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)