Why does data i push into a queue not come out on the other end?

Question

I am writing a multithreaded communication interface where one function (in my main thread) pushes data into a queue (VxWorks msgQLib) and another function in my communication task, fetches data from the queue to forward to the hardware interface. Now, it seems like my push data doesn't align with my fetch data (or vice versa). I inserted some debugging messages to find out what's going on but have failed to find the culprit thus far. The code I have:

a little explanation. I built the functions so, that I insert a message tag with some meta data before the actual message i.e. each message will insert two items into the queue. Please don't worry about the "reply" part yet, that still needs some work on either side. I'm now just worried about the outgoing part of the equation.

The function that pushes data into the Queue (in the application task):

static STATUS SPIQWriteRead_TDM(UINT8 cmdlen, UINT8 *cmd, UINT8 retdatlen, UINT8 *retdat)
{
    UINT8 BytesRead;
    UINT8 *msgTag[MSGTAGLEN]={0};
    static UINT spi_init = 0;
    UINT i=0;

    //assemble message tag
    msgTag[0] = 0x01; //FrameID
    msgTag[1] = cmdlen/255; //MsgLen MSB
    msgTag[2] = cmdlen%255; //MsgLen LSB
    msgTag[3] = CS_TDM; // ChipSelect
    msgTag[4] = 0; //SlotID


    //copy message tag into queue
    for(i=0;i<MSGTAGLEN;i++)
            printf("<= msgTag[%d] 0x%x\n",i,msgTag[i]);
    msgQSend(TDMTxQ,msgTag,MSGTAGLEN,NO_WAIT,MSG_PRI_NORMAL);
    printf("<= TxQueue(0x%x) contains %d items\n",TDMTxQ, msgQNumMsgs(TDMTxQ));

    for(i=0;i<cmdlen;i++)
        printf("<= msg[%d] 0x%x\n",i,cmd[i]);
    // copy message into queue
    msgQSend(TDMTxQ,cmd,cmdlen,NO_WAIT,MSG_PRI_NORMAL);
    printf("<= TxQueue(0x%x) contains %d items\n",TDMTxQ, msgQNumMsgs(TDMTxQ));

    // wait for a maximum of 10 ticks (~1600ms) for the Reply
    BytesRead = msgQReceive(TDMRxQ,retdat,retdatlen,100);
    for(i=0;i<retdatlen;i++)
            printf("retdat[%d] 0x%x\n",i,retdat[i]);
    printf("BytesRead 0x%x\n",BytesRead);
    // compare reply and return
    if (BytesRead != retdatlen)
        return ERROR;
    else
        return OK;
}

and the function that fetches data from the queue (in the communication task):

static FROM_Q_DAT *GetFromQueue(MSG_Q_ID TxQId)
{
    UINT8 msgTag[MSGTAGLEN]= {0};
    UINT8 FrameID = 0;
    UINT16 MsgLen = 0;
    UINT8 ChipSelect = 99;
    UINT8 SlotID = 99;
    static UINT8 *cmd=NULL;
    UINT8 retdat[2]={0};
    UINT8 cs=99;
    static FROM_Q_DAT Qdat;
    int retval = 0;
    UINT8 i=0;

    // read message tag from queue
    msgQReceive(TxQId, msgTag, MSGTAGLEN, NO_WAIT);
    for(i=0;i<MSGTAGLEN;i++)
            printf("=> msgTag[%d] 0x%x\n",i,msgTag[i]);

    // parse received message tag 
    FrameID = msgTag[0];
    MsgLen = (((UINT16)msgTag[1])<<8) | (msgTag[2]);
    ChipSelect = msgTag[3];
    SlotID = msgTag[4];
    printf("FrameID: %d\n",FrameID);
    printf("MsgLen: %d\n",MsgLen);
    printf("ChipSelect: %d\n",ChipSelect);
    printf("SlotID: %d\n",SlotID);

    printf("=> TxQueue(0x%x) contains %d items\n",TxQId, msgQNumMsgs(TxQId));

    cmd = (UINT8*) calloc(MsgLen, sizeof(UINT8));
    // Get the message from the queue 
    msgQReceive(TxQId, cmd, MsgLen, NO_WAIT);
    for(i=0;i<MsgLen;i++)
                printf("=> cmd[%d] 0x%x\n",i,cmd[i]);

    printf("=> TxQueue(0x%x) contains %d items\n",TxQId, msgQNumMsgs(TxQId));

    Qdat.cs = ChipSelect;
    Qdat.len = MsgLen;
    Qdat.data = cmd;
    return &Qdat;
}

And a screen capture looks like this:

<= msgTag[0] 0x1
<= msgTag[1] 0x0
<= msgTag[2] 0x9
<= msgTag[3] 0x2
<= msgTag[4] 0x0
<= TxQueue(0x261c6010) contains 1 items
<= msg[0] 0x5e
<= msg[1] 0x8
<= msg[2] 0x40
<= msg[3] 0x6
<= msg[4] 0x0
<= msg[5] 0x0
<= msg[6] 0x0
<= msg[7] 0x0
<= msg[8] 0x0
<= TxQueue(0x261c6010) contains 2 items
//the next message (with '==' is from the communication engine that checks for content in the communication Queue:
== TxQueue(1) contains 2 items
=> msgTag[0] 0x0
=> msgTag[1] 0x0
=> msgTag[2] 0x0
=> msgTag[3] 0x1
=> msgTag[4] 0x0
FrameID: 0
MsgLen: 0
ChipSelect: 1
SlotID: 0
=> TxQueue(0x261c6010) contains 1 items
=> TxQueue(0x261c6010) contains 0 items

I don't see why my data that goes into the queue wouldn't come out...

Answer 1

Please check all return values of function you call, so you at least don't miss any errors. e.g. you are receiving messages with NO_WAIT , so if you happen to call msgQReceive before anything has been sent, you won't receive anything - which would be an important thing to handle. Similar when you send with NO_WAIT - you don't handle the case if the queue is full.

However, your sending function have:

 UINT8 *msgTag[MSGTAGLEN]

Which for some reason is an array of UINT8 pointers. There's nothing in the code that suggests this should be an array of pointers - and since the array differs in the receiver function, you'll interpret the bytes quite differently in the two functions. Your receiver function seems to have it correct, so make sure the sender function also has the same:

 UINT8 msgTag[MSGTAGLEN];

(also make sure your two functions here arn't called from several tasks, it has static variables and is not thread safe, and not re-entrant.)