This structure describes the http request body. If application specifies the data to send, the data must remain valid until the HTTP request is sent. Alternatively, application can choose to specify total_size as the total data size to send instead while leaving the data NULL (and its size 0). In this case, HTTP request will then call on_send_data() callback once it is ready to send the request body. This will be useful if application does not wish to load the data into the buffer at once.
Default is empty.