C language -c51 compilation warning "* * * warning l1: unresolved external symbol" and extern

C51 Compile warnings “--- WARNING L1: UNRESOLVED EXTERNAL SYMBOL”

 ----WARNING L1: UNRESOLVED EXTERNAL SYMBOL

    SYMBOL:  ?C_START

    MODULE:  .STARTUP.obj (?C_STARTUP)

----WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL

    SYMBOL:  ?C_START

    MODULE:  .STARTUP.obj (?C_STARTUP)

    ADDRESS: 000DH

          If you're using C51 The compiler has the above warning , And I searched all over the Internet and couldn't find out , Do you think this problem is difficult , It's so rare that no one can answer ？

         In fact, this is only a mistake made by beginners and careless people ： No C Files added to the project ！

/// in addition , It may also be because there are defined functions that have not been called .