如何将指针发送到另一个线程？

Question

I created a Rust wrapper for a C++ library for a camera using bindgen , and the camera handle in the C++ library is defined as typedef void camera_handle which bindgen ported over as:

pub type camera_handle = ::std::os::raw::c_void;

I'm able to successfully connect to the camera and take images, however I wanted to run code on a separate thread for temperature control of the camera, essentially changing the cooler power based on the current temperature of the camera, which I want to have run separately from the rest of the code. These calls require the camera handle, but when I spawn a new thread, I keep getting the error:

'*mut std::ffi::c_void' cannot be sent between threads safely

And underneath it, it mentions:

the trait 'std::marker::Send' is not implemented for '*mut std::ffi::c_void'

How can I send this to another thread so I can use this camera handle there as well? I have tried using fragile and send_wrapper , but have been unsuccessful with both of them.

Answer 1

Pointers do not implement Send or Sync since their safety escapes the compiler. You are intended to explicitly indicate when a pointer is safe to use across threads. This is typically done via a wrapper type that you implement Send and/or Sync on yourself:

struct CameraHandle(*mut c_void);

unsafe impl Send for CameraHandle {}
unsafe impl Sync for CameraHandle {}

Since implementing these traits manually is unsafe , you should be extra diligent to ensure that the types from the external library actually can be moved another thread ( Send ) and/or can be shared by multiple threads ( Sync ).

If you ever take advantage of the pointer's mutability without the protection of &mut self , it should probably not be Sync since having two &mut T at the same time is always unsound.

See:

• Send and Sync in the Rustonomicon.
• Can a struct containing a raw pointer implement Send and be FFI safe?