Usage of static [detailed explanation]

Introduction to this issue

This paper mainly introduces ： What is the static,static modification local variable 、 Global variables 、 function Changes that will occur when , What are the essential reasons for these changes ..

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Catalog

1. summary
2. usage
   • Modify local variables
   • Modify global variable
   • Modify function
3. summary

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summary

Static, As the name suggests, it is static 、 Static meaning . stay C In language static Is used to modify variables and functions keyword , Some properties of the modified object will be fundamentally changed , In a sense, these changes seem to fit “ static state ” This means . Then let's talk about it in detail static The usage of this keyword .

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usage

static There are three different uses ：1. Modify local variables ,2. Modify global variable ,3. Modify function . Here is a detailed explanation and analysis of each usage in turn .

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Modify local variables

By static Modified local variables are called ： Static variables . Now there's a piece of code , Try to think about what the execution results are ：

#include<stdio.h>
void test()
{
    
	int a = 1;
	a++;
	printf("%d ", a);
}
int main()
{
    
	int i = 0;
	while (i < 10)
	{
    
		test();
		i++;
	}
	return 0;
}

We know the of local variables Life cycle That's where it is Local scope , To put it simply, enter this range variable to create , If you leave this range, the variable will be destroyed . From the above example, this sentence can also be well confirmed , Every time you call a function test, local variable a create , When the call is over a The destruction , So every print is 2. But if I use static Modify the variables here a, What kind of change will happen ？

#include<stdio.h>
void test()
{
    
	static int a = 1;
	a++;
	printf("%d ", a);
}
int main()
{
    
	int i = 0;
	while (i < 10)
	{
    
		test();
		i++;
	}
	return 0;

Why is this so ？ Someone might say ： because static Modified variables are executed only once in a program , Then it won't change . But I want to ask ： Why is that ？ Essentially because static The storage location of modified local variables has changed , Local variables are stored in The stack area Inside , Static variables are stored in Static zone Inside . And the impact of this change is to make local variable Of Life cycle Become as long as the whole program , namely ： If a local variable is out of its scope, it will not be destroyed , Unless the program ends . It is worth noting that although the declaration cycle of local variables has been greatly improved , But his scope hasn't changed , Or it can only be used in that local scope .

Memory is roughly divided into three areas ： The stack area , Heap area , Static zone . Here is just a simple mention , Not deep .

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Modify global variable

We know that global variables can be used in different source files , Just need to use extern Make an external statement . for example ：


But if use static Decorating this global variable will result in an execution error ：


The reason for the error is ： Unable to resolve external instruction , This phenomenon occurs The root cause yes ： Global variables are variables with External link properties Of , and static When modifying the global variable, put this External link properties Turned into Internal link properties . This makes it impossible for other source files to use the global variable . That's why you feel as if the scope of global variables has become smaller , But the life cycle remains the same , Still the whole project .

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Modify function

static Modifier functions are actually similar to global variables , Because functions also have External link properties , By static After modification, it becomes Internal link properties 了 , Other source files can no longer be used .

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summary

Local variables are static When decorating, the life cycle will become as long as the whole program , But the scope has not changed . and static When modifying global variables and functions , It can only be used inside the source file , Isolate from the outside .

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