feat: support Option<fn()> in WrapperApi

[zitsen]

use dlopen2::wrapper::{Container, WrapperApi};
use std::os::raw::c_int;

struct Api<'a> {
    c_fun: unsafe extern "C" fn(),
    // A function may not exist in the library.
    c_option_fun: Option<unsafe extern "C" fn() -> c_int>,
}

fn main(){
    let mut cont: Container<Api> =
        unsafe { Container::load("libexample.so") }.expect("Could not open library or load symbols");
    unsafe{ cont.c_fun() };

    if cont.has_c_option_fun() {
      // do something
    }
    // option function returns Option<fn_return_type>, it's Option<c_int> here.
    unsafe{ cont.c_option_fun().map(|i| i == 0) };
}

[zitsen]

We use dlopen2 to load multiple versions of libraries. dlopen2_allow_null is helpful, but fn pointer is a bit more complex to use. Currently we use Option fn and extract each fn mannually with dlopen2 raw api. It's better to support Option types in WrapperApi derive impl.

Hope it can be accepted. Please have a look @OpenByteDev .

[OpenByteDev]

Published in 0.6.0