[Tinyos-devel] Limiting typedef parameters of generic modules to non-struct types

[Philip Levis]

On Sep 18, 2007, at 2:03 PM, Kevin Klues wrote:

> I recently came across a problem with using the QueueC component to
> define a queue of size greater than 256.  The problem has to do with
> limitations of the nesC syntax, and not of the component
> implementation itself.
>
> As it stands now, the QueueC component is defined as
>
> generic module QueueC(typedef queue_t, uint8_t QUEUE_SIZE) {
>   provides interface Queue<queue_t>;
> }
>
> and then its implementation has variables to manage the queue defined
> as follows:
>   uint8_t head = 0;
>   uint8_t tail = 0;
>   uint8_t size = 0;
>
> If I want to create a queue that can hold, say, 500 elements, I have
> to make a copy of the QueueC component, change its definition to
> contain a QUEUE_SIZE variable that is of type uint16_t,  modify head,
> tail, and size to be uint16_t's as well,  and change the Queue
> interface to have the return types for Queue.size() and
> Queue.maxSize() be uint16_t.  All of the logic in the actual QueueC
> implementation remains unchanged.
>
> One thought I had to get around this was to add another parameter to
> the definition of QueueC that would allow me to specify the type that
> should be associated with head, tail, and size, and then just set
> QUEUE_SIZE to be of type uint32_t.  Since QUEUE_SIZE is a constant and
> it isn't feasible that you would ever have a queue size of 2^32 or
> greater, choosing this width seemed reasonable.
>
> generic module QueueC(typedef queue_t, typedef queue_size_t, uint32_t
> QUEUE_SIZE) {
>   provides interface Queue<queue_t, queue_size_t>;
> }
>
> The problem with this is that I can't just do something like the
> following in the implementation of QueueC anymore:
>   queue_size_t head = 0;
>   queue_size_t tail = 0;
>   queue_size_t size = 0;
>
> The reason is that nesC believes that queue_size_t could be any type
> including a struct, and assignment of a struct can't be done in the
> way shown.  I would need a way of telling nesC that I am limiting
> queue_size_t to non-struct types (i.e. uint8_t, uint16_t, and
> uint32_t) so that it would not throw an error when assigning a
> variable of that type to a number.
>
> Can anyone think of a nice, elegant way to deal with this?  Or is the
> best solution to just create separate Queue8C, Queue16C and Queue32C
> components that only differ in the maximum size their queues can be.
> Seems like unneeded duplication of code to me though....


There's something like a @number attribute for this. Otherwise things  
like variable width timers become a pain to write.

Phil