如何从Rust访问在C中声明的以0结尾的函数指针数组? [英] How do I access a zero-terminated array of function pointers declared in C from Rust?

问题描述

我有以下带有零终止函数指针数组的C代码:

#include <stdio.h>

void hello_register(void) {
  printf("hello_register called\n");
}

void (*vlog_startup_routines[])() = {
    hello_register,
    0
};

使用Cargo构建脚本编译此代码并将其链接到我的Rust程序.如何从Rust调用数组中的每个函数指针?

解决方案

前两个答案的组合看起来更好:

extern crate libc;

type VlogStartupRoutine = Option<extern "C" fn()>;

extern "C" {
    // This array is NULL-terminated; set the length to zero to
    // prevent any uncontrolled access.
    static vlog_startup_routines: [VlogStartupRoutine; 0];
}

fn main() {
    unsafe {
        let routines = vlog_startup_routines.as_ptr();

        for i in 0.. {
            match *routines.offset(i) {
                Some(routine) => {
                    println!("Calling startup routine #{}", i);
                    routine();
                }
                None => break,
            }
        }
    }
}

---

符号vlog_startup_routines不是指向函数指针的指针，而是一个函数指针数组.在C代码中使用名称vlog_startup_routines时，数组左值被强制为指针.这并不意味着该变量存储了一个指针！

要在Rust中最紧密地表达这一点，我们可以将vlog_startup_routines定义为数组.问题在于我们不知道数组中有多少个元素，因为它是NULL终止的.为防止意外使用，我们将长度设置为零，并且仅通过原始指针的偏移量访问元素.

我们将Option<extern "C" fn()>用于可为空的函数指针，如

This code is compiled and linked to my Rust program using a Cargo build script. How can I call each of the function pointers in the array from Rust?

解决方案

A combination of the previous two answers looks nicer:

extern crate libc;

type VlogStartupRoutine = Option<extern "C" fn()>;

extern "C" {
    // This array is NULL-terminated; set the length to zero to
    // prevent any uncontrolled access.
    static vlog_startup_routines: [VlogStartupRoutine; 0];
}

fn main() {
    unsafe {
        let routines = vlog_startup_routines.as_ptr();

        for i in 0.. {
            match *routines.offset(i) {
                Some(routine) => {
                    println!("Calling startup routine #{}", i);
                    routine();
                }
                None => break,
            }
        }
    }
}

---

The symbol vlog_startup_routines is not a pointer to a function pointer, it's an array of function pointers. When you use the name vlog_startup_routines in C code, the array lvalue is coerced to a pointer. That doesn't mean that the variable stores a pointer!

To most closely express this in Rust, we can define vlog_startup_routines as an array. The problem is that we don't know how many elements are in the array because it's NULL-terminated. To prevent any accidental misuse, we set the length to zero and only access elements though offsets of the raw pointer.

We use Option<extern "C" fn()> for the nullable function pointer as described in the FFI chapter of The Rust Programming Language.

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