Having fun with a switch

Have you had fun with a switch lately?

There is a thing called the Duff device. This is a C programmers dream and the reviewers nightmare. Some say it's abusing the language and some say it's beautiful. Myself I see beauty in most things.

This is how the Duff device looks like

switch (count % 8)  /* count > 0 assumed */
{
 case 0:        do {  *to = *from++;
 case 7:              *to = *from++;
 case 6:              *to = *from++;
 case 5:              *to = *from++;
 case 4:              *to = *from++;
 case 3:              *to = *from++;
 case 2:              *to = *from++;
 case 1:              *to = *from++;
                   } while ((count -= 8) > 0);
}

So what good can you do with a thingy like this? You can actually produce your own lightweight threads library using this construction. Just look at this

#include "pt.h"

struct pt pt;
struct timer timer;

PT_THREAD(example(struct pt *pt))
{
PT_BEGIN(pt);

while(1) {
  if(initiate_io()) {
    timer_start(&timer);
    PT_WAIT_UNTIL(pt,
       io_completed()
       timer_expired(&timer));
    read_data();
  }
}
PT_END(pt);
}

Example protothreads code.

This is 100% pure C. The macros actually build a kind of Duff device that incorporates the __LINE__ macro in an ingenious way. If you are interested take a look at http://www.sics.se/~adam/pt/