does it work in a Windows cdylib?

[vpopescu]

Is using this in a cdylib (on windows) supported?

I am trying to follow the documentation, and although I can get it to work in an exe, when I try it in a cdylib I get linkage errors coming from the C++ compiler (the C++ functions cannot be found).

For example I have a class:

            class MyClass
            {
            public:
                MyClass();
                uint32_t something(uint32_t input) const;
                ...
           }

and on the rust side, I have:

mod ffi {
    unsafe extern "C++" {
        include!("myclass.h");
        type MyClass;
        
        fn new_cxx_test() -> UniquePtr<MyClass>;
        fn exec(&self, t: u32 ) -> u32;
    }
}

When I compile this, I get a link failure (trimmed down a bit):

cxx-bridge.lib(lib.rs.o) : error LNK2019: unresolved external symbol "public: unsigned int __cdecl com::my::area::MyClass::exec(unsigned int)const " (?exec@MyClass@area@my@com@@QEBAII@Z) referenced in function com$my$area$cxxbridge1$MyClass$exec
 
cxx-bridge.lib(lib.rs.o) : error LNK2019: unresolved external symbol "class std::unique_ptr<class com::my::area::MyClass,struct std::default_delete<class com::my::area::MyClass> > __cdecl com::my::area::new_cxx_test(void)" (?new_cxx_test@area@my@com@@YA?AV?$unique_ptr@VMyClass@area@my@com@@U?$default_delete@VMyClass@area@my@com@@@std@@@std@@XZ) referenced in function com$my$area$cxxbridge1$new_cxx_test

 C:\source\mytest.dll : fatal error LNK1120: 2 unresolved externals

[dtolnay]

I don't know anything about cdylib on Windows, but possibly it's related to #231. Is the error any different if you do this?

class MyClass {
public:
  __declspec(dllexport) MyClass();
  __declspec(dllexport) uint32_t something(uint32_t input) const;
};

[vpopescu]

I did not see any difference in the message when __declspec(dllexport) was added. I will keep trying a few permutations. The *.lib file seems correct, but I'm no expert. Incidentally it also fails as a dylib, but not as an rlib.

Also, as an observation, I tried to add a block like this in my cpp *.h file:

#ifdef CXX_DLL
# define EXPORT __declspec(dllexport)
#else
# define EXPORT __declspec(dllimport)
#endif

and then in the *.cpp file, I added the following before the header file is included

#define CXX_DLL 1

However it seems like somewhere in the process of building the DLL, the header file is being read directly, and it compiles it with __declspec(dllimport) instead of __declspec(dllexport) since CXX_DLL is not set.

Will update if I learn anything new.

[dtolnay]

Sounds good. One other knob that may be fruitful to mess with (if you are using cxx-build / build.rs / Cargo as your build system) is right here:

cxx/gen/build/src/lib.rs

Lines 399 to 403 in 8b7643b

	let opt = Opt {
	allow_dot_includes: false,
	cfg_evaluator: Box::new(CargoEnvCfgEvaluator),
	..Opt::default()
	};

You can try adding:

cxx_impl_annotations: Some("__declspec(dllexport)".to_owned()),

in that struct to make cxx-build stick a given attribute onto every signature in the generated binding. (Obviously we would expose this in a better way if it turns out to work.)

[vpopescu]

I managed to get it to work. I don't think it has anything to do with linkage, there is probably something I don't understand in the sample. The root of the problem was probably a misunderstanding between my header file, and the code that was generated by the build crate from the cpp file. Once I managed to fix that, it works. I did not have to use the explicit export annotations.

The part I don't understand is this (from sample):

#include "demo/include/blobstore.h"
#include "demo/src/main.rs.h"
#include <algorithm>
...

In my case, I cannot include both "blobstore.h" (or my equivalent thereof) and "main.rs.h" because I end up with redefinition errors. I must be missing something very basic in the sample. I had to comment out main.rs.h (or in my case, lib.rs.h) to get it to compile.

Anyway, sorry for the false alarm, you can close this as "user error" or something. ;)

[dtolnay]

I cannot include both "blobstore.h" (or my equivalent thereof) and "main.rs.h" because I end up with redefinition errors.

Missing a #pragma once in your header maybe?

[vpopescu]

No, it's there, but I probably just don't understand the flow here, what causes the conflict below, why those two would conflict.

[... ]\plugin_cxx\src\cxx\defs.h(22): error C2011: '_Manifest': 'struct' type redefinition
[...]\build\plugin_cxx-f864598f2490960a\out\cxxbridge\crate\plugin_cxx/src/cxx/defs.h(21): note: see declaration of '_Manifest'

I have a declaration such as this in my defs.h:

typedef struct __Manifest
{
  //...
} Manifest, *PManifest;

[dtolnay]

It's unlikely there is any more guidance I would be able to provide on this, so I'll close the issue at this point. Good luck!