What is Double Pointer (Pointer to Pointer)?
As we know that, a  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable is used to store the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable of a variable in C.  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable reduces the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable time of a variable. However, In C, we can also define a pointer to  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable the address of another  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable. Such  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable is known as a  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable pointer (pointer to pointer). The first pointer is used to  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable the address of a  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable whereas the second  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable is used to store the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable of the first pointer. Let's understand it by the diagram given below.

Define Pointer Arithmetic in C?
We can perform arithmetic  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable on the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable like addition, subtraction, etc. However, as we know that  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable contains the address, the result of an  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable operation performed on the pointer will also be a pointer if the other operand is of type integer. In pointer-from- accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable, the result will be an  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable value. Following  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable operations are possible on the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable in C language:
Increment, Decrement, Addition, Subtraction, Comparison.

Define Dangling pointer ?
 accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable pointer occurs at the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable of the object destruction when the object is  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable or de-allocated from memory without modifying the value of the pointer. In this case, the pointer is pointing to the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable, which is de- accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable. The dangling pointer can point to the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable, which contains either the program code or the code of the operating  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable. If we assign the value to this  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable, then it overwrites the value of the program code or  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable system instructions; in such cases, the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable will show the undesirable result or may even crash. If the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable is re-allocated to some other process, then we dereference the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable pointer will cause the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable faults.

How can you Avoid Dangling pointer error?
The  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable pointer errors can be  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable by initializing the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable to the NULL value. If we assign the NULL value to the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable, then the pointer will  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable point to the de-allocated  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable. Assigning  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable value to the pointer means that the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable is not pointing to any memory location.

What is a Constant Pointers?
A  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable pointer in C cannot change the address of the  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable to which it is pointing, i.e., the address will remain  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable. Therefore, we can say that if a constant  accessaddressallocatedarithmeticavoidedconstantDanglingdeleteddoubleintegermemorynotNULLoperatingoperationspointerpointersprogramsegmentationstoresubtractionsystemtimevariable is pointing to some variable, then it cannot point to any other variable