This coap repository is based on master branch @b425b150 commit of libcoap.
For further information related to CoAP, see https://coap.technology.

CoAP package Path in menuconfig :
RT-Thread online packages/IoT/CoAP.

Config in menuconfig :
RT-Thread Components/Command shell/Using mode shell

Modify MSH consle buffer in menuconfig :
Set RT-Thread Kernel/Kernel Device Object/the buffer size for console log printf as 512

If there are some errors after compiled , please refer to the following Notes

Use coap_client to request coap.me/test

Using -m param to select coap client request method , it only realize get method.

msh />coap_client -m get coap:https://coap.me/test
uri.path.s = test; uri.host.s = coap.me/test 
server_host = coap.me
DNS lookup succeeded. IP=134.102.218.18
welcome to the ETSI plugtest! last change: 2018-01-15 13:55:19 UTC

msh />coap_server 

Default coap server uri is coap:https://yourserveripaddr/rtthread , use another coap_client to request this server as coap_client -m get coap:https://yourserveripaddr/rtthread , then client will return Hello rtthread! message.

Modify libcoap/include/coap/encode.h at line 13 as #if (BSD >= 199103) || defined(WITH_CONTIKI) || defined(RTTHREAD_VERSION)

Config in menuconfig :
Don't select RT-Thread Component/Device virtual file system/Enable BSD socket/Toolchain sys/select.h and Toolchain sys/socket.h.

Config in menuconfig :
The RT-Thread Component/Device virtual file system/The maximal number of opened files value need to greater or equal to RT-Thread Component/Network stack/light weight TCP/IP stack/The number of raw connection value.

1 CoAP Official website: https://coap.technology/
2 CoAP test server: coap.me