The device from which the data is to be read. The type must support the AsyncRandomAccessReadDevice concept.

The offset at which the data will be read.

A basic_streambuf object into which the data will be read. Ownership of the streambuf is retained by the caller, which must guarantee that it remains valid until the completion handler is called.

The function object to be called to determine whether the read operation is complete. The signature of the function object must be:

std::size_t completion_condition(
  // Result of latest async_read_some_at operation.
  const asio::error_code& error,

  // Number of bytes transferred so far.
  std::size_t bytes_transferred
);

A return value of 0 indicates that the read operation is complete. A non-zero return value indicates the maximum number of bytes to be read on the next call to the device's async_read_some_at function.

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(
  // Result of operation.
  const asio::error_code& error,

  // Number of bytes copied into the buffers. If an error
  // occurred, this will be the number of bytes successfully
  // transferred prior to the error.
  std::size_t bytes_transferred
);

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.