One or more data buffers to be sent to the remote endpoint. 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.

Flags specifying how the send call is to be made.

The remote endpoint to which the data will be sent. Copies will be made of the endpoint as required.

The completion token that will be used to produce a completion handler, which will be called when the send 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 asio::error_code& error, // Result of operation.
  std::size_t bytes_transferred // Number of bytes sent.
);

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.