We describe a full implementation of algebraic effects and handlers as a

library in standard and portable C99, where effect operations can be used

just like regular C functions. We use a formal operational semantics to

guide the C implementation at every step where an evaluation context

corresponds directly to a particular C execution context. Finally we show

a novel extension to the formal semantics to describe optimized tail

resumptions and prove that the extension is sound. This gives two orders

of magnitude improvement to the performance of tail resumptive operations

(up to about 150 million operations per second on a Core i7@2.6GHz)