Pointer arithmetic question

[Joe Wright]

Keith Thompson wrote:

[ too much snippage ]

Undefined behavior includes cases where the standard fails to
state what the behavior is.

This is too perfect for further comment. Go Keith!

 

[Keith Thompson]

Keith Thompson wrote:

[ too much snippage ]

Undefined behavior includes cases where the standard fails to
state what the behavior is.

This is too perfect for further comment. Go Keith!

Thanks -- but I'm going to comment further myself.

I was summarizing C99 4p2, which says:

If a "shall" or "shall not" requirement that appears outside of a
constraint is violated, the behavior is undefined. Undefined
behavior is otherwise indicated in this International Standard by
the words "undefined behavior" or by the omission of any explicit
definition of behavior. There is no difference in emphasis among
these three; they all describe "behavior that is undefined".

 

[Jim]

Actually, adding 0 to a null pointer invokes undefined behavior:

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.

It's likely to quietly yield a null pointer on most implementations,
but the standard doesn't require it.

I would say that the behaviour is defined for NULL+0. The standard
(C99 6.3.2.3 3) says any constant integer expression == 0 is
equivalent to a NULL pointer.

Jim

 

[Keith Thompson]

I would say that the behaviour is defined for NULL+0. The standard
(C99 6.3.2.3 3) says any constant integer expression == 0 is
equivalent to a NULL pointer.

NULL+0 is a null pointer constant only if NULL is an integer constant
expression. But we were discussing null pointers, not null pointer
constants.

Given:
int *ptr = NULL; /* or 0, or '-'-'-' */
the expression
ptr + 0
invokes undefined behavior.