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Although the gets function generally returns
a pointer to the first character in the character array, should you
redirect input to a file, and should the gets function encounter
the end of a file, it returns the null pointer, which is a pointer
to address zero.
You frequently see the null pointer written as NULL, a macro
symbol. By using NULL, instead of 0, you identify those
places where 0 is used as a special pointer, thereby increasing
program clarity.
Thus, the following expression tests the result of a call to gets to
ensure that the end of a file has not yet been encountered:
gets (input_buffer) != NULL