The role of keywords auto, static, register, const, volatile, extern in C language

1.auto

This keyword is used to declare that the lifetime of a variable is automatic, and that variables defined in any class, struct, enumeration, union, and function are not treated as global variables, whereas variables defined in a function are considered local variables . This keyword doesn't write much, because all variables are auto by default.

　

intfunc () {AutointA; return 1;} Autointb = A;intMainintargcChar Const*argv[]) {printf ("%d\n", B); return 0;}/*Mainc.c:7:10:error:file-scope declaration of ' B ' specifies ' auto ' auto int b = $;*/

2.register

This keyword command compiler as far as possible to have variables in the CPU internal registers rather than through memory addressing access to improve efficiency.

intMainintargcChar Const*argv[]) {Registerintb =Ten; printf ("%d\n", B); printf ("%p\n", &b); return 0;}/*mainc.c:in function ' main ': mainc.c:13:2: error:address of register variable ' B ' requested printf ("%p\n", &b); 
    */

3.static

The 3.1 static storage class instructs the compiler to maintain the existence of local variables for the lifetime of the program without having to create and destroy each time it enters and leaves the scope. Therefore, using the static modifier local variable can maintain the value of a local variable between function calls.

The 3.2 static modifier can also be applied to global variables. When static modifies a global variable, the scope of the variable is limited to the file in which it is declared.

3.3 Static is the default storage class for global variables

4.extern

The 4.1 extern storage class is used to provide a reference to a global variable that is visible to all program files. When you use ' extern ', the variable name is pointed to a previously defined storage location for variables that cannot be initialized.

4.2 When you have multiple files and you define a global variable or function that you can use in other files, you can use extern in other files to get a reference to a defined variable or function. As you can understand,extern is used to declare a global variable or function in another file.

The extern modifier is typically used when two or more files share the same global variable or function

Main.c
int count; extern void  int  main () {   5;    extern int  void write_extern (void) {   printf ("count is%d\n  ", Count);}

Reference: https://www.cnblogs.com/candyming/archive/2011/11/25/2262826.html

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The role of keywords auto, static, register, const, volatile, extern in C language