代码

#[no_mangle]
// extern "C" fn using(f: extern "C" fn()) {
extern "C" fn using(f: *const u8) {
    let a = (&f) as *const _ as *const fn();
    let b = unsafe { *a };
    println!("DLL开始");
    b();
    println!("DLL结束");
}

extern "C" fn a() {
    println!("a()...");
}

#[no_mangle]
extern "C" fn DllMain(hinstDLL: *mut u8, reason: u32, reserved: *mut u8) -> u32 {
    match reason {
        1 => {
            // DLL_PROCESS_ATTACH
            println!("连接到进程!");
            intro();
        }
        0 => {
            // DLL_PROCESS_DETACH
            println!("检测到进程退出");
        }
        _ => (),
    }
    return 1;
}

fn intro() {
    let p = a as extern "C" fn ();
    println!("p -> {:p}.", p); // 该地址正确，不过类型是fn()

    let p = &a as *const _ as *const u8;
    println!("p -> {:p}.", p); // 似乎是临时变量的地址

    let p = &(a as extern "C" fn ()) as *const _ as *const *mut u8;
    println!("p -> {:p}.", p); // 这个肯定是临时变量的地址，因为解引用之后正是a函数的地址

    println!("p -> {:p}.", unsafe { *p }); // a函数正确的地址
    unsafe {
        using(*p); // 该函数接收的是一个C函数的地址
    }
}