Q: My vendor provides header files that #define NULL as 0L. Why?
A: Some programs carelessly attempt to generate null pointers by using the NULL macro, without casts, in non-pointer contexts. (Doing so is not guaranteed to work; see questions 5.2 and 5.11.) On machines which have pointers larger than integers (such as PC compatibles in ``large'' model; see also question 5.17), a particular definition of NULL such as 0L can help these incorrect programs to work. (0L is a perfectly valid definition of NULL; it is an ``integral constant expression with value 0.'') Whether it is wise to coddle incorrect programs is debatable; see also question 5.6 and section 17.
References:
Rationale Sec. 4.1.5
H&S Sec. 5.3.2 pp. 121-2
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