When creating multiple c files in keil, *** ERROR LXX appears

There are many places in C/C++ language where extern is used. However, if you don't really understand its meaning, it will cause great trouble to programming. In order to help you avoid detours, we will explain it in detail.

For smaller programs, there is usually only one C file and one header file. We usually define global variables directly in the C file, and add the int i definition before the program. If you want to define it in the header file, there are two ways: use extern to declare: extern int i; This sentence only declares the variable i. Int i must be added before the C file program to define it. extern int i=0; This sentence does both declaration and definition.

For larger programs, there are many C files and header files. At this time, global variables must be declared in the header file (no initialization is required), and then defined in a C file (the initialized ones must be initialized). If defined in the header file, there will be a duplicate definition error when compiling. If it is only declared in the header file, there will be a warning that there is no definition.
   *** ERROR L104: MULTIPLE PUBLIC DEFINITIONS
       SYMBOL: K
       MODULE: 222.obj (222)
The above error occurs because the variable k is defined repeatedly. Add extern before the variable definition in your header file (it is just a variable declaration without initialization), and then define it before a program that calls a C file of the variable (note that the first C file called is responsible for initializing the variable, and other called C files do not need the initialization process).