Formatting numeric values with commas in C

This function reformats a numeric sequence to include grouping in the usual human readable format. "123456789" becomes "123,456,789", for example. The formatting is localized to the current running machine. In a German locale the previous example becomes "123.456.789". In a Hindi locale it becomes "12,34,56,789". Printing numbers for human consumption is a common request. C does not offer direct support for it, nor are there many examples that are complete for use outside of the US that I could find.

---