The problem of repeated declaration of global variables in embedded C programming

Jacktang

The problem of repeated declaration of global variables in embedded C programming [Copy link]

When writing programs in C language, we often encounter such a situation: we want to define a global variable in the header file, and then include it in two different c files, hoping that this global variable can be shared by the two files. For example, a global variable int i is declared in the header file led.h of the subfunction led.c, and led.h is included in main.c and led.c, so some people naively think that it can be used normally, but when you compile, the following error will appear: ..\OBJ\test.axf: Error: L6200E: Symbol TimingDelay multiply defined (by systick.o and main.o). That is to say, the compiler thinks that we have defined the variable i repeatedly. This is because the #include command moves the contents of the header file to the #include location intact, so it is equivalent to executing int i once in main.c and led.c. Global variables in C language are visible within the project (or engineering), which results in two variables i in the same project, and the compiler considers it a duplicate definition. The correct solution is as follows: [size=4 ](1)main.c #include"led.h" [color=# 000000] int i; //Define a global variable (2)led.c [size=4 ] #include"led.h" [color=#000000 ]extern int i;//Declare this variable as an external variable, which is a global variable defined in other c files. Note that declaration is different from definition. Definition will allocate variable space, while declaration will not allocate space. It just tells the compiler that this variable is somewhere else and I am just using it.