libcoap is a C implementation of a lightweight application-protocol for devices
that are constrained their resources such as computing power, RF range, memory,
bandwidth, or network packet sizes. This protocol, CoAP, is standardized by the
IETF as RFC 7252. For further information related to CoAP, see
http://coap.technology.

You might want to check out libcoap-minimal for usage examples.

WWW: https://github.com/obgm/libcoap