[Tinyos-help] Simple Question on CC2420PacketP Implementation

[Ali Shah]

Hi list, CC2420PacketP defines this command:

  async command cc2420_header_t * ONE CC2420PacketBody.getHeader( message_t*
ONE msg ) {
    return TCAST(cc2420_header_t* ONE, (uint8_t *)msg + offsetof(message_t,
data) - sizeof( cc2420_header_t ));
  }
My question is: why use the offsetof() macro when we know where the header
begins as message_t has a field devoted exactly for the message header:

tos/types/message.h:
typedef nx_struct message_t {
  nx_uint8_t header[sizeof(message_header_t)];
  nx_uint8_t data[TOSH_DATA_LENGTH];
  nx_uint8_t footer[sizeof(message_footer_t)];
  nx_uint8_t metadata[sizeof(message_metadata_t)];
} message_t;

Thanks,
Ali
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