Somewhere, I was receiving an integer and I had to return a filename based on it, where the integer was the sequence number, e.g. 0, 1, 2, etc. I needed to use that integer in a callback, so I was passing it as user_data with a GUINT_TO_POINTER (type casting with some sanity checks) and getting it back with GPOINTER_TO_UINT on the other side. The first file was being written normally, but the second failed. I tried inspecting why and saw this line in the callback:
MyElement *self = MY_ELEMENT (user_data);
where “self” wasn’t needed in that callback, I just added it out of habit, because usually user_data is self. It does have some sanity checks there, so the first time worked because user_data was just 0x0 – so self is NULL, alright. The second time, self was becoming (MyElement *) 0x1, so the sanity check was failing, and kaboom.