* Fixed SEGFAULT when calling remove_event_detect() from python callback #233
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| @zsserg thank you. I will review and try out on a BeagleBone. |
pdp7 approved these changes Feb 18, 2018
Testing of @zsserg's branch |
| thanks @zsserg |
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There is an old issue with the library which results in SEGFAULT if remove_event_detect() function is called from within the python callback.
More information can be found here:
https://sourceforge.net/p/raspberry-gpio-python/tickets/136/
https://raspberrypi.stackexchange.com/questions/73791/gpio-callbacks-throw-segmentation-fault
https://stackoverflow.com/questions/35632689/how-to-correctly-remove-interrupt-callback-in-beaglebone-black
http://schlierf.info/raspberry/pi/gpio/python/2016/04/15/RPi.GPIO_SegFault.html
The origin of the problem is the way of handling callbacks - C code stores them in a linked list structure, and, while it correctly removes/inserts new items it doesn't do any sanity checks inside the wrapper that iterates over the list. That results in dereferencing zero pointer if last callback in the chain was removed inside the callback function.
I provide a possible solution to this. Two places need to be patched. While the base C code (event_gpio.c) requires only minor alteration to prevent SEGFAULTs, upper-level python library (py_gpio.c) needs a bit more elaborate workaround to stop traversing the callback chain immediately if event detection was removed while running one of the callback functions.
Typical example when this functionality might be needed is programming interrupt-like behavior with event detection system (see 3rd link above).