Does using heap memory (malloc/new) create a non-deterministic program?
I started developing software for real-time systems a few months ago in C for space applications, and also for microcontrollers with C++. There's a rule of thumb in such systems that one should never create heap objects (so no malloc/new), because it makes the program non-deterministic. I wasn't able to verify the correctness of this statement when people tell me that. So, Is this a correct statement?
The confusion for me is that as far as I know, determinism means that running a program twice will lead to the exact, same execution path. From my understanding this is an issue with multithreaded systems, since running the same program multiple times could have different threads running in different order every time.
Answer
In the context of realtime systems, there is more to determinism than a repeatable "execution path". Another required property is that timing of key events is bounded. In hard realtime systems, an event that occurs outside its allowed time interval (either before the start of that interval, or after the end) represents a system failure.
In this context, usage of dynamic memory allocation can cause non-determinism, particularly if the program has a varying pattern of allocating, deallocating, and reallocating. The timing of allocations, deallocation, and reallocation can vary over time - and therefore making timings for the system as a whole unpredictable.