libcoap 4.3.1
Here is a list of all modules:
[detail level 12]
 Application APIApplication API Structures, Macros, Typedefs, Enums and Functions
 Application I/O HandlingAPI for Application Input / Output checking
 Asynchronous MessagingAPI for delayed "separate" messages
 Block TransferAPI for handling PDUs using CoAP Block options (RFC7959)
 Cache SupportAPI for Cache-Key and Cache-Entry
 Clock HandlingAPI for internal clock assess
 Context HandlingAPI for handling PDUs using CoAP Contexts
 DTLS SupportAPI for interfacing with DTLS libraries
 Encode / Decode APIAPI for endoding/decoding CoAP options
 Event HandlingAPI for event delivery from lower-layer library functions
 Logging SupportAPI for logging support
 Option FiltersAPI for access option filters
 Pseudo Random NumbersAPI for generating pseudo random numbers
 Rate ControlAPI for updating transmission parameters for CoAP rate control
 Resource ConfiguratonAPI for setting up resources
 Resource ObservationAPI for interfacing with the observe handling (RFC7641)
 SessionsAPI for CoAP Session access
 String handling supportAPI for handling strings and binary data
 URI Parsing FunctionsAPI for parsing URIs
 CoAP Debug APIInternal API for CoAP debug functions
 Libcoap Internal APILibcoap Internal API Structures, Macros, Typedefs, Enums and Functions
 ASN.1 SupportInternal API for CoAP ASN.1 handling
 Application I/O HandlingInternal API for Application Input / Output checking
 Asynchronous MessagingInternal API for CoAP Asynchronous processing
 Block TransferInternal API for Block Transfer (RC7959)
 Cache SupportInternal API for Cache-Key and Cache-Entry support
 Context HandlingInternal API for Context Handling
 DTLS SupportInternal API for DTLS Support
 Observe SubscriptionInternal API for handling CoAP Observe Subscriptions (RFC7641)
 PDUInternal API for PDUs
 ResourcesInternal API for handling resources
 SessionsInternal API for handling Sessions
 TCP SupportInternal API for handling CoAP TCP (RFC8283)