[cwzwarich]

The C standard basically only guarantees that pointer arithmetic works when the pointers involved all point to the same array object (it also allows for a pointer just off the end of an array object). Other pointer arithmetic or comparison is undefined behavior and an implementation can do whatever it pleases.

[mahmud]

So I implemented this stricter definition of C pointers and it's neither interesting, nor representative of Real World uses that I know of. Need to investigate a bit more.

[dkersten]

stricter definition of C pointers - this made me laugh, as the less-strict definition of C pointers is called undefined behavior.

This leads me to this statement based on what you said: interesting Real World C programs make use of undefined behavior

[Arelius]

> interesting Real World C programs make use of undefined behavior

I assume you've worked with a significant amount of real world C programs? Because they surprisingly often do. The difficulty with porting many programs to 64-bit for instance, is due to relying on implementation defined behavior.

[dkersten]

I assume you've worked with a significant amount of real world C programs?

Only embedded systems (AVR & PIC24). I have much more experience in C++, which I've used for both Desktop apps and telco server components.

The beauty of C (over C++) is that the standard is actually readable. C++ especially is a quagmire of undefined behavior. The scary thing about C/C++ is that its easy to hit undefined (or, at least, as you state, implementation defined) behavior and not even realize. Often the code looks valid, does what it looks like it does, yet is actually undefined or implementation defined and will break elsewhere.

With that said, while I don't expect everyone to have memorized the standard, I do hope most would have at least enough familiarity to avoid most cases of undefined behavior.

[coldgrnd]

Most of what I learned about C++ I got from books, blogs and some great C++ guys (mostly from the boost community). I have to admit I did not read the standard at all.

could you give an example of a case where you hit undefined behavior since I hardly seem to recall a case where that bit me in the past? (I'm mostly working on embedded systems (PPC & ARM))

The cases that come to my mind for C++ all involve initialization...

[dkersten]

Before I give you an example, I will define what the term undefined behavior means by quoting the standard - Section §1.3.12 (of the C standard, not the C++ standard which is horribly ginormous and hard to read):

    behaviour, such as might arise upon use of an erroneous program construct or erroneous data, for which this International Standard imposes no requirements 3.

    Undefined behaviour may also be expected when this International Standard omits the description of any explicit definition of behavior.

The reason undefined behavior is dangerous is that the standard does not guarantee any particular behavior and the implementation is free to do whatever it wants - ignore it, give an error message, delete everything on your hard drive.. whatever.

The two most commonly cited piece of undefined behavior is modifying a variable twice in one sequence point. The standard says:

    Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression.

This code snippet invokes undefined behavior:

    a = b++ * ++b;

because b is modified twice within one sequence point. More information here http://stackoverflow.com/questions/4176328/undefined-behavio...

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For more real world examples of where undefined behavior may bite you in the ass in C++, take a look at Washu's simple C++ quiz. It's only four questions: http://www.scapecode.com/2011/05/a-simple-c-quiz/

Take a moment to answer the questions before looking at the answers.

Once you've done that, here are three more quizzes by the same guy - these ones are about OOP in C++, so may be much more relevant to your question: http://www.scapecode.com/2011/05/c-quiz-2/ and http://www.scapecode.com/2011/05/c-quiz-3/ and http://www.scapecode.com/2011/05/c-quiz-4/