Thread: accessing outside of an array?

Hello everybody!

I have a quick question to ask: What happens when you access outside of an array?

I was reading my programming book today and saw this.

"If you were to declare an array size of [4] but request array[5] the operation proceeds at the memory location where array[5] would be if it existed. The result is a piece of data outside the array is overwritten which can create bugs that are very hard to track down."

Does this mean it only causes a bug inside the program and doesn't do any permanent damage to my computer? Such as "Hey! Array[5] would be located right where kernel32 is! *overwrite kernel*" or "Array[5]...nope don't see it...Might as well place it where the video card driver is located..."

I made a typo in an example program and it tried to access outside of an array but it ended up just saying "Card dealer#3 has stopped working searching for solution..."

Just wondering if thats all it does or if my computer is about to explode or something.

Anyways, Thank you to anyone who takes the time to answer my question!
I really appreciate it!

Alright thanks!

I was a little afraid that my computer might have been about to spontaneously combust or something else that would have been very bad.

It's undefined behavior. People often joke that undefined behavior means anything can happen, but in reality, modern OSs do a pretty good job of protecting the rest of the memory space from bad programs.

However, these kinds of mistakes are security vulnerabilities that can leave your program open to malicious attacks that would cause real harm.

Originally Posted by avaldi

Does this mean it only causes a bug inside the program and doesn't do any permanent damage to my computer? Such as "Hey! Array[5] would be located right where kernel32 is! *overwrite kernel*" or "Array[5]...nope don't see it...Might as well place it where the video card driver is located..."

As it has already been stated, nowadays this isn't likely to happen at all. That's not to say that crazy stuff *won't* happen, but your not going to cause permanent damage anymore. At least, not to hardware. Security risks and data corruption are still real problems. IIRC, this error was one of the primary means of attack when I used to follow such things.

That didn't used to be the case though. It's funny you spoke of the video card driver, because on the earlier IBM PC you could very well cause catastrophic damage if you were tinkering with the video and set the scan rate wrong by mistake. If you wrote to Bad Places(tm) through wild pointers or something, you could set the video chip scan rate to zero and the transformer that produced the voltage which drove the electrons to light up the phosphors on the monitor would be fed a constant voltage (not modulating to reduce heat). This, in turn, would make the transformer act as a resistor which would heat up considerably and burn the monitor out instantly. This did happen.

On modern PC's, this type of stuff is a thing of the past. On other systems (particularly embedded) it is still real, but not common. IOW, don't sweat it when you are learning on your computer. You are going to make this mistake more than once and the OS will save you by catching a bad access. But, that's no license to get sloppy either.

I'm just happy I don't have to worry about my computer imploding if I accessed outside of an array. The information here is pretty amazing, I really have to respect the amount of knowledge that everyone here has.