Fixed repeated variable definitions when multiple files are created in Keil

Fixed repeated variable definitions when multiple files are created in Keil Extern is used in many places in the C/C ++ language. However, if you do not really understand its meaning, it will bring a lot of trouble to programming, let's explain it in detail.
      For small programs, there is usually only one C file and one head file. We usually define global variables directly in the C file and add int I Definitions before the program. If you want to define the following two methods in the header file: Use extern to declare: extern int I; this statement only declares the variable I, int I must be added before the program in the C file. Extern int I = 0; both the Declaration and definition are done.
      For larger programs, there are many c files and header files. At this time, the global variables must be declared in the header file and then defined in a C file. If it is defined in the header file, a duplicate definition error occurs during compilation. If only the header file is declared, there will be a warning that the definition is not defined.
    * ** Error l104: multiple public Definitions
        Symbol: K
        Module: 222. OBJ (222)
  The above error occurs because the variable k is repeatedly defined. Add extern before the variable definition in your header file, and then define it before a program in a C file.
      * ** Warning L1: unresolved external symbol
      Symbol: K
      Module: 111. OBJ (111)
* ** Warning L2: reference made to unresolved external
      Symbol: K
      Module: 111. OBJ (111)
      Address: 000dh
* ** Warning L1: unresolved external symbol
      Symbol: K
      Module: 222. OBJ (222)
* ** Warning L2: reference made to unresolved external
      Symbol: K
      Module: 222. OBJ (222)
      Address: 001fh
    The four warnings indicate that the variable k is declared in the header file but not defined. You only need to define it before a program in a C file.Article reposted links: http://blog.sina.com.cn/s/blog_4d8890ad01000ajw.html