Bound checking and integer overflow

Question

The problem is very simple: given start_index and count, I want to see if the combinations can be used to safely accessed an array with length elements. What I have for the time being is the following:

uint32_t start_index = (value from somewhere);
uint32_t count = (value from somewhere);
uint32_t length = (value set earlier);
char *array = (memory allocated earlier);

if(start_index + count < length) {
    // access array starting at start_index
} else {
    // bailout
}

The check is, of course, inadequate since start_index + count can exceed the maximum possible value for an uint32_t and wrap around to a small value. To fix this, I wonder if it's more efficient to promote the variables to 64 bit or put in a second condition start_index + count > start_index. Or perhaps there's some other clever way to handle this?

There is a long list of methods to detect integer overflows before or after the fact here. — Richard
– Richard, Commented Feb 8, 2013 at 5:41

Mat · Accepted Answer · 2013-02-08 05:12:35Z

2

You can avoid overflows by doing things a bit differently: first check that count is smaller than length (bail out otherwise), then you can safely compare start_index with length - count.

answered Feb 8, 2013 at 5:12

Mat

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2 Comments

Alexey Frunze Over a year ago

2 people don't see an off by one error here. Come on, count smaller than OR EQUAL length.

Mat Over a year ago

If length == count, the original code (assuming no overflow issues) would bail out. My suggestion would bail out too. So I don't believe there is an OBOE here.

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