"do ... while ((NULL + 1) - 1);" -- valid C?

[Chris Torek]

>>                while ((s = strchr(s, ',') + 1) - 1)

In article <826 at ruso.UUCP> wolfgang at ruso.UUCP (Wolfgang Deifel) writes:
>Why should NULL + 1 not be valid ??? NULL is a pointer with the value 0
>and you can add the integer 1 to it ....

NULL is not a pointer with the value 0, and 1 is not being added to
NULL here, but rather to a nil-pointer-to-char in the case in question.

NULL is a preprocessor macro; it expands to either an integral constant
zero (whose type is one of the integral types, e.g., int or short or long,
and whose value is zero) or to such a value cast to pointer-to-void
(whose type is pointer-to-void and whose value is unknowable).

A nil-pointer-to-char has type pointer-to-char and an ineffable value.
There is no way to talk about its value other than to say `it is a
nil pointer to char'.  In particular, you cannot say what happens
when you add one to it.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris