rf12_canSend()
Call this when you have some data to send. If it returns true, then you can use rf12_sendStart() to start the transmission. Else you need to wait and retry this call at a later moment.
Don't call this function if you have nothing to send, because rf12_canSend() will stop reception when it returns true. IOW, calling the function indicates your intention to send something, and once it returns true, you should follow through and call rf12_sendStart() to actually initiate sending. See this weblog post.
Note that even if you only want to send out packets, you still have to call rf12_recvDone() periodically, because it keeps the RF12 logic going. If you don't, rf12_canSend() will never return true.
#include <RF12.h>
#include <Ports.h> // needed to avoid a linker error :(
byte rf12_canSend();
Parameters
None.