Question

I am an electronics guy so, treat me well, please.

I was reading this article about real time operating systems for 8-bit microcontrollers. I came across this function with a weird looking argument. I could not understand what it is doing. I know that void means “no type”. I am guessing that (*Task) is the casting. I really have no idea what those brackets do after that.

What does this function’s argument include?

Also, I couldn’t understand what does *(int*)((NewTCB->Stack) + (STACK_DEPTH-2)) = (int)Task; do?

Answer 1

void (*Task)() is actually a function pointer. Basically it is saying: “the parameter name is Task and it is a function which returns void and (since this is c not c++) takes any number of arguments.

So you could call it like this:

void my_task() {
    /* do something */
}

TaskCreate(my_task);

Of course, it would also be safe to write void my_task(void) { as well. When coding c, I personally prefer to explicitly say “there are no parameters”

Finally, *(int*)((NewTCB->Stack) + (STACK_DEPTH-2)) = (int)Task; is doing some casting magic.

Let’s disect it:

(int)Task is first converting Task to an int (which is questionable, but probably ok for your particular architecture/OS. Personally, I would use a long to be safe).

((NewTCB->Stack) + (STACK_DEPTH-2)) is just doing some simple arithmetic on NewTCB->Stack to get a pointer to a location in the TCB‘s stack.

*(int*) says “convert this to an int * and then deference (read or write) the location it points to.

We could write this more simply as follows:

int f = (int)Task;
int s = ((NewTCB->Stack) + (STACK_DEPTH-2)); /* I don't know the type of `NewTCB->Stack`, so we'll pretend 'int' for now */
int *stack_ptr = (int*)s;
*stack_ptr = f;

which is probably more clear.

FOLLOW UP: I’d like to point out how I use tend to use function pointers since the syntax can be a bit confusing sometimes. And I find this approach to be very helpful. Basically I like to create a typedef for the function pointer and use that instead, I find it a lot easier to get right:

For example:

/* typedef func_t to be a pointer to a function taking no arguments and returning void */
typedef void (*func_t)(void); 

Then later…

void CreateTask(func_t task) {
    /* same work as your example, just a little easier to read */
}