Precautions for using C language global variables (global variables in C and H files, static global variables)

C Language contains global variables in header files in three forms ：

1 Declare a global variable in the header file , In a C Global variables are defined in the file , And then in all the references to this header file C The global variable value can be used in the file .
If in main.c Definition
int a = 3;
Then define a main.h, Then all references main.h Of c In file , All accessible a And change its value .

2 Define a static global variable in the header file . Then all references to the header file C file , Each has a static global variable with the same name whose scope is within the scope of this file . The variable in different files has the same name , But it's a different variable .
For example a.h In the definition of
static int a = 3;
So in a.c I quote a.h, And will a Value is modified to 4.
stay b.c Also quoted in a.h, incorrect a Value to modify , Print a value , Still 3, Not because a.c To change the value .

3 Define global variables in the header file . This header file can only be used by one C File reference once .
such as a.h In the definition of
int a = 3;
stay a.c Reference can be made to a.h, Also on a Access and modify .
If there are under the same project b.c Refer to the a.h, The compiler will report the error of global variable with the same name when linking , Resulting in compilation failure .

Reference article ：c The language header file contains 、 Use of global variables