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If a variable contains NULL value and if you try to do some operation on it. it will throw a null pointer assignment error
We said that the value of a pointer variable is a pointer to some other variable. There is one other value a pointer may have: it may be set to a null pointer. A null pointer is a special pointer value that is known not to point anywhere. What this means that no other valid pointer, to any other variable or array cell or anything else, will ever compare equal to a null pointer. #include
int *ip = NULL;
In general, C does not initialize pointers to null for you, and it never tests pointers to see if they are null before using them. If one of the pointers in your programs points somewhere some of the time but not all of the time, an excellent convention to use is to set it to a null pointer when it doesn't point anywhere valid, and to test to see if it's a null pointer before using it. But you must use explicit code to set it to NULL, and to test it against NULL. (In other words, just setting an unused pointer variable to NULL doesn't guarantee safety; you also have to check for the null value before using the pointer.) On the other hand, if you know that a particular pointer variable is always valid, you don't have to insert a paranoid test against NULL before using it.