Background

Under certain conditions, it may be necessary to create a kernel thread whose stack likes in some custom memory.  This page provides and example of how that would be done:

Example

Here is the body of some function.  It expects to have the following inputs:

1. taskname:  The name of the task to be started
2. stacksize:  The size of the custom stack
3. priority:  The priority of the task to be started
4. entry_point:  The entry point of the task to be started
5. argv:  An optional array of argument strings passed to the task

  /* Allocate a TCB for the new task.  kmm_zalloc() is used so
   * that all fields of the new TCB will be zeroed.
   */

  tcb = (FAR struct task_tcb_s *)kmm_zalloc(sizeof(struct task_tcb_s));
  if (tcb == NULL)
    {
      return -ENOMEM;
    }

  /* Indicate (1) that this is a kernel thread and that (2) a custom
   * stack will be used.
   */

  tcb->flags = TCB_FLAG_TTYPE_KERNEL | TCB_FLAG_CUSTOM_STACK;

  /* Allocate the custom stack for the new task.
   *
   * Do whatever it takes to get a reference to the custom stack.
   * Here custom_alloc() is used as a placeholder for whatever
   * that may be.
   */

  stack = (FAR uint32_t *)custom_alloc(stacksize);
  if (stack == NULL)
    {
      kmm_free(tcb);
      return -ENOMEM;
    }

  /* Initialize the TCB.  This will initialize all remaining
   * fields of the TCB, associate the stack to the TCB, allocate
   * any additional resources needed by the task, and place the
   * TCB in a list of inactive tasks.
   */

  ret = task_init((FAR struct tcb_s *)tcb, progname, priority,
                  stack, stacksize, entry_point, argv);
  if (ret < 0)
    {
      kmm_free(tcb);
      custom_free(stack);
      return ret;
    }

  /* Then activate the task at the provided priority */

  ret = task_activate((FAR struct tcb_s *)tcb);
  if (ret < 0)
    {
      nxtask_uninit(tcb);
      custom_free(stack);
      return ret;
    }

  return OK;