luaopen_ functions should be extern "C"

[phoppermann]

No description provided.

[alerque]

PRs welcome, plus a bit more explanation or links to documentation for why.

[phoppermann]

Compare e.g. https://github.com/lunarmodules/luafilesystem/blob/ff2cbc21f1a2fd34d30c0840ff2abb0121d46131/src/lfs.h#L34.

This is required if you want to compile as C++ (not C), otherwise the symbols are not found in require. This has been advised for Lua itself (see ThePhD/sol2#739), so I think it should be possible for the modules as well.

[alerque]

Thanks for the explanation and links. Yes I think this makes sense. Again PRs welcome. As a bonus some test tooling to show it working (or failing) for C and C++ builds would help too.

[daurnimator]

Lua itself has no support for throwing with C++ exceptions, anyone doing that must be using custom patches against lua itself.

Lua itself also does not support declaring luaopen functions inside of extern "C", see https://www.lua.org/source/5.4/luaconf.h.html#LUAMOD_API
Instead I believe you have to include the entire lua header(s) inside of an extern "C".

Lua modules such as luasocket should similarly be included inside of an extern "C" block.

[phoppermann]

Instead I believe you have to include the entire lua header(s) inside of an extern "C".

Lua modules such as luasocket should similarly be included inside of an extern "C" block.

How would I do that in

local socket = require("socket")

?

[daurnimator]

How would I do that in

local socket = require("socket")

?

huh? It's a question of how you compile (or include) luasocket; not how you load it into a running interpreter

[phoppermann]

For instance, when requiring a.b.c, it will search for a C library for a. If found, it looks into it for an open function for the submodule; in our example, that would be luaopen_a_b_c.

https://www.lua.org/manual/5.4/manual.html

So when you require from a C++ Lua, it will not work without extern "C" because of different name mangling.

[daurnimator]

So when you require from a C++ Lua, it will not work without extern "C" because of different name mangling.

Do you mean the library will have the wrong symbol name? or Lua will have the wrong symbol names?

Lua's C module loader uses dlsym to find symbols, it doesn't know about C++ name mangling

[phoppermann]

Well, I've encountered the issue when require luasocket as is from C++ Lua. If you tell me that this cannot happen because the module loader doesn't care about C/C++, I have no clue what's going on. Also, if this is the case, why do other modules use extern "C" at all?

[daurnimator]

@phoppermann how did you compile luasocket?

[phoppermann]

Also as C++ (like Lua itself to be aligned). Maybe that's not the right way?