Troubles with typedef on the Arduino

Why doesn't this work on the Arduino!?

typedef struct {
   int a;
   int b;
} TTallywag;


void DillyDally(TTallywag *tw){
// stuff...
}

In short, I don't actually know! But I know how to get around it!

To make it work, you have to change how you use your new data type - infact, your function parameters can't use types that you've typedef'ed, but you can use the type if you do so thusly:

typedef struct _tallywag {
   int a;
   int b;
} TTallywag;


void DillyDally(struct _tallywag *tw){
   // stuff...
}

According to the following page that I found, you should be able to use the TTallywag if you define it in a header file and including that into you sketch.

Beware the Preprocessor

Note to self: Google 'Using headers in Arduino sketches'

I suppose technically defining types in header files is the right thing to do, but when you're dealing with a sketch that's only short, it seems such an overkill.

Now, the following does works fine. You can use the types inside functions no problemo. Which is partly why I spent so much time tearing out my hair last night!

typedef struct _tallywag{
   int a;
   int b;
} TTallywag;


void *ShillyShally(int alpha, int beta){
   static TTallywag tw;

   // stuff...


   tw.a = alpha;
   tw.b = beta;

   return &tw;
}

Must be some namespacing, or name mangling going on.

So it's not a huge problem, but if you're planning to share code modules between two devices, say one half in Linux written using GCC and the other half Arduino: Define your functions the latter way! That will work on both sides.