The function of extern, static, register, volatile keywords in C language; Nanny level teaching!

Catalog

Preface ：

One .extern

  Two .static

  3、 ... and .register

Four .volatile 

Last

Rome wasn't built in a day . Since we choose the distance , When you live up to your youth , Self-motivated forward , Every crack is an effort to reveal light , Only the ultimate struggle , To deserve the ultimate scenery . Gentlemen , See you at the top of the mountain ！

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Preface ：

stay C In language , These are the key words , They all have their own personalities .static and extern It's used a lot , especially static At school C Language tests often get .register and volatile These two keywords are seldom used , If you already know the role of these two keywords , Then Xiaobian will give you a thumb , The young man is amazing . Xiaobian himself has also been in the exam static This keyword is suffocating , To think oneself C In the case of good linguistics , see register and volatile I'm also confused , Today, I want to talk about these keywords .

One .extern

extern Declare external symbols , Used to declare that it is not in the same source file （ .c file ） The variable of , This is very similar to the reference header file , But the reference header file is limited to , Variable in    .h In file , We can use #include"xxx.h", But if it's true .c Header files cannot be used in files .

See code example , Here is the case of referencing header files .

Let's demonstrate again , External variables are .c In file ;

  Be careful ： there a adopt extern To declare , Here, the external files are pressed a, Must be a global variable , Because the scope of a local variable is only in its local scope , The life cycle of local variables is also the beginning of entering the scope and the end of leaving the scope , And global variables The scope of action is the whole project , Life cycle is the life cycle of the whole program , If a It's a local variable , Declared externally a, and a The life cycle of is over , The compiler will not compile .

extern It's the same for functions , The usage is the same as that of ordinary global variables .

But functions are generally OK without adding , But there may be an undeclared warning .

  Two .static

1.static The role of is , By static Modify local variables , Will change the storage type of local variables , For ordinary local variables, they are stored in the stack , By static Decorated variables are stored in static areas . The variables stored in the static area will not be destroyed when they are out of scope , There is still a . So the life cycle of static variables is the life cycle of the whole program , Only when the program ends, the variables in the static area will be recycled . Is the variable description of the static area here very similar to the global variable , ha-ha , Yes, global variables are also stored in static areas . In fact, in addition to the static area and stack area , There's also a heap area .

  Write a piece of code to see static How to prolong the life cycle of variables ：

#include<stdio.h>
void text()
{
	int a = 0;
	a++;
	printf("%d ", a);
}
int main()
{
	for (int i = 0; i < 5; i++)
	{
		text();
	}
	return 0;
}

  Friends can calculate the running result of this code .

Are you guys right , Because of the a It's a local variable ; So every time you enter the function text（）,a Variables will be recreated , So every time a The values of variables are all a=0+1; Out of function  a The variable will be destroyed .

  Now let's take a look at this code ：

#include<stdio.h>
void text()
{
	static int a = 0;
	a++;
	printf("%d ", a);
}
int main()
{
	for (int i = 0; i < 5; i++)
	{
		text();
	}
	return 0;
}

Let's calculate the result, buddy

  Why is this so ？

  thus it can be seen , The life cycle of local variables is extended , But the scope remains the same .

2. If static Modify global variable , What will it be like ？ Let's have a look .

First, global variables have external link attributes ,static After modification , The external connection attribute becomes , Internal connection properties . This global variable can only be in its own .c See the document , Other source files cannot be accessed .

  Modifying a function will also make the external connection attribute of the function become the internal connection attribute . Functions can only be in their own .c See the document , Other source files cannot be accessed .

  3、 ... and .register

register be called register , The variable modified by it is called , Register variables . What is a register? Let's briefly introduce , The storage location of the computer is , Hard disk , Memory , Cache , register .

  When a variable , Later, when it needs a lot of use , Every time CPU To register , Cache , Memory level by level to find , In this way, the access speed will be very slow , In this case , If we just store such variables in registers , that CPU Get it directly from the register every time , The access speed will be greatly improved . therefore register Keywords appear here , however register Modifying variables , It is only recommended to store variables in registers , It will depend on the situation , Not by register If you decorate it, you must put it in the register , Pay special attention to this . The compiler will intelligently add , The variables that need to be processed are put into registers . And being register Decorated variables cannot take addresses , Because & Only the address in memory can be retrieved , Regardless of being register Decorated variable , Is it really stored in the register , Can't take the address .

Four .volatile 

volaile There are unstable , Easy to change meaning . When we have a variable that needs a lot of use , This variable will , It may be stored in registers , Now let's see , It seems nothing, but there is a certain risk , Because the code we write now is single threaded , If the code has more than one thread , In another thread, variables will be modified , and CPU Still get variables from registers , What you get is not the modified variable , Will cause problems . and volatile Is to make the modified variable not to optimize , Always in memory , Improve access to special address locations , Make subsequent access more secure . here volatile and register A little opposite , Can compare memory .

It is worth noting that ,volatile Decorated variables also remind the compiler , This variable may change later , Take care to protect . Not by volatile Modification and certain changes . This and register Very much like .
 

Last

Rome wasn't built in a day . Since we choose the distance , When you live up to your youth , Self-motivated forward , Every crack is an effort to reveal light , Only the ultimate struggle , To deserve the ultimate scenery . Gentlemen , See you at the top of the mountain ！