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Guarded Resources

Posted by Mathias Zenger on July 10, 2009
Hi,
In my application I have several tasks which need to share data. So far I have just defined some global variables (e.g. byte, word or long size variables and structures) without considering about MUTEX. In the different C files I have referenced the variables with the key word external.

Since I know that this is no proper way for a multitasking system I would like to clean up my design. The resources should be accessed exclusively. Of course I would like to avoid priority inversion or dead locks. Do you think it could be a possible solution to write "setter" and "getter" functions for each resource? Example:

static xSemaphoreHandle xMyTaskMutex = xSemaphoreCreateMutex();
static unsigned long ulMyTaskSharedLong = 0;

void ulMyTaskSetSharedLong(unsigned long param) {
xSemaphoreTake(xMyTaskMutex, portMAX_DELAY) {
ulMyTaskSharedLong = param;
}
xSemaphoreGive(xMyTaskMutex);
}

unsigned long ulMyTaskGetSharedLong() {
return ulMyTaskSharedLong;
}

Do you think this could work? Any better (simpler, saver) solutions? Thanks!
Regards, M.

RE: Guarded Resources

Posted by Dave on July 10, 2009
First consider if the variables need guarding at all. This will depend on their size and where they are read from and written to. If the size is the natural word size of the MCU, and the variable is only written to from one place, then you might not need any guards in place.

RE: Guarded Resources

Posted by Mathias Zenger on July 13, 2009
Thanks for the comment. Perhaps I am chasing shadows. You are right: if word size matches then the operations become atomic. On my AVR32 system the context switches performed by FreeRTOS save/restore all 32 general purpose registers, the staus register, the program counter and the two stack pointer registers. Obviously the kernel does the main work anyway. However, do you think using mutex for writing shared resources is appropriate? What cases do you consider as critical and how do you handle them in your projects?


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