If you define this function, it is called by the C library during program start-up.

When __user_setup_stackheap() is called, sp has the same value it had on entry to the application. If this was set to a valid value before calling the C library initialization code, it can be left at this value. If sp is not valid, __user_setup_stackheap() must change this value before using any stack and before returning.

__user_setup_stackheap() returns the:

• Heap base, if the program uses the heap.
  • In AArch32 state, register R0 contains the heap base.
  • In AArch64 state, register X0 contains the heap base.

• Stack base in sp.

• Heap limit, if the program uses the heap and uses two-region memory.
  • In AArch32 state, register R2 contains the heap limit.
  • In AArch64 state, register X2 contains the heap limit.

If this function is re-implemented, it must:

• Preserve the registers required by the PCS, except for SP.
• Ensure alignment of the heap:
  • In AArch32 state, maintain 8-byte alignment of the heap by ensuring that the heap base is a multiple of 8.
  • In AArch64 state, maintain 16-byte alignment of the heap by ensuring that the heap base is a multiple of 16.

To create a version of __user_setup_stackheap() that inherits sp from the execution environment and does not have a heap:

• In AArch32 state, set r0 and r2 to zero and return.
• In AArch64 state, set x0 and x2 to zero and return.

There is no limit to the size of the stack. However, if the heap region grows into the stack, malloc() attempts to detect the overlapping memory and fails the new memory allocation request.