nanocoap, yacoap, and gcoap compared

The table below shows the functions used by nanocoap, yacoap, and gcoap to handle requests and responses.

Incoming Request

nanocoap	yacoap	gcoap and Notes
`coap_parse()`	`coap_parse()`	gcoap uses the nanocoap `coap_parse()`, which differs from yacoap in the CoAP PDU struct and order of params.
`coap_handle_req()` Callback provides a single coap_pkt_t for read and write.	`coap_handle_request()` Callback provides separate pkts for read and write, and includes an array of resource structs.	nanocoap uses an internal array of 'endpoints', which are like resources. Both libraries assume that the server provides resources only for itself. There is no notion of an external application registering endpoints with the server. gcoap allows an external application to register endpoints with `gcoap_register_listener()`. gcoap uses its own internal `_handle_req()`, which finds the callback from its list of registered arrays of endpoints.
Callback must set content type with `coap_put_option_ct()`, then writes the payload, and finally executes `coap_build_reply()`.	`coap_make_response()`	If a payload, callback executes `gcoap_resp_init()`, writes the payload, and then executes `gcoap_finish()`. If no payload, callback executes a single function, `gcoap_response()`. These functions accept a code for content type, which gcoap then uses to write the actual CoAP option.
N/A	`coap_build()` The server executes this function after the request callback.	N/A
`sock_udp_send()`	`sendto()`	gcoap calls its internal `_send_buf()`. All three functions require only buffer, length, and destination address/port.

nanocoap	yacoap	Notes and gcoap
Client executes `coap_build_hdr()`, and then `coap_put_option_url()`. Does not support a request payload yet.	`coap_make_request()`	If a payload, client executes `gcoap_req_init()`and writes the payload. If no payload, callback executes a single function, `gcoap_response()`.
N/A Since does not support request payload.	`coap_build()`	`gcoap_finish()` if wrote a payload, otherwise N/A.
`sock_udp_send()`	`send_to()`	`gcoap_req_send()` The send function for all three libraries requires buffer, length, and destination address/port. `gcoap_req_send()` also expects a handler callback for the response, and stores a memo of the request, including the callback, to match against the expected response.

nanocoap	yacoap	Notes and gcoap
`coap_parse()`	`coap_parse()`	gcoap uses the nanocoap `coap_parse()`.
N/A Does not support generic response handling.	`coap_handle_response()` Executes the callback set in coap_resource_t. Expects only a single outstanding request.	gcoap finds the memo for the request, with its internal `_find_req_memo()`. gcoap then executes the callback handler passed in to `gcoap_req_send()`. Supports a compile-time defined number of outstanding requests. Also supports a compile-time defined timeout for the response, and executes the same callback in this case.