OpenAT enviroment is very easy to use, helping you when missing brakets, parameters… other errors like type incompatibility will show you when building.
Every C standard library is very complete on SDK, try to write “strstr()” on SDK and will change of color, meaning that is available to use. Also you can use wm_strstr().
If not, make sure you know the library that contains your function or procedure, strstr() is a string.h library. And you can search all librarys in your SDK project tree -> Includes, search inside, when found read the library to see if function is available.