[Freeswitch-dev] OpenZAP Question re: Q931Rx23 (from Q931.c)

Michael Collins mcollins at fcnetwork.com
Tue Jun 24 01:33:47 EDT 2008

I have question about the two different buffers that are accessed in
this function:




  Function:     Q931Rx23


  Description:  Receive message from layer 2 (LAPD). Receiving a message

                is always done in 2 steps. First the message must be 

                interpreted and translated to a static struct. Secondly

                the message is processed and responded to.


                The Q.931 message contains a static header that is 

                interpreted in this function. The rest is interpreted

                in a sub function according to mestype.


  Parameters:   pTrunk  [IN]    Ptr to trunk info.

                buf     [IN]    Ptr to buffer containing message.

                Size    [IN]    Size of message.


  Return Value: Error Code. 0 = No Error, < 0 :error, > 0 : Warning

                see q931errors.h for details.



L3INT Q931Rx23(Q931_TrunkInfo_t *pTrunk, L3UCHAR * buf, L3INT Size)


    L3UCHAR *Mes = &buf[Q931L2HeaderSpace];

    L3INT RetCode = Q931E_NO_ERROR;

    Q931mes_Generic *m = (Q931mes_Generic *) pTrunk->L3Buf;

    L3INT ISize;


    L3INT IOff = 0;


    /* Protocol Discriminator */

    m->ProtDisc = Mes[IOff++];


    /* CRV */

    m->CRVFlag = Mes[IOff + 1] & 0x80;

    m->CRV = Q931Uie_CRV(pTrunk, Mes, m->buf, &IOff, &ISize);


    /* Message Type */

    m->MesType = Mes[IOff++];


    /* Call table proc to unpack codec message */

    /*debug */

    printf("\n\nQ931Rx23-  Dialect: %d, MsgType: %d\nCause:

    printf("Message: %s\n",Mes);

    RetCode = Q931Umes[pTrunk->Dialect][m->MesType](pTrunk, Mes,
(Q931mes_Generic *)pTrunk->L3Buf, Q931L4HeaderSpace + IOff , Size -
Q931L4HeaderSpace - IOff + 1);


    if(RetCode >= Q931E_NO_ERROR)


        RetCode = Q931Proc[pTrunk->Dialect][m->MesType](pTrunk,
pTrunk->L3Buf, 2);



    return RetCode;



What's the difference between pTrunk->L3Buf and *buf in this function?
I'm trying to understand where the q931 message is coming from as it is
passed up the stack from layer 2 to layer 3.  I may have found a clue
about the phantom cause IE issue but I won't know for sure until I know
what exactly I'm looking at.


Also, can you tell me what is accomplished by creating *Mes as opposed
to *m?  I am familiar with the Q931mes_Generic data struct but the line:

    L3UCHAR *Mes = &buf[Q931L2HeaderSpace];

... is confusing me.  What does that do?


Thanks for your time!!



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