Some Interview Questions and Answers for fresher level on Pointers

What is a void pointer?

Void pointer is a special type of pointer which can reference or point to any data type.  This is why it is also called as Generic Pointer.

As a void pointer can point to any data type, neither it is possible to dereference a void pointer, nor is it possible to do any arithmetic operations on void pointer. Hence an explicit type casting is absolutely must for using a void pointer.

The best use of void pointer can be exhibited in the case of malloc and calloc. This is discussed in details in this post.

What is a null pointer?

A null pointer is also a special type of pointer which points definitively to nothing.

As per official C language description, every valid type of pointer has a special value called “Null Pointer” and this value is always distinguishable from all other pointer values and it is guaranteed to compare unequal to a pointer of any object or function.  

Hold it for a second! So what you are saying is there are different versions of null pointers for every pointer type. Is it so?

No. We cannot call it as a “different versions” rather their internal values are different. But as a programmer, we need not to worry about it because compilers take care of and track the internal values (may be differently depending on the compiler types)

So why don’t you say that a null pointer is a pointer which is not initialized yet?

I cannot go with this idea because an uninitialized pointer may point to any thing. There might be an uninitialized char pointer, an uninitialized int pointer and so on. But a null pointer is definitely something which is there not to point anything.

Ok. Now tell me what is a dangling pointer?

Dangling pointers are the pointers which point to already de allocated or de referenced memory locations. Suppose I write a piece of code like this

int*p;

p = (int*)malloc(n*sizeof(int);

----

----

free(p);

In this piece of code, after allocating some memory to p, we are de allocating the memory which was initially pointed by p. Hence now onwards, p acts as dangling pointer.

How to rectify this?

In this case, after de allocation that is after using free() we should set the pointer p to null as p = NULL   

Could you please exhibit any other source of dangling pointer problems?

Well, there might be other sources also, like

Suppose I have declared int*p, and within a specific scope, I have defined another integer x as 10; and then within that specific scope I am making the pointer p to hold the base address of x. Now outside this specific scope the p will act as a dangling pointer. I can write the code as

void foo (void)

{

   int *p;

   {

          {

int x = 10;

                 p = &x;

          }

          // here p will act as dangling pointer

   }

}

Another case might occur when from a function, we return an address of any variable and a corresponding pointer in the receiver section receives it, then the receiving pointer might get into dangling pointer problem, as just after the returning, as the variable has passed it scope, the memory stack (where the variable is stored currently) might be refreshed. But this phenomenon is not obvious, this might happen.

Fine, so how to deal with this second case?

To deal with this, to be in safer side, we must have the scope of the variable intact out of the function call; declaring it as static might be an option.