt_init_stubs.c
+    _onexit.c
+    atexit.c
     mainCRTStartup.cpp
     wmainCRTStartup.cpp
 )

sdk/lib/vcruntime/_onexit.c

@@ -0,0 +1,34 @@
+//
+// _onexit.c
+//
+//      Copyright (c) 2024 Timo Kreuzer
+//
+// Implementation of _onexit.
+//
+// SPDX-License-Identifier: MIT
+//
+
+#include <stdlib.h>
+#include <internal_shared.h>
+#include <corecrt_startup.h>
+#include <assert.h>
+
+_onexit_table_t module_local_atexit_table;
+int module_local_atexit_table_initialized = 0;
+
+int __cdecl __scrt_initialize_onexit(void)
+{
+    _initialize_onexit_table(&module_local_atexit_table);
+    module_local_atexit_table_initialized = 1;
+    return 0;
+}
+
+// CRT startup initializer
+_CRTALLOC(".CRT$XIAA") _PIFV const __scrt_onexit_initializer = __scrt_initialize_onexit;
+
+_onexit_t __cdecl _onexit(_In_opt_ _onexit_t _Func)
+{
+    assert(module_local_atexit_table_initialized == 1);
+    int result = _register_onexit_function(&module_local_atexit_table, _Func);
+    return (result == 0) ? _Func : NULL;
+}

sdk/lib/vcruntime/atexit.c

@@ -0,0 +1,18 @@
+//
+// atexit.c
+//
+//      Copyright (c) 2024 Timo Kreuzer
+//
+// Implementation of atexit.
+//
+// SPDX-License-Identifier: MIT
+//
+
+#include <stdlib.h>
+
+int __cdecl atexit(void (__cdecl* _Func)(void))
+{
+    // Go through _onexit, so that the initializer is pulled in.
+    _onexit_t result = _onexit((_onexit_t)_Func);
+    return (result == NULL) ? -1 : 0;
+}

sdk/lib/vcruntime/inc/internal_shared.h

@@ -31,6 +31,7 @@ extern "C" {
 #pragma section(".CRT$XPXA", long, read)
 
 #pragma section(".CRT$XIA", long, read)
+#pragma section(".CRT$XIAA", long, read) // CRT startup
 #pragma section(".CRT$XIZ", long, read)
 #pragma section(".CRT$XCA", long, read)
 #pragma section(".CRT$XCZ", long, read)