Here is a list of libcoap API manual pages, some of which have code examples:

coap-client(5)	coap-client(5) - CoAP Client based on libcoap
coap-rd(5)	coap-rd(5) - A CoAP Resource Directory based on libcoap
coap-server(5)	coap-server(5) - CoAP Server based on libcoap
coap(7)	coap(7) - Overview of the libcoap library
coap_attribute(3)	coap_attribute(3) - Work with CoAP attributes
coap_context(3)	coap_context(3) - Work with CoAP contexts
coap_handler(3)	coap_handler(3) - work with CoAP handlers
coap_observe(3)	coap_observe(3) - work with CoAP observe
coap_pdu_setup(3)	coap_pdu_setup(3) - Work with CoAP PDUs
coap_recovery(3)	coap_recovery(3) - Work with CoAP packet transmissions
coap_resource(3)	coap_resource(3) - Work with CoAP resources
coap_session(3)	coap_session(3) - Work with CoAP sessions
coap_tls_library(3)	coap_tls_library(3) - Work with CoAP contexts
coap_encryption(3)	coap_encryption(3) - Work with CoAP tls/dtls
coap_logging(3)	coap_logging(3) - Work with CoAP logging