[424] two C language pointer

Reference: C pointer to pointer

Pointer is a pointer to a multi-stage form of indirect addressing, or is a chain of pointers. Typically, a pointer contains the address of a variable. When we define a pointer to a pointer, the first pointer contains the address of the second pointer, the second pointer to an actual position values.

A pointer to a pointer variable must be declared as follows, namely placing two asterisks in front of the variable name. For example, the following declaration of a pointer pointing to a pointer of type int:

int ** were ; 

When a target value is a pointer pointing to another pointer indirectly, to access the values ​​requires two asterisks operator, as shown in the following example:

• var: int variables
• ptr: Pointer to the var
• pptr: the pointer ptr
  
  
• Var value represents three methods:
  • where
  • * Ptr: ptr pointer corresponding to a value, i.e., var
  • ** pptr: * pptr corresponding pointer value PTR i.e., so ** pptr = * ptr = var

• Ptr represents the value (var address, pptr value corresponding to) four methods:
  • & Var: var represents an address, i.e. ptr
  • ptr
  • & (* Ptr): * ptr corresponds var
  • * Pptr: * pptr corresponding pointer value PTR i.e., so * pptr = ptr = & var

• It represents three methods pptr value (PTR address):
  • & Ptr: indicates the address of ptr, that pptr
  • pptr
  • & (* Pptr): * pptr equivalent to ptr

• It represents pptr address ways:
  • &pptr

#include <stdio.h>

int main() {
	int var;
	int *ptr;
	int **pptr;
	
	var = 3000;
	
	ptr = &var;
	
	pptr = &ptr;
	
	printf("Value of var = %d\n", var );
	printf("Value available at *ptr = %d\n", *ptr );
	printf("Value available at **pptr = %d\n", **pptr);
	
	printf("\n");
	
	printf("Address of var: &var = %p\n", &var);
	printf("Address of var: ptr = %p\n", ptr);
	printf("Address of var: &(*ptr) = %p\n", &(*ptr));
	printf("Address of var: *pptr = %p\n", *pptr);
	
	printf("\n");
	
	printf("Address of ptr: &ptr = %p\n", &ptr);
	printf("Address of ptr: pptr = %p\n", pptr);
	printf("Address of ptr: &(*pptr) = %p\n", &(*pptr));
	
	printf("\n");
	
	printf("Address of pptr: &pptr = %p\n", &pptr);
	
	return 0;
}

Outputs：

Value of var = 3000
Value available at *ptr = 3000
Value available at **pptr = 3000

Address of var: &var = 0x7ffff21a1b64
Address of var: ptr = 0x7ffff21a1b64
Address of var: &(*ptr) = 0x7ffff21a1b64
Address of var: *pptr = 0x7ffff21a1b64

Address of ptr: &ptr = 0x7ffff21a1b68
Address of ptr: pptr = 0x7ffff21a1b68
Address of ptr: &(*pptr) = 0x7ffff21a1b68

Address of pptr: &pptr = 0x7ffff21a1b70