🚀 go-pugleaf

darcy@druid.uucp (D'Arcy J.M. Cain) writes:
>In fact I don't think the standard even forbids "fn2(foo[-1000])"
>although your OS may complain (core dump) when you reference the data.

It is "forbidden":
In the section on Additive Operators, discussing the Semantics of pointer
arithmetic:  "If both the pointer operand and the result point to elements
of the same array object, or one past the last element of the array object,
the evaluation shall not produce an overflow; otherwise, the behavior is
undefined."

>>    } foo [10000];
>>    #define bar (foo - 8000)
>>        fn2(bar[9000]);		/* <---- */
>Chapter 3.3.2.1 says that E1[E2] is identical to "(*(E1+(E2)))."  After
>expanding macros the above becomes "(foo - 8000)[9000]" which is
>identical to "(*((foo - 8000)+(9000)))."  Removing superfluous parens
>that gives "*(foo - 8000 + 9000)" or "*(foo + 1000)" which looks
>perfectly legitimate to me.

The C abstract machine must perform the calculations as written in the order
specified by the syntax (3.3 Expressions, see footnote 35).  Implementations
are not obligated to simplify expressions when they can.  This expression
asks for the calculation of "foo - 8000" as an intermediate result, which is
illegal.