As a matter of style, many programmers prefer not to have unadorned 0's scattered through their programs. Therefore, the preprocessor macro NULL is #defined (by or ) with the value 0, possibly cast to (void *)
A programmer who wishes to make explicit the distinction between 0 the integer and 0 the null pointer constant can then use NULL whenever a null pointer is required.
Using NULL is a stylistic convention only; the preprocessor turns NULL back into 0 which is then recognized by the compiler, in pointer contexts.

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