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使用C回调用户数据存储盒装铁锈封口时的分段错误

ffi segmentation-fault rust

1

ItsTimmy · 技术社区 · 7 年前

Test::trigger_callback(...)

extern crate libc;

use libc::c_void;
use std::mem::transmute;

struct Test {
    callback: extern "C" fn(data: i32, user: *mut c_void) -> (),
    userdata: *mut c_void,
}

extern "C" fn c_callback(data: i32, user: *mut libc::c_void) {
    unsafe {
        println!("Line {}. Ptr: {}", line!(), user as u64);
        let func: &mut Box<FnMut(i32) -> ()> = transmute(user);
        println!("Line {}. Data: {:?}", line!(), data);
        (*func)(data);
        println!("Line {}", line!());
    }
}

impl Test {
    fn new<F>(func: F) -> Test
    where
        F: FnMut(i32) -> (),
        F: 'static,
    {
        let func = Box::into_raw(Box::new(Box::new(func)));
        println!("Line: {}, Ptr: {}", line!(), func as u64);

        Test {
            callback: c_callback,
            userdata: func as *mut c_void,
        }
    }

    fn trigger_callback(&self, data: i32) {
        (self.callback)(data, self.userdata);
    }
}

fn main() {
    let test = Test::new(|data: i32| {
        println!("Inside callback! Data: {}", data);
    });

    test.trigger_callback(12345);
}

如链接答案所述, Box 盒子

运行时,此代码打印出来:

Line: 29, Ptr: 140589704282120
Line 13. Ptr: 140589704282120
Line 15. Data: 12345
Segmentation fault (core dumped)

extern "C"

为什么?据我所知,在然后使用 Box::into_raw(...)

1 回复 | 直到 7 年前

1

Shepmaster Tim Diekmann 7 年前

Box::into_raw(Box::new(Box::new(func)));

   = note: expected type `()`
              found type `*mut std::boxed::Box<F>`

let func: &mut Box<FnMut(i32) -> ()> = transmute(user);

// Trait object with a stable address
let func = Box::new(func) as Box<FnMut(i32)>;
// Thin pointer
let func = Box::new(func);
// Raw pointer
let func = Box::into_raw(func);

Box<FnMut(i32) -> ()>

返回类型 () Box<FnMut(i32)>

让func:&mut box<fnmut(i32)->()>=transmute(用户);

很难避免 transmute

extern "C" fn c_callback(data: i32, user: *mut libc::c_void) {
    let user = user as *mut Box<FnMut(i32)>;
    unsafe {
        (*user)(data);
    }
}

类型别名

type CallbackFn = Box<FnMut(i32)>;

let user = user as *mut CallbackFn;

let func = Box::new(func) as CallbackFn;

另请参见:

The Rust FFI Omnibus