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

[Anthony Minessale]

buf is the data buffer you are reading the q931 message into.
pTrunk->L3Buf is the buffer in the trunk that you are reading it from it has
the last received message in it.


*Mes is a pointer being declared to point at &buf[Q931L2HeaderSpace];

buf is a raw buffer
Q931L2HeaderSpace is a constant of the 921 header space which i think is 2
bytes.

so it's a pointer to the beginning of the actual message which is 2 bytes
into buf.



On Tue, Jun 24, 2008 at 12:33 AM, Michael Collins <mcollins at fcnetwork.com>
wrote:

>  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:
> [%x]\n",pTrunk->Dialect,m->MesType,m->Cause);
>
>     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!!
>
>
>
> -MC
>
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>


-- 
Anthony Minessale II

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