Hi ! I just had a look to those C libraries and I like both of them because they enable you to write cleanly clients/servers through so-called co-routines.
Both are made by Martin Sustrik.

libmill

libmill is a port to C of the concurrent construction in the Go language.

• Check out this nice tutorial : http://libmill.org/tutorial.html
• Documentation: http://libmill.org/documentation.html
• Devel repo: https://github.com/sustrik/libmill

libdill

libdill is the successor to the previous library, it is a concurrency model a bit different that is called structured concurrency that allows clean nesting and to provide a good encapsulation.

• Check out both tutorials : http://libdill.org/tutorial-basics.html, http://libdill.org/tutorial-protocol.html
• Documentation: http://libdill.org/documentation.html
• Devel repo: https://github.com/sustrik/libdill