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Start an asynchronous write at the specified offset.

    typename ConstBufferSequence,
    typename WriteToken = DEFAULT>
DEDUCED async_write_some_at(
    uint64_t offset,
    const ConstBufferSequence & buffers,
    WriteToken && token = DEFAULT);

This function is used to asynchronously write data to the random-access handle. It is an initiating function for an asynchronous operation, and always returns immediately.



The offset at which the data will be written.


One or more data buffers to be written to the handle. 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 completion handler is called.


The completion token that will be used to produce a completion handler, which will be called when the write 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 boost::system::error_code& error, // Result of operation.
  std::size_t bytes_transferred // Number of bytes written.

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.

Completion Signature
void(boost::system::error_code, std::size_t)

The write operation may not write all of the data to the file. Consider using the async_write_at function if you need to ensure that all data is written before the asynchronous operation completes.


To write a single data buffer use the buffer function as follows:

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

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

Per-Operation Cancellation

This asynchronous operation supports cancellation for the following cancellation_type values: