Type-casting with Character Pointer
====================================

* In this section, you are going to learn

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	How to manipulate integer using character pointer ?

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	How to manipulate short integer using character pointer ?

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	How to manipulate structure using character pointer ?

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	How to swap bytes using character pointer ?

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	How to manipulate heap using character pointer ?

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         Topics in this section,

                * :ref:`Example 1 : Manipulate Integer : Using character pointer pointing to an integer <typecasting_n_ptr_sp_char_ptr_ex_1>`
                * :ref:`Example 2 : Manipulate Short Integer : Using character pointer pointing to a short integer <typecasting_n_ptr_sp_char_ptr_ex_2>`
                * :ref:`Example 3 : Manipulate Structure : Using character pointer pointing to a structure <typecasting_n_ptr_sp_char_ptr_ex_3>`
                * :ref:`Example 4 : Manipulate Integer Array : Using character pointer pointing to an integer array <typecasting_n_ptr_sp_char_ptr_ex_4>`
                * :ref:`Example 5 : Swap two bytes in an Integer using character pointer <typecasting_n_ptr_sp_char_ptr_ex_5>`
                * :ref:`Example 6 : Swap two bytes in an Integer using character pointer : Bitwise logic <typecasting_n_ptr_sp_char_ptr_ex_6>`
                * :ref:`Example 7 : Swap two bytes in an Integer using character pointer : Bitwise logic <typecasting_n_ptr_sp_char_ptr_ex_7>`
                * :ref:`Example 8 : Swap two bytes in a Short Integer using character pointer : Bitwise logic <typecasting_n_ptr_sp_char_ptr_ex_8>`
                * :ref:`Example 9 : Swap two bytes in a Short Integer using character pointer : Bitwise logic <typecasting_n_ptr_sp_char_ptr_ex_9>`
                * :ref:`Example 10 : Integer pointer arithmetic changes after type conversion to Chracter pointer <typecasting_n_ptr_sp_char_ptr_ex_10>`
                * :ref:`Example 11 : Manipulate Heap : Compare heap access with char and int pointer <typecasting_n_ptr_sp_char_ptr_ex_11>`
                * :ref:`Summary <typecasting_n_ptr_sp_char_ptr_ex_12>`

.. _typecasting_n_ptr_sp_char_ptr_ex_1:

.. tab-set::

    .. tab-item:: Example 1 : Manipulate Integer : Using character pointer pointing to an integer

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		How to change content of an Integer using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 5, 7, 10

		#include <stdio.h>

		int main(void)
		{
		        unsigned int x = 0x11223344;

		        unsigned char *ptr;

		        // Step 1 : ptr is pointing to a block of 4 bytes
		        ptr = (unsigned char *) &x;

		        // Step 2 : ptr can access all 4 bytes for read
		        printf("\nPrint before change : Using character pointer\n");
		        for (int i = 0; i < sizeof(int); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nPrint before change : Using integer\n");
		        printf("x = %x\n", x);

		        // Step 3 : ptr can access all 4 bytes for write
		        ptr[0] = 0xaa;
		        ptr[1] = 0xbb;
		        ptr[2] = 0xcc;
		        ptr[3] = 0xdd;

		        printf("\nPrint after change : Using character pointer\n");
		        for (int i = 0; i < sizeof(int); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nPrint after change : Using integer\n");
		        printf("x = %x\n", x);

		        return 0;
		}



	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:
			:emphasize-lines: 1, 7, 10, 16

			Print before change : Using character pointer
			ptr[0] = 44
			ptr[1] = 33
			ptr[2] = 22
			ptr[3] = 11

			Print before change : Using integer
			x = 11223344

			Print after change : Using character pointer
			ptr[0] = aa
			ptr[1] = bb
			ptr[2] = cc
			ptr[3] = dd

			Print after change : Using integer
			x = ddccbbaa

.. _typecasting_n_ptr_sp_char_ptr_ex_2:

.. tab-set::

    .. tab-item:: Example 2 : Manipulate Short Integer : Using character pointer pointing to a short integer

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		How to change content of a Short Integer using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 5, 7, 10

		#include <stdio.h>

		int main(void)
		{
		        unsigned short int x = 0x1122;

		        unsigned char *ptr;

		        // Step 1 : ptr is pointing to a block of 2 bytes
		        ptr = (unsigned char *) &x; 

		        // Step 2 : ptr can access all 2 bytes for read
		        printf("\nPrint before change : Using character pointer\n");
		        for (int i = 0; i < sizeof(unsigned short int); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nPrint before change : Using integer\n");
		        printf("x = %x\n", x); 

		        // Step 3 : ptr can access all 2 bytes for write
		        ptr[0] = 0xaa;
		        ptr[1] = 0xbb;

		        printf("\nPrint after change : Using character pointer\n");
		        for (int i = 0; i < sizeof(unsigned short int); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nPrint after change : Using integer\n");
		        printf("x = %x\n", x); 

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:
			:emphasize-lines: 1, 8, 12

			Print before change : Using character pointer
			ptr[0] = 22
			ptr[1] = 11

			Print before change : Using integer
			x = 1122

			Print after change : Using character pointer
			ptr[0] = aa
			ptr[1] = bb

			Print after change : Using integer
			x = bbaa

.. _typecasting_n_ptr_sp_char_ptr_ex_3:

.. tab-set::

    .. tab-item:: Example 3 : Manipulate Structure : Using character pointer pointing to a structure

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		How to change content of a Structure using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 12, 14, 17

		#include <stdio.h>

		struct ABC
		{
		        int a;
		        int b;
		        int c;
		};

		int main(void)
		{
		        struct ABC x = { .a = 0x11223344, .b = 0x55667788, .c = 0x99aabbcc };

		        unsigned char *ptr;

		        // Step 1 : ptr is pointing to a block of 12 bytes
		        ptr = (unsigned char *) &x;

		        // Step 2 : ptr can access all 12 bytes for read
		        printf("\nPrint before change : Using character pointer\n");
		        for (int i = 0; i < sizeof(struct ABC); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nPrint before change : Using Structure\n");
		        printf("x.a = %x\n", x.a);
		        printf("x.b = %x\n", x.b);
		        printf("x.c = %x\n", x.c);

		        // Step 3 : ptr can access all 12 bytes for write

		        // Step 3.1 : Change x.a using ptr
		        ptr[0] = 0x11;
		        ptr[1] = 0x11;
		        ptr[2] = 0x11;
		        ptr[3] = 0x11;

		        // Step 3.2 : Change x.b using ptr
		        ptr[4] = 0x22;
		        ptr[5] = 0x22;
		        ptr[6] = 0x22;
		        ptr[7] = 0x22;

		        // Step 3.3 : Change x.c using ptr
		        ptr[8] = 0x33;
		        ptr[9] = 0x33;
		        ptr[10] = 0x33;
		        ptr[11] = 0x33;

		        printf("\nPrint after change : Using character pointer\n");
		        for (int i = 0; i < sizeof(struct ABC); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nPrint after change : Using Structure\n");
		        printf("x.a = %x\n", x.a);
		        printf("x.b = %x\n", x.b);
		        printf("x.c = %x\n", x.c);

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:
			:emphasize-lines: 1, 15, 20, 34

			Print before change : Using character pointer
			ptr[0] = 44
			ptr[1] = 33
			ptr[2] = 22
			ptr[3] = 11
			ptr[4] = 88
			ptr[5] = 77
			ptr[6] = 66
			ptr[7] = 55
			ptr[8] = cc
			ptr[9] = bb
			ptr[10] = aa
			ptr[11] = 99

			Print before change : Using Structure
			x.a = 11223344
			x.b = 55667788
			x.c = 99aabbcc

			Print after change : Using character pointer
			ptr[0] = 11
			ptr[1] = 11
			ptr[2] = 11
			ptr[3] = 11
			ptr[4] = 22
			ptr[5] = 22
			ptr[6] = 22
			ptr[7] = 22
			ptr[8] = 33
			ptr[9] = 33
			ptr[10] = 33
			ptr[11] = 33

			Print after change : Using Structure
			x.a = 11111111
			x.b = 22222222
			x.c = 33333333

.. _typecasting_n_ptr_sp_char_ptr_ex_4:

.. tab-set::

    .. tab-item:: Example 4 : Manipulate Integer Array : Using character pointer pointing to an integer array

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		:column: col-lg-12 p-2
		:card: shadow

		How to change content of an Integer array using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 5, 7, 10

		#include <stdio.h>

		int main(void)
		{
		        unsigned int x[] = {0x11111111, 0x22222222, 0x33333333, 0x44444444 };

		        unsigned char *ptr;

		        // Step 1 : ptr is pointing to a block of 16 bytes
		        ptr = (unsigned char *) x;

		        // Step 2 : ptr can access all 16 bytes for read
		        printf("\nBefore Change : Print using character pointer\n");
		        for (int i = 0; i < sizeof(x); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        // Step 3 : ptr can access all 16 bytes for write
		        for (int i = 0; i < sizeof(x); i++)
		        {
                		ptr[i] = 0xaa;
		        }

		        printf("\nAfter Change : Print using character pointer\n");
		        for (int i = 0; i < sizeof(x); i++)
		        {
                		printf("ptr[%d] = %x\n", i, ptr[i]);
		        }

		        printf("\nAfter Change : Print using integer array\n");
		        printf("x[0] = %x\n", x[0]);
		        printf("x[1] = %x\n", x[1]);
		        printf("x[2] = %x\n", x[2]);
		        printf("x[3] = %x\n", x[3]);

		        return 0;
		}


	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:
			:emphasize-lines: 1, 19, 37

			Before Change : Print using character pointer
			ptr[0] = 11
			ptr[1] = 11
			ptr[2] = 11
			ptr[3] = 11
			ptr[4] = 22
			ptr[5] = 22
			ptr[6] = 22
			ptr[7] = 22
			ptr[8] = 33
			ptr[9] = 33
			ptr[10] = 33
			ptr[11] = 33
			ptr[12] = 44
			ptr[13] = 44
			ptr[14] = 44
			ptr[15] = 44

			After Change : Print using character pointer
			ptr[0] = aa
			ptr[1] = aa
			ptr[2] = aa
			ptr[3] = aa
			ptr[4] = aa
			ptr[5] = aa
			ptr[6] = aa
			ptr[7] = aa
			ptr[8] = aa
			ptr[9] = aa
			ptr[10] = aa
			ptr[11] = aa
			ptr[12] = aa
			ptr[13] = aa
			ptr[14] = aa
			ptr[15] = aa

			After Change : Print using integer array
			x[0] = aaaaaaaa
			x[1] = aaaaaaaa
			x[2] = aaaaaaaa
			x[3] = aaaaaaaa

.. _typecasting_n_ptr_sp_char_ptr_ex_5:

.. tab-set::

    .. tab-item:: Example 5 : Swap two bytes in an Integer using character pointer

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		:column: col-lg-12 p-2
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		How to swap two bytes in an Integer using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 13

		/* Program to swap byte 0 and byte 3 of an integer */
		#include <stdio.h>

		int main(void)
		{
		        unsigned int x = 0x11223344;

		        unsigned char *ptr;

		        unsigned char temp;

		        // Step 1 : ptr is pointing to a block of 4 bytes
		        ptr = (unsigned char *) &x; 

		        printf("Before swap : x  = %x\n", x); 

		        temp = ptr[0];
		        ptr[0] = ptr[3];
		        ptr[3] = temp;

		        printf("After swap : x  = %x\n", x); 

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			Before swap : x  = 11223344
			After swap  : x  = 44223311

.. _typecasting_n_ptr_sp_char_ptr_ex_6:

.. tab-set::

    .. tab-item:: Example 6 : Swap two bytes in an Integer using character pointer : Bitwise logic

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		:column: col-lg-12 p-2
		:card: shadow

		How to swap two bytes in an Integer using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 11

		/* Program to swap byte 0 and byte 3 of an integer */
		#include <stdio.h>

		int main(void)
		{
		        unsigned int x = 0x11223344;

		        unsigned char *ptr;

		        // Step 1 : ptr is pointing to a block of 4 bytes
		        ptr = (unsigned char *) &x; 

		        printf("Before swap : x  = %x\n", x); 

		        ptr[0] ^= ptr[3];
		        ptr[3] ^= ptr[0];
		        ptr[0] ^= ptr[3];

		        printf("After swap : x  = %x\n", x); 

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			Before swap : x  = 11223344
			After swap  : x  = 44223311

.. _typecasting_n_ptr_sp_char_ptr_ex_7:

.. tab-set::

    .. tab-item:: Example 7 : Swap two bytes in an Integer using character pointer : Bitwise logic

	.. panels::
		:container: container pb-4
		:column: col-lg-12 p-2
		:card: shadow

		How to swap two bytes in an Integer using character pointer ?

	.. code-block:: c
		:linenos:

		/* Program to swap byte 0 and byte 3 of an integer */
		#include <stdio.h>

		int main(void)
		{
		        unsigned int x = 0x11223344;

		        printf("Before swap : x  = %x\n", x); 

		        ((unsigned char *) &x)[0] ^=  ((unsigned char *) &x)[3];
		        ((unsigned char *) &x)[3] ^=  ((unsigned char *) &x)[0];
		        ((unsigned char *) &x)[0] ^=  ((unsigned char *) &x)[3];

		        printf("After swap  : x  = %x\n", x); 

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			Before swap : x  = 11223344
			After swap  : x  = 44223311

.. _typecasting_n_ptr_sp_char_ptr_ex_8:

.. tab-set::

    .. tab-item:: Example 8 : Swap two bytes in a Short Integer using character pointer : Bitwise logic

	.. panels::
		:container: container pb-4
		:column: col-lg-12 p-2
		:card: shadow

		How to swap two bytes in a Short Integer using character pointer ?

	.. code-block:: c
		:linenos:
		:emphasize-lines: 11

		/* Program to swap byte 0 and byte 1 of an integer */
		#include <stdio.h>

		int main(void)
		{
		        unsigned short int x = 0x1122;

		        unsigned char *ptr;

		        // Step 1 : ptr is pointing to a block of 2 bytes
		        ptr = (unsigned char *) &x; 

		        printf("Before swap : x  = %x\n", x); 

		        ptr[0] ^= ptr[1];
		        ptr[1] ^= ptr[0];
		        ptr[0] ^= ptr[1];

		        printf("After swap  : x  = %x\n", x); 

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			Before swap : x  = 1122
			After swap  : x  = 2211

.. _typecasting_n_ptr_sp_char_ptr_ex_9:

.. tab-set::

    .. tab-item:: Example 9 : Swap two bytes in a Short Integer using character pointer : Bitwise logic

	.. panels::
		:container: container pb-4
		:column: col-lg-12 p-2
		:card: shadow

		How to swap two bytes in a Short Integer using character pointer ?

	.. code-block:: c
		:linenos:

		/* Program to swap byte 0 and byte 1 of an integer */
		#include <stdio.h>

		int main(void)
		{
		        unsigned short int x = 0x1122;

		        printf("Before swap : x  = %x\n", x); 

		        ((unsigned char *) &x)[0] ^=  ((unsigned char *) &x)[1];
		        ((unsigned char *) &x)[1] ^=  ((unsigned char *) &x)[0];
		        ((unsigned char *) &x)[0] ^=  ((unsigned char *) &x)[1];

		        printf("After swap  : x  = %x\n", x); 

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			Before swap : x  = 1122
			After swap  : x  = 2211

.. _typecasting_n_ptr_sp_char_ptr_ex_10:

.. tab-set::

    .. tab-item:: Example 10 : Integer pointer arithmetic changes after type conversion to Chracter pointer

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		:column: col-lg-12 p-2
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		Observe that Integer Pointer increments by 4 Bytes

	.. code-block:: c
		:linenos:
		:emphasize-lines: 9, 11

		#include <stdio.h>

		int main(void)
		{
		        unsigned int x[] = { 0x11223344, 0x55667788, 0x99aabbcc };

		        unsigned int *p; 

		        p = x; // p is pointing to x[0]

		        p++;   // p is pointing to x[1]

		        printf("*p = %x\n", *p);

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			*p = 55667788

	.. panels::
		:container: container pb-4
		:column: col-lg-12 p-2
		:card: shadow

		Observe that Integer Pointer is typecasted to Char Pointer and hence increments by 1 Byte

	.. code-block:: c
		:linenos:
		:emphasize-lines: 11, 13

		#include <stdio.h>

		int main(void)
		{
		        unsigned int x[] = { 0x11223344, 0x55667788, 0x99aabbcc };

		        unsigned int *p; 

		        unsigned char *cp;

		        p = x; // p is pointing to x[0]

		        cp = (unsigned char *)p + 1;   // cp is pointing second byte of x[0]

		        printf("*cp = %x\n", *cp);

		        return 0;
		}

	.. tab-set::

	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			*cp = 33

.. _typecasting_n_ptr_sp_char_ptr_ex_11:

.. tab-set::

	.. tab-item:: Example 11 : Manipulate Heap : Compare heap access with char and int pointer

		.. panels::
			:container: container pb-4
			:column: col-lg-12 p-2
			:card: shadow

			* Both pointers ip and cp are pointing to a single block of 40 bytes in heap
			* ip can index from 0 to 9
			* cp can index from 0 to 39

		.. code-block:: c
			:linenos:
			:emphasize-lines: 12, 16

			#include <stdio.h>
			#include <stdlib.h>

			int main(void)
			{
			        unsigned int *ip;

			        unsigned char *cp;

				// Allocate heap memory of 40 bytes
				// ip views this as 10 integers
			        ip = malloc(40);

				// cp is also pointing to memory allocated in previous line
				// cp views this as 40 characters
			        cp = (unsigned char *) ip; 

				// Change bytes 36, 37, 38, 39 using ip
			        ip[9] = 0x11223344;
			        printf("Changed using ip : ip[9] = %x\n", ip[9]);

				// Change bytes 36, 37, 38, 39 using cp
			        cp[36] = 0xaa;
			        cp[37] = 0xbb;
			        cp[38] = 0xcc;
			        cp[39] = 0xdd;
			        printf("Changed using cp : ip[9] = %x\n", ip[9]);

			        return 0;
			}

		.. tab-set::

			.. tab-item:: Output

				.. code-block:: c
					:linenos:

					Changed using ip : ip[9] = 11223344
					Changed using cp : ip[9] = ddccbbaa

.. _typecasting_n_ptr_sp_char_ptr_ex_12:

.. tab-set::

	.. tab-item:: Summary

		============================= ==============================================================================
		Learning                      Description
		============================= ==============================================================================
		ptr = (unsigned char \*) &x;  * Use ptr to manipulate variable x
					      * Where x can be short integer
					      * Where x can be integer
					      * Where x can be structure
					      * Where x can be union
		ptr = (unsigned char \*) x;   * Use ptr to manipulate array x
					      * Where x can be char array
					      * Where x can be short integer array
					      * Where x can be integer array
					      * Where x can be structure array
					      * Where x can be union array
		((unsigned char \*) &x)[3];   * Access Byte 3 of variable x
					      * Where x can be integer
					      * Where x can be structure (of size 4 bytes atleast)
					      * Where x can be union (of size 4 Bytes atleast)
		cp = (unsigned char \*)p + 1; * Access Byte 1 of memory pointed by p
					      * Where p can be an array of any type
					      * Where p can be a pointer of any type (pointing to memory of atleast 2 Bytes)
		============================= ==============================================================================

.. card:: See Also

        * Current Module

                * :doc:`typecasting_n_ptr`

        * Previous Module

                * :doc:`../malloc_ptr/malloc_ptr`

        * Next Module

                * :doc:`../funcs_n_ptrs/funcs_n_ptrs`

        * Other Modules

                * :doc:`../variable_and_ptr/variable_and_ptr`
                * :doc:`../array_n_ptrs/array_n_ptrs`
                * :doc:`../memcpy_ptr/memcpy_ptr`
                * :doc:`../const_ptr/const_ptr`
                * :doc:`../void_ptr/void_ptr`
                * :doc:`../array_of_ptr/array_of_ptr`
                * :doc:`../ptr_to_array/ptr_to_array`
                * :doc:`../function_ptr/function_ptr`
                * :doc:`../pre_incr_ptr/pre_incr_ptr`
                * :doc:`../post_incr_ptr/post_incr_ptr`
                * :doc:`../pre_decr_ptr/pre_decr_ptr`
                * :doc:`../post_decr_ptr/post_decr_ptr`