>>     I thought NULL was just a constant zero, not implicitly or
 >> explicitly a pointer value. Correct me if I am wrong, if I make below
 >> statements {...}

They will, but the preferred method is to use NULL for pointers and 'real' zero
for variables.  There were a couple of compilers at one time that had NULL
equal to -1 (I think ... but then again ...)

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NULL (not necessarily the macro by the same name) is a pointer.  Any use
of a constant 'zero' in a pointer context must generate a NULL pointer.
Because of this, some implementations define NULL as '0', when they
should more properly use '(void *)0'.  The latter definition aids the
compiler in generating appropriate warning messages.  Still, while a
zero generates a NULL, it is not advisable to assume that NULL will
generate a zero.

*May*, not will.  (Although they usually will).

NULL is always represented as (some kind of) constant 0.  The actual bit
pattern for a NULL need not be all zeroes, and both all ones and some
other patterns are used on various machines.

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As Arjan correctly states NULL is allowed to be defined in the headers as
((void *) 0). in general the result of converting this quantity back to an
arithmentic type is undefined.

NULL has always been 0 or 0 converted to a suitable type as far as source
code goes. A compiler is allowed to use any value it likes to represent
NULL behind the scenes but it has to perform suitable conversions so that
no ANSI conforming (its difficult to specify the required limitations
on non-ANSI) code can tell this.

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Strictly it is just as proper to define NULL as 0 as to define it as
((void *) 0), however code should never make any assumptions about which it
is.

In a few situations but I don't believe it makes much difference in
practice.

Indeed it is quite wrong to do so.

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