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##### basic_stream::async_read_some

Read some data asynchronously.

###### Synopsis

    template<
        class MutableBufferSequence,
        class ReadHandler = net::default_completion_token_t<executor_type>>
    DEDUCED
    async_read_some(
        MutableBufferSequence const& buffers,
        ReadHandler&& handler = net::default_completion_token_t< executor_type >{});

###### Description

This function is used to asynchronously read data from the stream.

This call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- One or more bytes are read from the stream.
- An error occurs.

The algorithm, known as a composed asynchronous operation, is implemented in terms of calls to the next layer's `async_read_some` function. The program must ensure that no other calls to `read_some` or `async_read_some` are performed until this operation completes.

If the timeout timer expires while the operation is outstanding, the operation will be canceled and the completion handler will be invoked with the error `error::timeout`.

###### Parameters

Name | Description
--- | ---
`buffers` | The buffers into which the data will be read. If the size of the buffers is zero bytes, the operation always completes immediately with no error. 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 handler is called.

`handler`

The completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. The equivalent function signature of the handler must be:

    void handler(
        error_code error,               // Result of operation.
        std::size_t bytes_transferred   // Number of bytes read.
    );

If the handler has an associated immediate executor, an immediate completion will be dispatched to it. Otherwise, the handler will not be invoked from within this function. Invocation of the handler will be performed by dispatching to the immediate executor. If no immediate executor is specified, this is equivalent to using `net::post`.

###### Remarks

The `async_read_some` operation may not receive all of the requested number of bytes. Consider using the function `net::async_read` if you need to ensure that the requested amount of data is read before the asynchronous operation completes.

###### Per-Operation Cancellation

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

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

if they are also supported by the socket's `async_read_some` operation.