A protothread runs inside a single C function and cannot “span” on other functions. It can, of course, call normal C functions but, at the same time, it cannot block inside a called function.

This code, therefore, will never be able to run or compile correctly:

  void f( ) {
      ...
      MR_PTI_YIELD();
      ...
  }

  MR_PT_THREAD(g) {
      ...
      f();
      ...
  }

This is because protothread g() will try to block execution when it is in function f(), which itself is not a protothread. To overcome this limit, a separate protothread can be generated for each potentially blocking function and, consequently, a variable mr_protothread is allocated for it.

  MR_PT_THREAD(f) {
      ...
      MR_PTI_YIELD();
      ...
  }
  
  mr_protothread fThread;
      
  MR_PT_THREAD(g) {
      ...
      MR_PTI_SPAWN(&fThread, f(&fThread));
      ...
  }

The advantage of this approach is that the blocking is explicit, and therefore the programmer knows exactly which functions can block and which ones they cannot block, thus simplifying debugging by simply reading the code.

Move to HOW TO MANAGE LOCAL VARIABLES.