C language - keyword 1

auto

Scope of variable ：

• The range that variables can act on . It's usually in curly brackets .

Life cycle of variable ：

• The time it takes for a variable to be created and destroyed .

It can be considered that all local variables default to auto Of , But you don't have to write a compiler to know . This keyword is hardly used .

register

When you use register When decorating a variable , The compiler will place the variable in as far as possible CPU And not in memory , therefore CPU The speed of accessing this variable is very fast .
Some attention ：

• 1,register The best decoration is local variables . Because global variables always occupy registers .
• 2, Will not be written . Writing is to write back to memory . Then the variable is set to register It doesn't make sense .
• It is better to read and write in high frequency .
• Can't be & Symbolic address .
• Don't be overused , Because registers are limited .

Why have header files

A lot of times we have this demand ： Using functions or variables from another source file in one source file . Can it be used directly ？

• A function can , But it will warn , Function as long as the link is found .( It is limited to the functions written by yourself , To use library functions or other functions, you must include declarations ).
• Variables must be declared before they can be used , And it cannot be assigned at the time of declaration . Because the statement does not open up space .

But if I have 100 Source file , Each source file must use the same global variable , So what to do ？ Should I write 100 Do you want to make a statement ？

therefore , A good solution is , Extract these declarations into 1 Shared files , To facilitate the maintenance of the project . This file is the header file . And every source file where you want to use variables or functions , Just include the header file .

• What the header file contains ：
  

static

We know that global variables and functions can span files . Functions can be used without even declaration . Is there any way to ensure that your functions cannot be directly used by other source files ？

• static Modify global variable , Indicates that this variable can only be used inside the source file . Changed the scope of the variable . But it can be used indirectly .
• static Modify function , Indicates that the function can only be used inside the source file . But it can be used indirectly in a nested way .
• static The characteristics of for C Language provides the possibility of encapsulation . Ensure the safety of the project .

/************* test.c file  ***************/
static int g_val = 100;  //g_val Cannot be directly accessed by external files . because static
int show_val() {
    
	printf("%d\n", g_val); // But it can go through show Function access ; because show Functions can be accessed by other source files 
}

static void test(){
     // By static modification , Cannot be accessed directly from outside 
 // do something...
}
void show_test(){
    
	test(); // But you can nest , Indirect access to 
}

• static Modify local variables , The life cycle of the variable

extern

• In front of the variable , Indicates that the variable is declared . Global variable declarations must carry extern, Otherwise, the compiler may be ambiguous . It is impossible to tell whether it is a definition or a declaration without it .
• Function can be without , Because the compiler can distinguish the definition and declaration of functions , But to keep good habits , It is suggested that you take it with you .