kernel: Add C++ main() support

The C++ standard requires the main() function to have the return type
of 'int' and does not allow the main() to be defined with the 'void'
return type. Moreover, GCC goes as far as to emit a hard error when the
'::main()' has the return type of `void`.

This commit introduces an option to instruct the Zephyr kernel to call
the 'int main(void)' instead of the 'void main(void)' in case a Zephyr
application defines main() in a C++ source file.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>

kernel/init.c

@@ -321,9 +321,13 @@ static void bg_thread_main(void *unused1, void *unused2, void *unused3)
 	z_mem_manage_boot_finish();
 #endif /* CONFIG_MMU */
 
+#ifdef CONFIG_CPP_MAIN
+	extern int main(void);
+#else
 	extern void main(void);
+#endif
 
-	main();
+	(void)main();
 
 	/* Mark nonessential since main() has no more work to do */
 	z_main_thread.base.user_options &= ~K_ESSENTIAL;

kernel/main_weak.c

@@ -21,8 +21,16 @@
 
 #include <kernel_internal.h>
 
+#ifdef CONFIG_CPP_MAIN
+int __weak main(void)
+#else
 void __weak main(void)
+#endif
 {
 	/* NOP default main() if the application does not provide one. */
 	arch_nop();
+
+#ifdef CONFIG_CPP_MAIN
+	return 0;
+#endif
 }

subsys/cpp/Kconfig

@@ -54,6 +54,17 @@ config STD_CPP2B
 
 endchoice
 
+config CPP_MAIN
+	bool "C++ main() function definition"
+	help
+	  This option instructs the Zephyr kernel to call the 'int main(void)'
+	  instead of the 'void main(void)', which is the default main() type
+	  for Zephyr.
+
+	  C++ does not allow the main() to be defined with 'void' return type,
+	  and any applications defining its main() in a C++ source file must
+	  enable this option.
+
 config LIB_CPLUSPLUS
 	bool "Link with STD C++ library"
 	depends on !MINIMAL_LIBC