The Portable Coroutine Library (PCL) implements the low level
functionality for coroutines. For a definition of the term coroutine
see The Art of Computer Programming by Donald E. Knuth. Coroutines
are a very simple cooperative multitasking environment where the
switch from one task to another is done explicitly by a function
call.

WWW: http://www.xmailserver.org/libpcl.html