> So, what would be your inference in the case of architectures where
> the null pointer is not designated by 0(zero) (lot of them are
> mentioned there on the C-faq page). What would then ((struct a *)0L)-
> field refer to and how will the compiler know that
(null-pointer)->field is required to evaluate the offset of field in
the
> structure "a" ?
You seem to have ignored parts of the FAQ that answer your question.
I quote from the answer to question 5.2:
According to the language definition, a constant 0 in a pointer
context is converted into a null pointer at compile time. That
is, in an initialization, assignment, or comparison when one
side is a variable or expression of pointer type, the compiler
can tell that a constant 0 on the other side requests a null
pointer, and generate the correctly-typed null pointer value.
and from 5.3:
Substituting the trivial pointer expression "p" for "expr", we
have
if(p) is equivalent to if(p != 0)
and this is a comparison context, so the compiler can tell that
the (implicit) 0 is actually a null pointer constant, and use
the correct null pointer value. There is no trickery involved
here; compilers do work this way, and generate identical code
for both constructs. The internal representation of a null
pointer does *not* matter.
And if that doesn't settle the argument, here is one more:
5.10: But wouldn't it be better to use NULL (rather than 0), in case
the value of NULL changes, perhaps on a machine with nonzero
internal null pointers?
A: No. (Using NULL may be preferable, but not for this reason.)
Although symbolic constants are often used in place of numbers
because the numbers might change, this is *not* the reason that
NULL is used in place of 0. Once again, the language guarantees
that source-code 0's (in pointer contexts) generate null
pointers. NULL is used only as a stylistic convention. See
questions 5.5 and 9.2.
Also, of all the architectures mentioned in the FAQ that use non-zero
values for null pointers, how many have Linux ports??
-Ravi.
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