• To know where exactly the value of a variable is stored in the memory, you need to be aware of the concept called pointers.
• A pointer is a powerful construct of the C programming language.
• A pointer is a variable which holds the address of other variables such as arrays, structures and functions that are used in a program.
• It contains only the memory location of the variable rather than its content.

Advantages

• To point to different data structures.
• Manipulation of data at different memory locations is easier.
• To achieve clarity and simplicity.
• More compact and efficient coding.
• To return multiple values via functions.
• Dynamic Memory Allocation.

Operations used with pointers

• The address operator – &(ampersand)
• The indirection operator – *(asterix)
• An address operator gives the address of a variable(not constants or expression); while the indirection operator gives the value of the variable that the pointer is pointing to.
• Both the pointer variable and the variable to which pointer points to, must be declared.
• Also, a pointer variable must be assigned the address of the declared variable before it is used in the program.

Pointer Dereferencing

• This is a process of referencing the contents of the variables using pointers, indirectly.
• Steps involved - Address of a variable, whose value is to be referenced is assigned to a pointer. Then, this pointer points to the value.

Operations on Pointers

• A pointer variable holds the address of the other variables. This address can be incremented or decremented.
• However, the pointer variables cannot be multiplied or divided.
• A pointer variable can be assigned the address of other variable.
• If two pointer variables are pointing to the object of the same data type, then they can be assigned.
• A pointer variable can be assigned a NULL value(NULL = 0).
• An integer value can be added to and subtracted from a pointer variable.
• One pointer variable can be subtracted from another pointer variable, if both are pointing to the elements of the same array.
• If two pointer variables are pointing to the objects of the same data type, then they can be compared with one another.

Pointer Initialization

• As ordinary variables are initialized within the declaration part of a program, the pointer variables can also be initialized by assigning the addresses of another variables that are used in the program.
• data_type *ptr_var = expression;
• A pointer can also be initialized by assigning a NULL value.

Pointers and Functions

• Like other parameters, pointers can also be passed to functions.
• This is carried out by the call-by-reference parameter passing mechanism.

Call-by-Reference

• This is a mechanism by which the pointers can be passed as arguements to the functions.
• Thus, the data items of the calling program can be assessed by the called program(function).
• No value is copied when the pointers are passed as arguements, as in the call-by-value method.
• Another important point is that if the values are changed in the functions, this will modify the original contents of the original parameters.
• In the calling program, the function is invoked with the function name and addresses of the actual parameters enclosed within the parantheses.
• function_name(&var1,&var2,…&varn);
• In the parameter list of the called program each and every formal parameter(pointers) must be preceded by an indirection operator(*).
• data_type function_name(*var1,*var2,…*varn);

Pointers and Arrays

• Pointers can be used with arrays for efficient programming.
• One-dimensional Arrays
• The name of an array itself designates some memory location and that location in main memory is the address of the very first element of the array.
• &aarray[0] or simply array.
• The addressof the i-th element of the array - &array[i-1] or (array+(i-i))

Dynamic Memory Allocation

• This is a process of dynamically allocating memory to any variable that is being used in your C program.
• When the program is terminated, the dynamically allocated memory is deallocated.
• malloc(), calloc(), free().
• malloc() – This function is used to allocate the memory in bytes.
• malloc(nob);
• free() – This function frees a block of allocated memroy.
• free(p);

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