ThreadX, using new and delete with no static allocation

Question

On the STM32H753, I have configured ThreadX with static allocation. When doing a new and delete, where does the memory come from? As well, where does the memory come from when a, for example, std::vector is created on the stack and then increased in size with a concatenation?

TDK · Accepted Answer

The memory allocated by tx_byte_allocate is from a statically-allocated pool as it mentions in the note, not part of the heap. It dynamically allocates memory from a statically allocated pool. In that manner, you can always be assured that at least X bytes are available to allocate per thread.

New/delete (or malloc and free) come from the heap, which is typically after static allocation and before the stack.

> My linker file does not specify a heap location/size

You sure? It probably does. At least the location is probably specified, not the size. Look at the _sbrk implementation to see where it grabs memory from. Sometimes there's a check so it doesn't overflow the stack.

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TDK · Answer

Unless you use tx_byte_allocate to allocate the memory, it will come from the normal place. In this case, the heap for std::vectors.

I would argue this is not really static allocation, despite what they are calling it. It still comes from a pool and is assigned as needed at runtime. That's just the heap with extra steps.

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