Wild pointer recurrence

One 、 I don't know if the pointer is used incorrectly

I believe everyone has heard of wild pointer , I also know and I also know the harm . But I don't know that I always use wild pointer , Ashamed to speak .

Before using pointer ↓

// function ： Read the contents of the pointer 
// Realization ： Output the contents of the pointer to the serial port assistant 
// author ：maosql
// Time ：2021.9.9
void DisplayPointer(void)
{
    
	char *p;			//u8 *p;
	const u8* zfc="masoql"; 	// Connect the password 
	
	sprintf((char*)p,"AT+XXX\"%s\"",zfc);// Set wireless parameters :ssid, password  

}

Take it for granted that this string is stored in the pointer p among ： First address , Save it back one by one , When I visit, the value in it should be the value I saved , In fact, it should be so , But there is a lot of uncertainty , Because such use is the wrong operation , Pointers cannot be assigned this way ;
error ： You cannot assign a value like this without allocating memory space to the pointer , If p Later memory is used , The string cannot be assigned and the program crashes
consequence ： This kind of program will appear , Maybe it's still good now , After a day , Nothing , You may not be able to run

Two ： Wrong lead

Although the harm is great , But I don't know before , There is no problem with this use , Possible problems , I didn't think in this direction . When one day I need to access and output the content inside, I find that I can't access , I thought at first printf The output pointer is in the wrong format , I Baidu others said the output method is still wrong , Then the accidental experiment can be output , But it makes me puzzled

1. Read the error content

Correct printf Output pointer format , But the content read is h mouth （ My God . What is this ！！ See the screenshot below ）

// function ： Read the contents of the pointer 
// Realization ： Output the contents of the pointer to the serial port assistant 
// author ：maosql
// Time ：2021.9.9
void DisplayPointer(void)
{
    
	char *p;			//u8 *p;
	const u8* zfc="masoql"; 	// Simple string （？？？ Why can we assign values again  ↓↓↓ ）
	/* eg: image char *p = "hello";  Such , It's not about defining a pointer p, Then give this p assignment , The content inside is filled with hello.  It is hello Such a string , Exists in a place in memory , Then define a pointer , Point here  */
	
	sprintf((char*)p,"AT+XXX,\"%s\"",zfc);// hold  AT+XXX,"maosql"  Store in p In the memory of the first address 

	printf("%s\r\n",(char *)p);	// || printf(p); || printf("%s\r\n",p);// Format can output content correctly 

// printf("%p\r\n",&p); 
}

result ：（ The code format is the same. For the convenience of explanation, the original code is not used ）

2. Why read the content

The same output format as above is followed by a sentence , You can display the desired results （why, The following statement can also affect the previous statement ？ See the screenshot below ）

// function ： Read the contents of the pointer 
// Realization ： Output the contents of the pointer to the serial port assistant 
// author ：maosql
// Time ：2021.9.9
void DisplayPointer(void)
{
    
	char *p;			//u8 *p;
	const u8* zfc="masoql"; 	// Connect the password 
	
	sprintf((char*)p,"AT+XXX\"%s\"",zfc);// Set wireless parameters :ssid, password  

	printf("%s\r\n",(char *)p);	// printf(p); || printf("%s\r\n",p);
	printf("%p\r\n",&p);	
}

result ： This result can be output correctly , But that's why , Why is it unreasonable to add this sentence after it , But it can output the results correctly , Until now, I haven't understood why . If someone understands , Please also comment on the area guidance 12 .（ But I realized , The wrong starting point , There are strange problems , So I directly learn how to use it correctly ）

3、 ... and ： Wrong solution

Give up the wrong problem caused by the wrong use above , Directly use the method of correctly assigning values to pointers

1. Method 1 ：

// function ： Read the contents of the pointer 
// Realization ： Output the contents of the pointer to the serial port assistant 
// author ：maosql
// Time ：2021.9.9
void DisplayPointer(void)
{
    
	char p[64] = {
    0};// The way to declare an array is to realize the function of a pointer , By manipulating the p Realize the use of pointers , As we all know, arrays 、 The function itself is the first address of the pointer 
	const u8* zfc="masoql"; 	// Simple string （？？？ Why can we assign values again  ↓↓↓ ）
	/* eg: image char *p = "hello";  Such , It's not about defining a pointer p, Then give this p assignment , The content inside is filled with hello.  It is hello Such a string , Exists in a place in memory , Then define a pointer , Point here  */

	sprintf((char*)p,"AT+XXX\"%s\"",zfc);	// hold  AT+XXX,"maosql"  Store in p In the memory of the first address 
	printf("%s\r\n",(char *)p);	// || printf(p); || printf("%s\r\n",p);// Format can output content correctly 
	
}

2. Method 2 ：

// function ： Read the contents of the pointer 
// Realization ： Output the contents of the pointer to the serial port assistant 
// author ：maosql
// Time ：2021.9.9
void DisplayPointer(void)
{
    
	char *p;			//u8 *p;
	const u8* zfc="masoql"; 	// Simple string （？？？ Why can we assign values again  ↓↓↓ ）
	/* eg: image char *p = "hello";  Such , It's not about defining a pointer p, Then give this p assignment , The content inside is filled with hello.  It is hello Such a string , Exists in a place in memory , Then define a pointer , Point here  */

	p=mymalloc(SRAMIN,32);								// apply 32 Byte memory  
	sprintf((char*)p,"AT+XXX\"%s\"",zfc);	// hold  AT+XXX,"maosql"  Store in p In the memory of the first address 
	printf("%s\r\n",(char *)p);	// || printf(p); || printf("%s\r\n",p);// Format can output content correctly 

	myfree(SRAMIN,p);											// Free memory  
	p=NULL;																// Avoid becoming a wild pointer 
	
}

result ： Output correctly , There are no strange problems

Four ： Understanding and perception after solution

Pointer to an address , Always for easy reading existing The contents of the address . If you want to point to, point to the address value of the existing memory address , If you want to assign a value to the pointer, use the above two methods , One is used as an array , A dynamic application for memory space （ Remember to release and set NULL）.