undefined reference to `rt_thread_join'

This error message typically occurs during the compilation process of a program that uses the Real-Time (RT) Thread library. It indicates that the linker is unable to find the definition of the function `rt_thread_join` in the library.

To resolve this issue, you need to ensure that the RT Thread library is properly installed and linked to your project. You can do this by adding the appropriate linker flags to your build configuration.

For example, if you are using GCC as your compiler, you can add the following linker flags to your Makefile:

-L/path/to/rt_thread/lib -lrtthread

This tells the linker to search for the RT Thread library in the specified directory and link it to your program.

If you are using a different compiler or build system, you will need to consult the documentation for that tool to determine the appropriate linker flags to use.

Once you have added the linker flags, recompile your program and the error message should be resolved.