Update: Use typedef on a structure tag to avoid the need for void pointer on the...

Update: Use typedef on a structure tag to avoid the need for void pointer on the callback for f_state_t.

Using a typedef on the structure tag allows for having the structure reference itself.
In this case, the callback that is part of the structure accepts its own structure as one of the callback parameters.

I previously did not know that I could do this and opted to use a void pointer to provide this.
With this approach I get more safety, integrity, and less memory usage (for not having to add a variable that is a cast of the void pointer to the f_state_t).