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How to know if an ISR is executing?

Posted by Owen on October 6, 2011
Hi there,

I have a small chunk of code below that needs to be atomic and may be called from ISR or task.
Variable bool bFromIsr is true if the section is called from ISR false if from task.
Two questions:
1. Is there any OS variable I can examine to know if I am presently in an ISR? (E.g. is ISR nesting level tracked somewhere, I could not find it.) Or do I have to independently provide a value for bFromIsr?
2. Have I chosen the correct macros to do this? I had thought that I should not have to call directly any functions prefixed by "port...". What is the correct macro for "suspend interrupts from an interrupt"?

Thanks and regards,


//because this can be called indirectly from multiple tasks or ISRs the action needs to be atomic
if (bFromIsr) {
uxSavedInterruptStatus = portSET_INTERRUPT_MASK_FROM_ISR();
} else {
// BEGIN section of atomic code ////////////////
//check and update the high water mark
if (i>logPoolHwm)
//since we start scanning from zero, i represents the hwm
logPoolHwm = i;
//mark it as in use
logPool.inUse = 1;
// END section of atomic code ////////////////
if (bFromIsr) {
portCLEAR_INTERRUPT_MASK_FROM_ISR( uxSavedInterruptStatus );
} else {

RE: How to know if an ISR is executing?

Posted by Richard Damon on October 6, 2011
FreeRTOS doesn't have any flag like that built in. Depending on the hardware architecture, you may be able to check a hardware register to detect this, or you could just make your bFromIsr a global, and require that ISRs begin with a bFromIsr++ and end with a bFromIsr-- (and make it really an int and not a bool like the name implies).

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