How to check if function pointer passed from C is non-NULL

You can use Option<...> to represent nullable function pointers. It is incorrect to have a NULL value for a value of type fn(...) so the Option wrapper is required for cases like this.

For example,

#[no_mangle]
pub extern "C" fn call_c_function(value: i32, fun: Option<fn(i32) -> i32>) -> i32 {
    if let Some(f) = fun {
        f(value)
    }
}

However, there's one extra point: fun is a C function, but the type fn(...) is a Rust function. They're not directly compatible (e.g. their calling conventions differ). One needs to use the extern "C" fn(...) (a.k.a. extern fn(...)) type when interacting with C function pointers:

#[no_mangle]
pub extern "C" fn call_c_function(value: i32, fun: Option<extern "C" fn(i32) -> i32>) -> i32 {
    if let Some(f) = fun {
        f(value)
    }
}