Here are a list of libcoap API summary man pages, some of which have code examples, and Examples:

coap(7)	Overview of the libcoap library
coap_async(3)	Work with CoAP async support
coap_attribute(3)	Work with CoAP attributes
coap_block(3)	Work with CoAP Blocks
coap_cache(3)	Work with CoAP cache functions
coap_context(3)	Work with CoAP contexts
coap_encryption(3)	Work with CoAP TLS/DTLS
coap_endpoint_client(3)	Work with CoAP client endpoints
coap_endpoint_server(3)	Work with CoAP server endpoints
coap_handler(3)	Work with CoAP handlers
coap_io(3)	Work with CoAP I/O to do the packet send and receives
coap_keepalive(3)	Work with CoAP keepalive
coap_logging(3)	Work with CoAP logging
coap_observe(3)	work with CoAP observe
coap_pdu_access(3)	Accessing CoAP PDUs
coap_pdu_setup(3)	Setting up CoAP PDUs
coap_recovery(3)	Work with CoAP packet transmissions
coap_resource(3)	Work with CoAP resources
coap_session(3)	Work with CoAP sessions
coap_string(3)	Work with CoAP string functions
coap_tls_library(3)	Work with CoAP TLS libraries
coap-client(5)	CoAP Client based on libcoap
coap-rd(5)	A CoAP Resource Directory based on libcoap
coap-server(5)	CoAP Server based on libcoap