add c ffi so people can call casbin-rs from c code

[xcaptain]

Writing a pure C version casbin is quite difficult, but using rust ffi can be very easy. This commit adds C-bindings, so C/C++ programmer can easily integrate casbin into their projects.

[xcaptain]

A simple link demo

[0x8f701]

I think we should continue from this PR and make it possible to call rust from c/c++, it's a great idea!

[hsluoyz]

I have a question. If this idea (PR) works, do we still need to develop: https://github.com/casbin/casbin-cpp ?

[DevinR528]

I bet it could be done, this was a really good idea @xcaptain!! Here's a few links I found while looking around cbindgen more helpful info

All of the C binding stuff could be in a new crate that imports the nessesary stuff from casbin-rs and with a build.rs file generates cpp files. According to the second link it is possible using the cpp crate to take the cdylib and generate cpp bindings.

[xcaptain]

@DevinR528 I did some experiments with cbindgen but sadly it doesn't work, because we have used

Box<dyn Adapter>
// or
*mut dyn Adapter

According to cbindgen's doc

cbindgen also cannot support wide pointers like &dyn Trait or &[T], as their layout and ABI is not guaranteed. In the case of slices you can at least decompose them into a pointer and length, and reconstruct them with slice::from_raw_parts.

but a hand written

pub extern "C" fn new_adapter(dsn: *const c_char) -> Box<dyn Adapter>

can be converted to

extern struct Adapter *new_adapter(char *dsn);

It's a little tricky but we were lucky it worked.

I also tried FFI diesel-adapter and again we were lucky, it worked.

Later I will do some experiments on multi-thread scenarios to see if rust's RWLock works in C code, if every thing goes well then I will start porting more APIs to ffi/c.rs.

As for the cpp crate, it used to embed C++ code into rust project, seems no relation with this pull request. I don't think a C++ binding is needed, because a C binding is enough, also write a well structured C++ binding may be hard.

[divy9881]

Hello everyone, this will be a fruitful feature for rust users where they do not require a authorization engine as its C/C++ client project's main requirement but as a RFC(Remote function call) interface like requirement where they do not need to use the whole heavy-weight set-up for C++. Obviously this engine won't be as fast as the main C/C++ library but can be easy to plugin in projects.

[hsluoyz]

Any update?

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