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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.
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Dynamic memory allocation tends to be non-deterministic; the time taken to allocate memory may not be predictable and the memory pool may become fragmented, resulting in unexpected allocation failures.
malloc allocates memory to the heap to the program for it's use, free releases the memory from use back to the heap (and can be reused by a future call to malloc )
The malloc( ) (memory allocate) function can be used to dynamically allocate an area of memory to be used at run time. Heap memory is used for these variables that use malloc( ). Include <stdlib. h> whenever using malloc( ).
Dynamic allocations with new/delete are said to take place on the free-store, while malloc/free operations use the heap.
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.
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The comment, as stated, is incorrect.
Using a heap manager with non-deterministic behavior creates a program with non-deterministic behavior. But that is obvious.
Slightly less obvious is the existence of heap managers with deterministic behavior. Perhaps the most well-known example is the pool allocator. It has an array of N*M bytes, and an available[] mask of N bits. To allocate, it checks for the first available entry (bit test, O(N), deterministic upper bound). To deallocate, it sets the available bit (O(1)). malloc(X) will round up X to the next biggest value of M to choose the right pool.
This might not be very efficient, especially if your choices of N and M are too high. And if you choose too low, your program can fail. But the limits for N and M can be lower than for an equivalent program without dynamic memory allocation.
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