asio C++ library



Start an asynchronous read at the specified offset.

    typename MutableBufferSequence,
    typename ReadHandler>
void-or-deduced async_read_some_at(
    uint64_t offset,
    const MutableBufferSequence & buffers,
    ReadHandler handler);

This function is used to asynchronously read data from the random-access handle. The function call always returns immediately.



The offset at which the data will be read.


One or more buffers into which the data will be read. 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.


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(
  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 handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using asio::io_service::post().


The read operation may not read all of the requested number of bytes. Consider using the async_read_at function if you need to ensure that the requested amount of data is read before the asynchronous operation completes.


To read into a single data buffer use the buffer function as follows:

handle.async_read_some_at(42, asio::buffer(data, size), handler);

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