In my program, compiled according to CanSendMainkey this variable in key. C is not defined in this document, however right you can find it in the can. C file defines, search baidu said in the wrong file and define the header file, but I'm in the key. The added can. C h header file, it's no use said, because the can. There is no definition CanSendMainkey h this variable, ask colleagues to know that there is no in the key. The separate statement calling this variable in c, so in the key. C with extern under-16 CanSendMainkey [3] is good (I am beginner, there is wrong, please point out)