#### posix::basic_descriptor::async_wait

Asynchronously wait for the descriptor to become ready to read, ready to write, or to have pending error conditions.

    template<
        typename WaitToken = DEFAULT>
    DEDUCED async_wait(
        wait_type w,
        WaitToken && token = DEFAULT);

This function is used to perform an asynchronous wait for a descriptor to enter a ready to read, write or error condition state. It is an initiating function for an asynchronous operation, and always returns immediately.

###### Parameters

w

  Specifies the desired descriptor state.

token

  The completion token that will be used to produce a completion handler, which will be called when the wait completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        const asio::error_code& error // Result of operation.
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `post`.

###### Completion Signature

    void(asio::error_code)

###### Example

    void wait_handler(const asio::error_code& error)
    {
      if (!error)
      {
        // Wait succeeded.
      }
    }

    ...

    asio::posix::stream_descriptor descriptor(my_context);
    ...
    descriptor.async_wait(
        asio::posix::stream_descriptor::wait_read,
        wait_handler);

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`
- `cancellation_type::total`