Context Switching
A context switch is not a function call. A function call returns to its caller; a context switch enters on one task's stack and leaves on another's, and the code that resumes has no idea it was ever stopped. There is no C construct for that. What makes it possible on a Cortex-M is that the processor already performs most of it, for free, every time an exception is taken — and that an exception return reads its destination out of memory rather than from a register, so pointing it at a different stack points it at a different task.