GCC: program doesn't work with compilation option -O3

Question

I\'m writing a C++ program that doesn\'t work (I get a segmentation fault) when I compile it with optimizations (options -O1, -O2, -O3, etc.), but it works just fine when I

Answer 1

Here's some code that seems to work, until you hit -O3...

#include 

int main()
{
    int i = 0, j = 1, k = 2;
    printf("%d %d %d\n", *(&j-1), *(&j), *(&j+1));
    return 0;
}

Without optimisations, I get "2 1 0"; with optimisations I get "40 1 2293680". Why? Because i and k got optimised out!

But I was taking the address of j and going out of the memory region allocated to j. That's not allowed by the standard. It's most likely that your problem is caused by a similar deviation from the standard.

I find valgrind is often helpful at times like these.

EDIT: Some commenters are under the impression that the standard allows arbitrary pointer arithmetic. It does not. Remember that some architectures have funny addressing schemes, alignment may be important, and you may get problems if you overflow certain registers!

The words of the [draft] standard, on adding/subtracting an integer to/from a pointer (emphasis added):

"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."

Seeing as &j doesn't even point to an array object, &j-1 and &j+1 can hardly point to part of the same array object. So simply evaluating &j+1 (let alone dereferencing it) is undefined behaviour.

On x86 we can be pretty confident that adding one to a pointer is fairly safe and just takes us to the next memory location. In the code above, the problem occurs when we make assumptions about what that memory contains, which of course the standard doesn't go near.