constant pointer and pointer to constant and both

Syntax:

• constant pointer
• <type of pointer> * const <name of pointer>        ( int * const ptr;)
• once address assigned then that's it. address  is constatnt. but that address can have any value
• thinking like used to store particular hardware register using this kind of pointer and never changed that pointer address/restrict to change that pointer address throughout the program
• pointer to constant
• const <type of pointer>* <name of pointer>         (const int* ptr;)
• once address assigned then address's value is constant but can change address
• thinking like used in the receving function parameter and not allowing the function to change the value (void func(const int* ptr))
• constant pointer to constant
• const <type of pointer>* const <name of pointer>(const int* const ptr;)
• both the assigned address cannot be changed and address's value also cannot be changed
• combination of above both

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Purpose of const qualifer :

• ows the compiler to catch errors in which code accidentally changes the value of a variable
• he compiler might be able to make certain optimizations to the code generated if it knows that a variable will not be changed.
• example 1:

• while (*str = 0) /*instead of *str != 0  mentioned as *str = 0 by progmer*/
  {
      /* some code here */
      str++;
  }

• constant pointer
• <type of pointer> * const <name of pointer>        ( int * const ptr;)
• once address assigned then that's it. address value is constatnt

Example 1a:

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# cat constant_pointer.c 

#include<stdio.h>

int main()

{

int a=10;

int b=20;

int * const ptr=&a;

ptr=&b;

return 0;

}

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# gcc constant_pointer.c 

constant_pointer.c: In function ‘main’:

constant_pointer.c:8:2: error: assignment of read-only variable ‘ptr’

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# 

Example 1b:

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# cat constant_pointer.c 

#include<stdio.h>

/* */

int main()

{

int a=10;

int b=20;

int * const ptr=&a;

printf("\nvalue before changing  value of a = %d\n",*ptr);

*ptr = 20;

printf("\nvalue after changing of value of a = %d\n",*ptr);

//ptr=&b;

return 0;

}

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# gcc constant_pointer.c 

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# ./a.out 

value before changing  value of a = 10

value after changing of value of a = 20

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# 

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• pointer to constant
• const <type of pointer>* <name of pointer>         (const int* ptr;)
• once address assigned then address's value is constant but can change address

Example 2a:

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# cat pointer_to_constant.c 

#include<stdio.h>

int main()

{

int a=10;

int b=20;

const int *ptr=&a;

*ptr = 20;

return 0;

}

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# gcc pointer_to_constant.c 

pointer_to_constant.c: In function ‘main’:

pointer_to_constant.c:8:2: error: assignment of read-only location ‘*ptr’

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# 

Example 2b:

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# cat pointer_to_constant_2.c 

#include<stdio.h>

/* */

int main()

{

int a=10;

int b=20;

const int *ptr=&a;

ptr = &b;

printf("\n%d\n",*ptr);

return 0;

}

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# gcc pointer_to_constant_2.c 

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# ./a.out 

20

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# 

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• constant pointer to constant
• const <type of pointer>* const <name of pointer>(const int* const ptr;)
• both the assigned address cannot be changed and address's value also cannot be changed

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# cat constant_pointer_to_constant.c 

#include<stdio.h>

int main()

{

int a=10;

int b=20;

const int * const ptr=&a;

*ptr = 30;

ptr = &b;

printf("\n%d\n",*ptr);

return 0;

}

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# gcc constant_pointer_to_constant.c 

constant_pointer_to_constant.c: In function ‘main’:

constant_pointer_to_constant.c:7:2: error: assignment of read-only location ‘*ptr’

constant_pointer_to_constant.c:8:2: error: assignment of read-only variable ‘ptr’

root@selvakumar-Lenovo-B460e:/C_pgms/const_pointer# 

Reference:

http://www.thegeekstuff.com/2012/06/c-constant-pointers/
https://stackoverflow.com/questions/1143262/what-is-the-difference-between-const-int-const-int-const-and-int-const