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template< typename MutableBufferSequence, typename ReadToken = default_completion_token_t<executor_type>> DEDUCED async_read_some( const MutableBufferSequence & buffers, ReadToken && token = default_completion_token_t< executor_type >());
This function is used to asynchronously read one or more bytes of data from the stream. It is an initiating function for an asynchronous operation, and always returns immediately.
The buffers into which the data will be read. Although the buffers object may be copied as necessary, ownership of the underlying buffers is retained by the caller, which must guarantee that they remain valid until the completion handler is called.
                The completion
                token that will be used to produce a completion handler, which
                will be called when the read 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 read. );
                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.
              
void(asio::error_code, std::size_t)
          The async_read_some operation may not read all of the requested number
          of bytes. Consider using the async_read function if you need
          to ensure that the requested amount of data is read before the asynchronous
          operation completes.
        
          This asynchronous operation supports cancellation for the following cancellation_type
          values:
        
cancellation_type::terminal
            cancellation_type::partial
            
          if they are also supported by the Stream
          type's async_read_some
          and async_write_some operations.