How to use tokio lib with dll in windows?

help
1 
#[tokio::DllMain]
#[no_mangle]
extern "system" async fn DllMain(_: *const u8, _: u32, _: *const u8) -> u32{ 
    let resp = reqwest::get("http://httpbin.org/ip")
        .await?
        .json::<HashMap<String, String>>()
        .await?;
    println!("{:#?}", resp);
    Ok(())
}
2

Where are you getting that tokio::DllMain attribute from? I can't see anything in the docs for its Windows build mentioning that.

The tokio::main attribute is just shorthand for creating a new runtime and blocking until the function body completes.

From the attribute's examples, this snippet

#[tokio::main(flavor = "current_thread")]
async fn main() {
    println!("Hello world");
}

expands to

fn main() {
    tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .unwrap()
        .block_on(async {
            println!("Hello world");
        })
}
1 Like
3

I mean

#[no_mangle]
extern "system" fn DllMain(_: *const u8, _: u32, _: *const u8) -> u32{ 
resolve_http().await // how to call await in Dllmain    
0
}

#[tokio::main]
async fn resolve_http() -> reqwest::Result<()>{
let resp = reqwest::get("http://httpbin.org/ip")
        .await?
        .json::<HashMap<String, String>>()
        .await?;
    println!("{:#?}", resp);
    Ok(())
}
4

The same as in any other sync function - by explicitly creating the runtime and calling block_on or equivalent. An example for Tokio was provided above.

2 Likes
5

You can't use .await in any function that isn't a Rust's native async fn or async {} block. Functions that use .await don't directly execute the code, but rather they transform it to be a state machine encoded in a Future struct. DllMain is supposed to be a function, not a Future struct, so this can't work.

However, you can wait for Futures in a regular function, in a blocking way (non-async) by using a Tokio runtime handle and its block_on method.

tokio::main macro doesn't do much. It's basically this:

  tokio::runtime::Builder::new()
                    .basic_scheduler()
                    .enable_all()
                    .build()
                    .unwrap()
                    .block_on(async { function body goes here })
1 Like
6

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