[Tinyos-help] AMSend, T2 tutorial 3, K&R C, nesC language questions

[John Griessen]

I"ve been working through the T2 tutorial 3, and use AMsend instead of 
Packet  just to self teach,
and hit a cryptic C part I want to ask for alternative  ways to write.  
It comes up using
the Packet or AMSend interfaces.  If you use Packet.getPayLoad(&pkt, 
NULL) or
AMSend.getPayLoad(&pkt) to find out the address of the next packet, you 
can use
a line from the tutorial and it compiles, but is so opaque to me I want 
to find other ways to write it.

      BlinkToRadioMsg* btrpkt = (BlinkToRadioMsg*) (call 
AMSend.getPayload(&pkt));
      //cast result of .getPayload to pointer of nx_struct BlinkToRadioMsg*
      btrpkt->nodeid = TOS_NODEID;    // stuff data in struct member
      btrpkt->counter = counter;            // stuff data in struct member

where the header file includes this struct definition:

     typedef nx_struct BlinkToRadioMsg {
       nx_uint16_t nodeid;
       nx_uint16_t counter;
     } BlinkToRadioMsg;

Is there any way to write the top line more like:

BlinkToRadioMsg* btrpkt ;   // declare type of btrpkt is address of struct
btrpkt = (call AMSend.getPayload(&pkt));

or maybe just:
BlinkToRadioMsg* btrpkt ;   // declare type of btrpkt variable
call AMSend.getPayload(&btrpkt)




What is the meaning of
(BlinkToRadioMsg*)  (call AMSend.getPayload(&pkt));
 in the top line?  Is this the syntax for coercing to type BlinkToRadioMsg*?
Where in the docs is this syntax defined? 

It's not like K&R C...where
you would declare *btrpkt to get a pointer you could use as btrpkt->nodeid


John Griessen

Ecosensory