In - http://arduino.cc/forum/index.php/topic,143584.0.html - I posted code to be able to print int64_t and uint64_t for the UNO.
Not tested on other platforms but the code is very straightforward,
Drawback is it adds 8KB to a sketch so placed some #ifdefs in the proposed code.
What's the status on this? Any updates?
If this does not end up in the core I'd be happy to add it to my PrintEx lib. Thus giving you and easy way to use it without having to add it to the core on each update.
Is the 8Kb a flat addition or only when used? If it is added without using it, I think I may be able to mitigate this in PrintEx.
That code is from long long ago ;)
As far as I recall it only adds 8K when actually used. The linker is smart enough.
I had a quick look at the code again and I (still) propose the print9() version which has imho a good balance between footprint and speed and understandability.
Feel free to add the code to your printEx library so it can be used. Please add a link to the forum thread above.