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 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 handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the 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 handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using asio::io_context::post().