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What is a null pointer?

There are times when it’s necessary to have a pointer that doesn’t
 point to anything. The macro NULL, defined in , has a value that’s
 guaranteed to be different from any valid pointer. NULL is a literal zero,
 possibly cast to void* or char*. Some people, notably C++
 programmers, prefer to use 0 rather than NULL.
 The null pointer is used in three ways:
 1) To stop indirection in a recursive data structure
 2) As an error value
 3) As a sentinel value