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Avoid crashes when leaving a task procedure

Posted by kilo-sierra on January 22, 2014

Hi,

In our university lab we intensively use FreeRTOS. Students frequently write code that allows the flow of control to leave a task function. On the ARM Cortex (as in other families) this creates a crash that may be hard to track down.

To help debugging I use a hook-function that is called whenever this problem arises. This hook's address must be put into the stack-frame when setting up the task's stack on creation of a task.

Alternatively you can also link a function that let's the task delete itself on exit with the same mechanism.


Avoid crashes when leaving a task procedure

Posted by kilo-sierra on January 22, 2014

void vApplicationReturnFromTaskProcedureHook(void);

portSTACKTYPE *pxPortInitialiseStack( portSTACKTYPE pxTopOfStack, pdTASKCODE pxCode, void *pvParameters, portBASETYPE xRunPrivileged, portBASE_TYPE xUseFPU) { / Simulate the stack frame as it would be created by a context switch interrupt. */

pxTopOfStack--; /* Offset added to account for the way the MCU uses the stack on entry/exit of interrupts. / *pxTopOfStack = portINITIAL_XPSR; / xPSR / pxTopOfStack--; *pxTopOfStack = ( portSTACK_TYPE ) pxCode; / PC / pxTopOfStack--; *pxTopOfStack = (portSTACK_TYPE *) vApplicationReturnFromTaskProcedureHook; / LR / pxTopOfStack -= 5; / R12, R3, R2 and R1. */ ...


Avoid crashes when leaving a task procedure

Posted by davedoors on January 22, 2014

The last few versions do what you want already. It was discussed here a while back and decided not to automatically delete the task because that needs vTaskDelete() to be in the build and hides the error from users.

Look for prvTaskExitError in the following source file http://sourceforge.net/p/freertos/code/HEAD/tree/tags/V7.6.0/FreeRTOS/Source/portable/GCC/ARM_CM3/port.c


Avoid crashes when leaving a task procedure

Posted by kilo-sierra on January 23, 2014

Hi, Correct - thanks for the info. I have overseen this because I am using my Cortex M4 FPU + MPU port which was branched from FreeRTOS V7.5.2


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