rosserial & sending str_msg

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Another really basic question but I cannot seem to figure it out:

I am trying to transmit data from a ROS node on an arduino. I figured that the best way to do this was through a str_msg. I have set the str_msg.data and I send that to my publisher which is set up to take in a str_msg. I am wondering if this is all that I need to do or if there are other fields that I need to populate before I even attempt to publish them?