What is the use of a pointer variable in c programme. State applications of pointers in c?

Answer 1

Pointer variables are no different to any other variable. The only difference is that value we store in a pointer variable is a memory address, a reference. Although it is possible to use references alone, there is no guarantee that a reference will always be valid. For instance, if we attempt to allocate a large amount of memory and there isn't a large enough block of free memory to meet that allocation, then we cannot return a reference. References must always refer to something valid. Thus we use a pointer variable instead: if the allocation succeeds, the pointer will contain a reference, otherwise it will contain the value 0 (NULL). Once we know for sure the memory was allocated successfully, we can reference the memory through the pointer.

Although dynamic memory allocation is the main reason for having pointer variables, we can also use pointer variables to point at any valid memory, and refer to that memory in different ways. For instance, if we point to an int variable's reference using a char pointer, then the char pointer can refer to that memory as if it were really just a 1 byte character, rather than a 4 byte integer (just as if we'd cast the int to a char), but we can also increment or decrement the pointer to access any of the four bytes within the integer, just as if it were a 4 element array of type char.

Pointers are also useful in linked structures such as linked lists and trees. Each element in the structure may be allocated non-contiguously and in any sequence (as opposed to an array which always allocates elements contiguously). However we can easily change the order of the elements simply by changing the pointer variables within each element. In an array we have to physically move the elements around, but with a linked structure we only need to update the links to change the order, where each element points to the next element in the sequence, or the previous element, or a parent element or one or more child elements, or any combination as befits the structure. Thus highly complex networks can be generated.

Although working with references is much easier than working with pointer variables, the fact a pointer can be NULL or can point to any reference makes it that much more powerful than a reference. References cannot be changed once instantiated, but pointer variables can. Thus pointers can "walk" through references.

Since functions consume memory just as variables consume memory, pointers can indirectly refer to functions, making it possible to pass functions into functions just as easily as passing variables into functions.

And since pointers are variables with memory allocations of their own, pointers can refer to other pointers. This makes it possible to allocate complex, multi-dimensional arrays in non-contiguous memory, using arrays of pointers to pointers. Although this consumes more memory than a contiguous allocation would, the fact the allocation is broken up into a series of smaller allocations means those allocations will have a far greater chance of being allocated than a single allocation would.