如何检查从C传递的函数指针是否为非NULL

问题描述：

下面的示例代码

锈部分:

#[no_mangle]
pub extern fn call_c_function(value: i32, fun: fn(i32) -> i32) -> i32 {
    fun(value)
}

还有C部分:

int32_t call_c_function(int32_t value, int32_t (*fun)(int32_t));

int32_t triple(int32_t x)
{
    return x*3;
}

int main(int argc, char *argv[])
{
    int32_t value = 3;
    int32_t result = call_c_function(value, triple);

    printf("%d tripled is %d\n", value, result);

    call_c_function(0, NULL);  // Crash here

    return EXIT_SUCCESS;
}

当然，call_c_function的第二次调用将崩溃. Rust编译器不会抱怨call_c_function内部的不安全代码，因为从rust的角度来看，该代码是安全的.另外，也不能简单地写:

Of course second call of call_c_function will crash. Rust compiler will not complain about unsafe code inside call_c_function, because from rust point of view this code is safe. Also it's not allowed to simply write:

if !fun.is_null() {
    fun(value)
}

因为fun类型是fn(i32) -> i32(它不是指针).

because fun type is fn(i32) -> i32 (it's not a pointer).

所以我的问题是，如何保护call_c_function防止NULL指针取消引用?有什么方法可以检查从C传递的回调是否无效?

So my question is, how I can protect call_c_function against NULL pointer dereference? Is there any way to check if callback passed from C is not valid?

也许我必须更改call_c_function定义?

答

您可以使用Option<...>表示可为空的函数指针.对于类型为fn(...)的值具有NULL值是不正确的，因此在这种情况下需要使用Option包装器.

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.

例如，

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

但是，还有一个要点:fun是C函数，而类型fn(...)是Rust函数.它们不直接兼容(例如，它们的调用约定不同).与C函数指针进行交互时，需要使用extern "C" fn(...)(也称为extern fn(...))类型:

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)
    }
}

如何检查从C传递的函数指针是否为非NULL

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