[dish of learning notes dog learning C] initial level of pointer

What is the pointer

   A pointer is relative to a memory unit , Refers to the address of the unit , The content of the unit contains data . stay C In language , Allow pointer variables to hold pointers , therefore , The value of a pointer variable is The address of a memory unit .
   Pointers describe the location of data in memory , Indicates an entity occupying storage space , The relative distance value of the starting position in this space .

Pointer types

   The meaning of pointer type ：
   1. The pointer type determines the permission of pointer dereference ;
   2. The type of pointer determines , The pointer takes one step , How far can I go （ step ）.

int main() {
    
	int a = 0x11223344;
	int* pa = &a;
	char* pc = &a;

	printf("%p\n", pa);
	printf("%p\n\n", pa + 1);

	printf("%p\n", pc);
	printf("%p\n\n", pc + 1);

	printf("%0x\n", a);
	*pc = 0;
	printf("%0x\n", a);
	*pa = 0;
	printf("%0x\n", a);

	return 0;
}

Wild pointer

   A wild pointer means The position pointed by the pointer is unknown （ Random 、 incorrect 、 There is no definite limit to ）.
    1. Pointer not initialized

int main() {
    
	// Pointer not initialized 
	int* p;//p Is a local pointer variable , Local variables are not initialized , The default is a random value 
	*p = 10;

	return 0;
}

    2. Pointer access is out of bounds

int main() {
    
	// Pointer access is out of bounds 
	int arr[10] = {
     0 };
	int* pa = arr;
	int i = 0;
	for (i = 0; i <= 10; i++) {
    
		*pa = i;
		pa++;
	}

	return 0;
}

    3. Pointer to space release

int* test() {
    
	int a = 10;
	return &a;
}
int main() {
    
	// Pointer to space release 
	int* p = test();
	*p = 20;

	return 0;
}

   Avoid wild pointer ：
    1. Pointer initialization - Clearly know the initialization value
    2. Watch out for the pointer
    3. The pointer points to the space release and is set to NULL
    4. Check the validity of the pointer before using it

Pointer arithmetic

   The pointer +- Integers ;
   The pointer - The pointer , obtain Number of elements between two pointers ; The premise of pointer subtraction is that two pointers point to The same space ;
   Pointer relation operation
     The standard specifies that the pointer to the array element can be the same as the pointer to Pointer to the memory location after the last element of the array / Address Compare , But it is not allowed to point to Pointer to the memory location before the first element / Address Compare .

int main() {
    
	int arr[] = {
     1,2,3,4,5,6,7,8,9,10 };
	int* p = arr;
	int* pend = arr + 9;
	while (p <= pend) {
    
		printf("%d\n", *p);
		p++;
	}

	printf("%d\n", &arr[9] - &arr[0]);

	return 0;
}

int my_strlen(char* str) {
    
	char* start = str;
	while (*str != '\0') {
    
		str++;
	}
	return str - start;
}
int main() {
    
	int len = my_strlen("abc");
	printf("%d\n", len);

	return 0;
}

Arrays and pointers

int main() {
    
	int arr[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	//[] Is an operator ,2 and arr Are two operands , The order of the two operands can be exchanged 
	//arr[2] It will be converted to *(arr+2),arr[2] <==> *(arr+2)
	//arr Equivalent p,*(arr+2) <==> *(p+2) <==> *(2+p) <==> *(2+arr)
	//arr[2] <==> *(arr+2) <==> *(p+2) <==> *(2+p) <==> *(2+arr) <==> 2[arr]
	//arr[2] --> *(arr+2) -->*(2+arr) --> 2[arr]
	int* p = arr;
	printf("%d\n", arr[2]);
	printf("%d\n\n", p[2]);

	printf("%d\n", 2[arr]);
	printf("%d\n", arr[2]);

	return 0;
}

Pointer array

   An array of pointers is essentially an array , Is an array specially used to store pointers .

The secondary pointer

   The essence of pointer is also a variable , Since it's a variable , Then it will also be stored in memory , There will also be a storage address , The variable used to store the address of a primary pointer is called a secondary pointer . Similarly, the variable used to store the secondary pointer address is the tertiary pointer （ No doll ,doge）.

int main() {
    
	int a = 10;
	int* pa = &a;
	//ppa It's just a secondary pointer variable 
	int** ppa = &pa;//pa It's also a variable ,&pa Take out pa Start address in memory 

	return 0;
}

  【 Learning notes Dog learn C】 Getting to know the pointer
  【 Learning notes Dog learn C】 Advanced pointer