Type-casting with Structure Pointer
====================================

* In this section, you are going to learn

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	How to extract formatted data from structure pointer pointing to character array ?

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         Topics in this section,

                * :ref:`Extract formatted data from character array using Structure Pointer <typecasting_n_ptr_sp_struct_ptr_ex_1>`
                * :ref:`Summary <typecasting_n_ptr_sp_struct_ptr_ex_2>`


.. _typecasting_n_ptr_sp_struct_ptr_ex_1:

.. tab-set::

    .. tab-item:: Extract formatted data from character array using Structure Pointer

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		How to extract formatted data from raw hex dump stored in char array ?

		* Step 1 : Define the desired structure

		* Step 2 : Define a structure pointer

		* Step 3 : Point structure pointer to start of char array

		* Step 4 : Now access data using structure members

	.. code-block:: c
		:linenos:
		:emphasize-lines: 20

		#include <stdio.h>

		// Step 1 : Define the desired structure
		struct ABC {
		        unsigned int a;
		        unsigned int b;
		        unsigned int c;
		        unsigned int d;
		};

		char data[16] = { 0xfe, 0x12, 0x45, 0x67,
                		  0x92, 0x90, 0x23, 0x11,
		                  0x22, 0x83, 0x81, 0xcc,
                		  0x37, 0xbe, 0xff, 0xad
		                };  

		int main(void)
		{
			// Step 2 : Define a structure pointer
		        struct ABC *sp;


			// Step 3 : Point structure pointer to start of char array
		        sp = (struct ABC *) data;

			// Step 4 : Now access data using structure members
		        printf("sp->a = %x\n", sp->a);
		        printf("sp->b = %x\n", sp->b);
		        printf("sp->c = %x\n", sp->c);
		        printf("sp->d = %x\n", sp->d);

		        return 0;
		}

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	    .. tab-item:: Output

		.. code-block:: c
			:linenos:

			sp->a = 674512fe
			sp->b = 11239092
			sp->c = cc818322
			sp->d = adffbe37


	* Now, let us suppose we want to split ``a`` into two fields

		* First  field is of 4 bits with value of ``0xe``

		* Second field is of 28 bits with value of ``0x674512f``

	* We can extract using bitwise operations like below

	.. code-block:: c
		:linenos:

	        f1 = sp->a & 0xf;

       		f2 = (sp->a >> 4);

	* Instead of performing bitwise operations we could redefine the structure with bitfields

	.. code-block:: c
		:linenos:

		struct ABC {
	        	unsigned int f1 : 4;
		        unsigned int f2 : 28; 
		        unsigned int b;
		        unsigned int c;
	        	unsigned int d;
		};

		char data[16] = { 0xfe, 0x12, 0x45, 0x67,
		                  0x92, 0x90, 0x23, 0x11,
		                  0x22, 0x83, 0x81, 0xcc,
		                  0x37, 0xbe, 0xff, 0xad
		                };

		int main(void)
		{
		        struct ABC *sp;

		        sp = (struct ABC *) data;
    
		        printf("sp->f1 = %x\n", sp->f1);
		        printf("sp->f2  = %x\n", sp->f2);
		        printf("sp->b = %x\n", sp->b);
		        printf("sp->c = %x\n", sp->c);
		        printf("sp->d = %x\n", sp->d);

		        return 0;
		}


	* Now, what if we want to extract bits 21 to 27 (inclusive of 21 and 27) ?
		
	.. code-block:: c
		:linenos:

		f3 = (sp->a & 0x0fd00000) >> 21; // is equal to 0x3a

	* Instead of performing bitwise operations we could redefine the structure with bitfields again !

	.. code-block:: c
		:linenos:

		#include <stdio.h>

		struct ABC {
		        // bits 0 to 20 inclusive
		        // Total number of bits in 0..20 is 21
		        unsigned int f1 : 21;

		        // bits 21 to 27 inclusive
		        // Total number of bits in 21..27 is 7
		        unsigned int f2 : 7;

		        // bits 28 to 31 inclusive
		        // Total number of bits in 28..31 is 4
		        unsigned int f3 : 4;
		        unsigned int b;
		        unsigned int c;
		        unsigned int d;
		};

		char data[16] = { 0xfe, 0x12, 0x45, 0x67,
                		  0x92, 0x90, 0x23, 0x11,
		                  0x22, 0x83, 0x81, 0xcc,
        		          0x37, 0xbe, 0xff, 0xad
                		};

		int main(void)
		{
		        struct ABC *sp;

		        sp = (struct ABC *) data;

		        printf("sp->f1 = %x\n", sp->f1);
		        printf("sp->f2 = %x\n", sp->f2);
		        printf("sp->f3 = %x\n", sp->f3);
		        printf("sp->b  = %x\n", sp->b);
		        printf("sp->c  = %x\n", sp->c);
		        printf("sp->d  = %x\n", sp->d);

		        return 0;
		}

	.. code-block:: c
		:linenos:

		sp->f1 = 512fe
		sp->f2 = 3a
		sp->f3 = 6
		sp->b  = 11239092
		sp->c  = cc818322
		sp->d  = adffbe37

.. _typecasting_n_ptr_sp_struct_ptr_ex_2:

.. tab-set::

	.. tab-item:: Summary

		============================= ==============================================================================
		Learning                      Description
		============================= ==============================================================================
		============================= ==============================================================================

.. 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`