如何獲得下一個指針？

Question

我有一個用C語言實現的函數，我想用Rust接口寫一個函數。該函數接收一個指向數組開頭（win8_t *）和數組長度的指針。我需要能夠遍歷數組。

必須有一個更好的方式來獲得下一個值，但現在我可以做這個奇怪的事情：

use std::mem; 
pub extern "C" fn print_next(i: *const u8) { 
    let mut ii = unsafe { mem::transmute::<*const u8, i64>(i) }; 
    ii += 1; 
    let iii = unsafe { mem::transmute::<i64, *const u8>(ii) }; 
    let jj = unsafe { *iii }; 
    println!("{}", jj); // jj is next value 
} 

Answer 1

正如Shepmaster說，你可能需要提供片的長度。

大多數時候你正在使用指針，你的函數將是不安全的（因爲你通常需要在某些時候解引用它）。將他們安全責任委派給調用者是不安全的，這可能是一個好主意。

下面是使用offset和from_raw_slice一些例子：

use std::mem; 
use std::slice; 

// unsafe! 
pub extern "C" fn print_next(i: *const u8) { 
    let mut ii = unsafe { mem::transmute::<*const u8, i64>(i) }; 
    ii += 1; 
    let iii = unsafe { mem::transmute::<i64, *const u8>(ii) }; 
    let jj = unsafe { *iii }; 
    println!("{}", jj); // jj is next value 
} 

// unsafe! 
pub unsafe extern "C" fn print_next2(i: *const u8) { 
    let j = *i.offset(1); 
    println!("{}", j); 
} 

// (less but still ...) unsafe! 
pub unsafe extern "C" fn print_next3(i: *const u8, len: usize) { 
    let slice = slice::from_raw_parts(i, len); 
    // we are not checking the size ... so it may panic! 
    println!("{}", slice[1]); 
} 

fn main() { 
    let a = [9u8, 4, 6, 7]; 
    print_next(&a as *const u8); 
    unsafe { 
     print_next2(&a[1] as *const u8); 
     print_next3(&a[2] as *const u8, 2); 
    } 

    // what if I print something not in a?? 
    print_next(&a[3] as *const u8); // BAD 
    unsafe { 
     print_next2(&a[3] as *const u8); // BAD 
     print_next3(&a[3] as *const u8, 2); // as bad as others, length is wrong 

     print_next3(&a[3] as *const u8, 1); // panic! out of bounds 
    } 
}