备注

AI Translation Notice

This document was automatically translated by Qwen/Qwen3-8B model, for reference only.

Source document: kernel/core_api/casting.md
Translation time: 2025-05-19 01:41:11
Translation model: Qwen/Qwen3-8B

Please report issues via Community Channel

Type Conversion Library API

The kernel provides some functions to help you convert between different types. These include the following types:

Numeric type conversion (using the num-traits library)
Arc type conversion

All functions not specially marked are implemented in kernel/src/libs/casting.rs.

1. Numeric Type Conversion

1.1 Conversion Between Integer Types and Enum Types

You can use macros provided by the num-traits library to convert between enum types and integer types. The SystemError enum type uses this approach, and you can find its usage in kernel/src/syscall/mod.rs.

It first inherits the FromPrimitive, ToPrimitive two traits, and then performs the conversion like this:

impl SystemError {
    /// @brief 把posix错误码转换为系统错误枚举类型。
    pub fn from_posix_errno(errno: i32) -> Option<SystemError> {
        // posix 错误码是小于0的
        if errno >= 0 {
            return None;
        }
        return <Self as FromPrimitive>::from_i32(-errno);
    }

    /// @brief 把系统错误枚举类型转换为负数posix错误码。
    pub fn to_posix_errno(&self) -> i32 {
        return -<Self as ToPrimitive>::to_i32(self).unwrap();
    }
}

These two functions well illustrate how to use these two traits.

2. Arc Type Conversion

2.1 Conversion from Arc to Arc

When we need to convert an Arc<dyn U> to a specific type pointer of Arc<T>, we need to implement the DowncastArc trait for U. This trait is defined in kernel/src/libs/casting.rs. It requires trait U to implement the Any + Sync + Send trait.

To implement the DowncastArc trait for trait U: Any + Send + Sync, you need to do the following:

impl DowncastArc for dyn U {
    fn as_any_arc(self: Arc<Self>) -> Arc<dyn Any> {
        return self;
    }
}

Using the DowncastArc trait, we can convert like this:

let arc: Arc<dyn U> = ...;
let arc_t: Arc<T> = arc.downcast_arc::<T>().unwrap();

If the specific type of arc is not Arc<T>, then downcast_arc::<T>() will return None.