Avoid crashes when leaving a task procedure

kilo-sierra wrote on Wednesday, January 22, 2014:


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.

kilo-sierra wrote on Wednesday, January 22, 2014:

void vApplicationReturnFromTaskProcedureHook(void);

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

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

davedoors wrote on Wednesday, 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

kilo-sierra wrote on Thursday, January 23, 2014:

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