IPC and Message Queue

  typedef struct MyMsg {
      MessageQ_MsgHeader header;
      ... [I need to add my own data here]
  } MyMsg;

And here is the header used:

typedef struct {
    Bits32       reserved0;         /*!< reserved for List.elem->next       */
    Bits32       reserved1;         /*!< reserved for List.elem->prev       */
    Bits32       msgSize;           /*!< message size                       */
    Bits16       flags;             /*!< bitmask of different flags         */
    Bits16       msgId;             /*!< message id                         */
    Bits16       dstId;             /*!< destination queue id               */
    Bits16       dstProc;           /*!< destination processor id           */
    Bits16       replyId;           /*!< reply id                           */
    Bits16       replyProc;         /*!< reply processor                    */
    Bits16       srcProc;           /*!< source processor                   */
    Bits16       heapId;            /*!< heap id                            */
    Bits16       seqNum;            /*!< sequence number                    */
    Bits16       reserved;          /*!< reserved                           */
} MessageQ_MsgHeader;


Who is populating the fields in the message? Is this done internally (dstProc, srcProc and all the reply information)
Should I assume that the writer set all these fields and the reader can use them?
The documentation is stating: Your application should not modify or directly access the fields in the MessageQ_MsgHeader but
I could not find an API to access all the fields (ex: the proc)
Is there an API to set msgSize or is it fixed to the size I asked to allocate?
What is the sequence number used for?

Thanks in advance

Aymeric

PS: the reason for all the questions is that I have all my code using messages. (I have inter thread, inter core and I will have inter proc)