#### basic_stream_file::async_write_some

Start an asynchronous write.

    template<
        typename ConstBufferSequence,
        typename WriteToken = DEFAULT>
    DEDUCED async_write_some(
        const ConstBufferSequence & buffers,
        WriteToken && token = DEFAULT);

This function is used to asynchronously write data to the stream file. It is an initiating function for an asynchronous operation, and always returns immediately.

###### Parameters

buffers

  One or more data buffers to be written to the file. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the completion handler is called.

token

  The completion token that will be used to produce a completion handler, which will be called when the write 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.
        std::size_t bytes_transferred // Number of bytes written.
      );

  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, std::size_t)

###### Remarks

The write operation may not transmit all of the data to the peer. Consider using the `async_write` function if you need to ensure that all data is written before the asynchronous operation completes.

###### Example

To write a single data buffer use the `buffer` function as follows:

    file.async_write_some(asio::buffer(data, size), handler);

See the `buffer` documentation for information on writing multiple buffers in one go, and how to use it with arrays, boost::array or std::vector.

###### Per-Operation Cancellation

On POSIX or Windows operating systems, this asynchronous operation supports cancellation for the following `cancellation_type` values:

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