C language: 6. Simple use and precautions of pointer

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1、 Simple instance of pointer

Through the pointer , It can be simplified C Execution of programming tasks , There are other tasks , Such as dynamic memory allocation , Can't execute without a pointer .
Each variable has a memory location , Each memory location defines the available memory & Address accessed by operator , It represents an address in memory

#include <stdio.h>
int main()
{
    
	int num = 10;
	int *p;      // Defining pointer variables 
	p = &num;
	printf("num The address of the variable ：%p\n", p);
	return 0;
}

The compilation and execution results of the above code are as follows ：

num The address of the variable ：0x7ffeeaae08d8

2、 What is a pointer

The pointer is the memory address , Pointer variables are variables used to store memory addresses .
Just like using other variables or constants , Before using pointers to store other variable addresses , It must be declared . The format is as follows ：

// data type  * Pointer variable name ;
int    *ip;    /*  Pointer to an integer  */
double *dp;    /*  One  double  Pointer to type  */
float  *fp;    /*  A floating point pointer  */
char   *ch;    /*  A character pointer  */

All actual data types , Whether it's an integer 、 floating-point 、 Character , Or other data types , The values of the corresponding pointers are of the same type , Is a long hexadecimal number representing the memory address .

The only difference between pointers of different data types is , The data type of the variable or constant that the pointer points to is different .

3、 How to use the pointer

The following operations are frequently performed when using pointers ：

• Define a pointer variable , Such as int *p;
• Assign variable address to pointer ,p = #
• Access the value of the address available in the pointer variable ,int val = *p;

#include <stdio.h>
int main()
{
    
	int var = 20;  // Actual variable declaration , Definition 
	int *ip;       // Declaration of pointer variables 

	ip = &var;     // Store... In pointer variable  var  The address of 
	
	printf("var  The address of the variable ：%p\n", &var);
	
	// Address stored in pointer variable 
	printf("ip  The address of the variable store ： %p\n", ip);

	// Use pointers to access values 
	printf("*ip  The value of the variable ：%d\n", *ip);
}

The above code compiles and executes , give the result as follows ：

var  The address of the variable : 0x7ffeeef168d8
ip  The address of the variable store : 0x7ffeeef168d8
*ip  The value of the variable : 20

4、NULL The pointer

When declaring pointer variables , If there is no explicit address to assign , Assign a value of NULL Is a good programming habit .
To assign as NULL A pointer to a value is called a null pointer .

NULL Pointer is a constant defined in the standard library with a value of zero .

#include <stdio.h>
int main()
{
    
	int *ptr = NULL;
	printf("ptr  The address is ：%p\n", ptr);
	return 0;
}

Run the above code and the result is ：

ptr  The address is  0x0

On most operating systems , The program does not allow access to address 0 Of memory , Because the memory is reserved by the operating system . then , Memory address 0 It is of great significance , It indicates that the pointer does not point to an accessible memory address . But by convention , If the pointer contains a null value （ Zero value ）, Suppose it doesn't point to anything .

Check null pointer mode ：

if (ptr)  // If  ptr  Non empty , Then finish 
if (!prt) // If  ptr  It's empty , Then finish 

5、 Pointer notes

5.1、 Do not hard code assignment to pointer

The following code assigns a value to a pointer by hard coding , It's a very dangerous thing （ This method has some usage scenarios , It is mainly reflected in the scenario that the starting address of embedded programming and other programs are written dead or the communication address is written dead ）.

    int *p = (int *)0XFDDFFBD4;
    PRINT_HEX(p);
    PRINT_INT(*p);

5.2、 Be careful to avoid generating wild pointers

int *point_bad = NULL;
void badPointDemo(){
    
    int a = 10;
    point_bad = &a;
    //....do some thing
    
    //1、 When badPointDemo() When the function finishes running , Variable a The memory address of will be recycled .
    //2、 At this time, if the pointer is not set to NULL,point_bad The variable becomes a wild pointer 
    //3、 The wild pointer cannot be judged by any method 
    point_bad = NULL;
}
int main(){
    
    badPointDemo();
    // Because the pointer variable is set to NULL 了 
    // So you can be right point_bad Pointer to judge 
    if(point_bad){
    
	}
    return 0;
}