How to create rust functions that take a C function pointer as argument?

[ananis25]

Hi - I am trying to write a python extension module using PyO3. The rust section orchestrates the computation while the actual function to execute is provided at runtime from python.

For regular python functions, I can pass them like any other arg and PyO3 casts it to a PyObject.

#[pyfunction]
fn rust_caller(py: Python, callback: PyObject, x: i64, y: i64) -> PyResult<PyObject> {
    println!("Orchestration logic");
    for n in 0..100 {
        callback.call(py, (x, y), None)
    }
}

However the computation is numerical, so I use the numba library to compile the callback function to native code. Though, calling the compiled routine through python would acquire and release the GIL each time.

Numba also produces a C function pointer that can be passed to native libraries - link. This repository illustrates using it - link.
Is it possible to create a rust function using PyO3 that takes an extern C fn(...) as an argument?

[davidhewitt]

Sorry for the very slow reply.

I don't think it will be possible to create a #[pyfunction] that will do this. Instead you should do the same as the example repository you link: create unsafe extern "C" fn and use ctypes to call it. Unfortunately, you should be aware of #1683 (hopefully we can fix this incompatibility eventually).