Thread: A pointer problem from a high school programming contest

I'm trying to follow this discussion, but I'm having a little difficulty understanding what it is that is "allowed" or "not allowed" by the standard. Are we saying that the following code could actually cause a meltdown?

Code:

int *p = (int *)rand();

Interesting.

As long as p isn't dereferenced, what would be the logic in disallowing a pointer that points to something invalid as long as it is never used (ie. dereferenced)?

Originally Posted by laserlight

That's correct. None of the four functions (main() excluded) has any net effect, assuming that this invalid pointer thing also has no net effect.

Some compilers will even optimize out things that have no effect, so they may not ever even get called.


Quzah.

Originally Posted by brewbuck

There is no logic in it. It seems like the designers of the C and C++ standards wanted to forge a language that could conceivably run anywhere, including architectures which are deliberately obtusely designed. I refuse to submit to that kind of stupidity.

It seems quite silly, especially considering the fact that no memory accesses are actually performed prior to dereferencing a pointer. How would this be enforced? Would the O/S or hardware actually keep records of all pointers inside a program handy in a table? LOL. That would seem to be such a waste of resources that it makes me sick to even think of it.

You're right when you said that such a system would catch a memory violation when it would occur. That's how a system should be protected imo. I don't understand the motivation to increase the protection anymore.

Anyway, does anyone know of a system that actually performs such checking?

Originally Posted by James Dow Allen

CBFalconer wrote:
> /* Making fatal hidden assumptions */
> /* Paul Hsiehs version of strlen.


I haven't read your entire treatise, and I surely wouldn't want to
defend
Mr. Hsieh (whose code, he's led us to understand, was written by his
dog)
but I do have some comments.


> #define SW (sizeof (int) / sizeof (char))


The fact that sizeof(char)==1 doesn't make this line bad *if* the
programmer feels that viewing SW as chars per int is important.
I don't defend this particular silliness, but using an expression for
a simple constant is often good if it makes the constant
self-documenting.


> Some sneaky hidden assumptions here:
> 1. p = s - 1 is valid. Not guaranteed. Careless coding.


Mr. Hsieh immediately does p++ and his code will be correct if then
p == s. I don't question Chuck's argument, or whether the C standard
allows the C compiler to trash the hard disk when it sees p=s-1,
but I'm sincerely curious whether anyone knows of an *actual*
environment
where p == s will ever be false after (p = s-1; p++).


Many of us fell in love with C because, in practice, it is so much
simpler
and more deterministic than many languages. Many of the discussions in
comp.lang.c seem like they'd be better in a new newsgroup:
comp.lang.i'd_rather_be_a_lawyer


:-) :-)


James Dow Allen

Thanks for the link.

ROFL, I love this post.