GDExtension C Example: ODR violations in `api.h`

[charjr]

Following the GDExtension C example leads to compilation errors with scons due to multiple definition warnings.

These errors are specific to the api, constructors and destructors variables defined in api.h which break the one definition rule by defining variables inside a header file:

godot-docs/tutorials/scripting/gdextension/gdextension_c_example.rst

Lines 291 to 324 in e4d463e

	.. code-block:: c
	#pragma once
	/*
	This file works as a collection of helpers to call the GDExtension API
	in a less verbose way, as well as a cache for methods from the discovery API,
	just so we don't have to keep loading the same methods again.
	*/
	#include "gdextension_interface.h"
	#include "defs.h"
	extern GDExtensionClassLibraryPtr class_library;
	// API methods.
	struct Constructors
	{
	GDExtensionInterfaceStringNameNewWithLatin1Chars string_name_new_with_latin1_chars;
	} constructors;
	struct Destructors
	{
	GDExtensionPtrDestructor string_name_destructor;
	} destructors;
	struct API
	{
	GDExtensionInterfaceClassdbRegisterExtensionClass2 classdb_register_extension_class2;
	} api;
	void load_api(GDExtensionInterfaceGetProcAddress p_get_proc_address);

My suspicion is that these variables should be static, however I cannot confirm that because I have not managed to compile a working extension.

[dsnopek]

Hm. I think the One Definition Rule is a C++ thing (not C), but perhaps these were meant to be extern?

I don't think we should be marking variables as static in a header - I think that would lead to a unique version of the variable existing in each translation unit that included the header.

[dsnopek]

Oh, it looks like @vnen merged a PR to his complete version of that tutorial which marked them as extern:

vnen/gdextension-c-example#1

Looks like it didn't make it's way back to the docs for some reason, though

[dsnopek]

I just posted PR #11389 to add extern at the variable declaration, and a definition in the .c file. I haven't tested it, but it seems right to me

@charjr Can you test if making the same change fixes the compilation errors you are seeing?

[charjr]

@dsnopek I confirm that it solved my issues with some additional changes: #11389 (comment).

I do still have one warning:

src/api.c: In function 'load_api':
src/api.c:23:52: warning: assignment to 'GDExtensionInterfaceStringNameNewWithLatin1Chars' {aka 'void (*)(void *, const char *, unsigned char)'} from incompatible pointer type 'GDExtensionInterfaceFunctionPtr' {aka 'void (*)()'} [-Wincompatible-pointer-types]
   23 |     constructors.string_name_new_with_latin1_chars = p_get_proc_address("string_name_new_with_latin1_chars");

But this has not stopped it from compiling, and is usable in the Godot editor.

[dsnopek]

Thanks for testing!

I do still have one warning:

Ah, yeah, that would be solved with a cast. It looks like these docs are inconsistent about using a cast or not when loading function pointers. Something else to fix!

[dsnopek]

I just posted PR #11390 which adds all the missing casts

[charjr]

Thanks for testing!

I just posted PR #11390 which adds all the missing casts

@dsnopek Thank you for answering! Honestly, the fact I had these issues and get to see how you fix it really helps.