stdlib.h should define NULL -- Ansi C, Section 7.10.

include/stdlib.h

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	from: @(#)stdlib.h	5.13 (Berkeley) 6/4/91
- *	$Id: stdlib.h,v 1.9 1993/10/22 17:52:28 jtc Exp $
+ *	$Id: stdlib.h,v 1.10 1993/11/13 01:44:59 jtc Exp $
  */
 
 #ifndef _STDLIB_H_
@@ -57,6 +57,10 @@ typedef struct {
 	long rem;		/* remainder */
 } ldiv_t;
 
+#ifndef	NULL
+#define	NULL	0
+#endif
+
 #define	EXIT_FAILURE	1
 #define	EXIT_SUCCESS	0