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Problem whith xQueueSend and array

Posted by Dennis Ignazio Virgo on October 26, 2008
Hello,

I'm a newbie in FreeRTOS and C. While trying out I had some trouble in understanding why source code A does work, but B and C do not work properly.

/*** simplified functions ***/
void vStringOutput (portCHAR *pcSomeString);

void vFunctionWithQueueReceive(void)
{
portCHAR *pcReceivedString;
xQueueHandle xQueue;

xQueue = xQueueCreate((20, sizeof(portCHAR *));

if(xQueue != 0)
{
if(xQueueReceive(xQueue, &pcReceivedString, 10))
{
xStringOutput(pcReceivedString);
}
}
}


A: This works perfectly
void vFunctionWithQueueSend(void)
{
...
portChar *pcAString = "abcdefg";
...
xQueueSend(xQueue, &pcAString1, 10);
...
}

B: This puts out wierd characters
void vFunctionWithQueueSend(void)
{
...
portChar cAnArray[] = "abcdefg";
...
xQueueSend(xQueue, &cAnArray, 10);
...
}

C: This puts out wierd characters
void vFunctionWithQueueSend(void)
{
...
xQueueSend(xQueue, (void *)"abcdefg", 10);
...
}

How can I manage to put a char array in a Queue?

Thank you in advance.

Dennis

RE: Problem whith xQueueSend and array

Posted by Richard on October 26, 2008
> A: This works perfectly
> void vFunctionWithQueueSend(void)
> {
> ...
> portChar *pcAString = "abcdefg";
> ...
> xQueueSend(xQueue, &pcAString1, 10);
> ...
> }

pcAString is a pointer to a const string, which is probably stored in Flash. The pointer contains the address of data that is in flash and is always there.

> B: This puts out wierd characters
> void vFunctionWithQueueSend(void)
> {
> ...
> portChar cAnArray[] = "abcdefg";
> ...
> xQueueSend(xQueue, &cAnArray, 10);
> ...
> }


You have declared an array as a local (stack) variable within vFunctionWithQueueSend(). Once vFunctionWithQueueSend() exits the variable will no longer exist and its value in RAM will probably be overwritten during the next function call.

>
> C: This puts out wierd characters
> void vFunctionWithQueueSend(void)
> {
> ...
> xQueueSend(xQueue, (void *)"abcdefg", 10);
> ...
> }

Here you are posting a character pointer (char *), but your queue receive function is expecting a pointer to a character pointer (char **).

These statements are just general C pointers, the same would be true if you were using FreeRTOS.org or not.

Regards.

RE: Problem whith xQueueSend and array

Posted by Dennis Ignazio Virgo on October 26, 2008
Thank you very much! I thought that my problem has to do with FreeRTOS, but now I understand that it is an C issue. I have to read more about pointer... :)

With best Regards
Dennis

RE: Problem whith xQueueSend and array

Posted by Dennis Ignazio Virgo on October 26, 2008
After thinking to understand the problem and after many trials I'm not able to pass the array in the queue like intendet in B.
If possible i want to avoid global arrays, so I have to use a local one.
I tryed to allocate dynamic memory for a pointer and copied each array element to the allocated memory, but it didn't work.
Without using a queue I would try to call the function tranferring a pointer and work with a reference or maybe I would try a function which returns a pointer.
Since I have to use xQueueSend() and I read that the items are passed by copy, I have no ideas how to continue.

Are there any hints?

What possibilities do I have to send the array from example B threadsave to the Queue?

Regards

Dennis


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