ptr::null() 与 null_mut() 函数

这两个函数用于创建空指针, 它指向的内存地址是 0, 常用它们来初始化或者重置原始指针.

它们的区别是:

• null() 用于创建只读的空指针, 即不能通过该指针修改它指向的内存, 返回的是 *const T 指针, 类似于C代码的写法: const i32* ptr = NULL;
• null_mut() 用于创建可改写的空指针 (mutable pointer), 返回的是 *mut T 指针, 类似于C代码的写法: i32* ptr = NULL;

use std::ptr;

fn main() {
    let ptr: *const i32 = ptr::null();
    assert!(ptr.is_null());
}

这一组函数常用于 FFI 相关的代码, 比如下面的代码片段, 调用 mmap(2) 时, 将 start_pointer 设置为空指针, 这样的话 linux 内核会自动选择合适的地址作为内存页的起始地址:

use std::ffi::c_void;
use std::ptr;

fn main() {
    let path = "/etc/passwd";
    let fd = unsafe { nc::openat(nc::AT_FDCWD, path, nc::O_RDONLY, 0o644) };
    assert!(fd.is_ok());
    let fd = fd.unwrap();

    let mut sb = nc::stat_t::default();
    let ret = unsafe { nc::fstat(fd, &mut sb) };
    assert!(ret.is_ok());

    let offset: usize = 0;
    let length: usize = sb.st_size as usize - offset;
    // Offset for mmap must be page aligned.
    let pa_offset: usize = offset & !(nc::PAGE_SIZE - 1);
    let map_length = length + offset - pa_offset;

    let addr = unsafe {
        nc::mmap(
            ptr::null(),
            map_length,
            nc::PROT_READ,
            nc::MAP_PRIVATE,
            fd,
            pa_offset as nc::off_t,
        )
    };
    assert!(addr.is_ok());
    let addr: *const c_void = addr.unwrap();

    let stdout = 1;
    // Create buffer slice.
    let buf: &[u8] = unsafe {
        &*ptr::slice_from_raw_parts(
            addr.wrapping_add(offset)
                .wrapping_sub(pa_offset)
                .cast::<u8>(),
            length,
        )
    };
    let n_write = unsafe { nc::write(stdout, buf) };
    assert!(n_write.is_ok());
    assert_eq!(n_write, Ok(length as nc::ssize_t));
    let ret = unsafe { nc::munmap(addr, map_length) };
    assert!(ret.is_ok());
    let ret = unsafe { nc::close(fd) };
    assert!(ret.is_ok());
}