From X-Plane SDK
XPLM_API void XPLMSetDatab( XPLMDataRef inDataRef, void * inValue, int inOffset, int inLength);
Write part or all of a byte array dataref. The values passed by inValues are written into the dataref starting at inOffset. Up to inCount values are written; however if the values would write "off the end" of the dataref array, then fewer values are written.
Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.