Does “Undefined Behavior” really permit *anything* to happen? [duplicate]

Answer 1

First, it is important to note that it is not only the behaviour of the user program that is undefined, it is the behaviour of the compiler that is undefined. Similarly, UB is not encountered at runtime, it is a property of the source code.

To a compiler writer, "the behaviour is undefined" means, "you do not have to take this situation into account", or even "you can assume no source code will ever produce this situation". A compiler can do anything, intentionally or unintentionally, when presented with UB, and still be standard compliant, so yes, if you granted access to your nose...

Then, it is not always possible to know if a program has UB or not. Example:

int * ptr = calculateAddress();
int i = *ptr;

Knowing if this can ever be UB or not would require knowing all possible values returned by calculateAddress(), which is impossible in the general case (See "Halting Problem"). A compiler has two choices:

• assume ptr will always have a valid address
• insert runtime checks to guarantee a certain behaviour

The first option produces fast programs, and puts the burden of avoiding undesired effects on the programmer, while the second option produces safer but slower code.

The C and C++ standards leave this choice open, and most compilers choose the first, while Java for example mandates the second.

---

Why is the behaviour not implementation-defined, but undefined?

Implementation-defined means (N4296, 1.9§2):

Certain aspects and operations of the abstract machine are described in this International Standard as implementation-defined (for example, sizeof(int) ). These constitute the parameters of the abstract machine. Each implementation shall include documentation describing its characteristics and behavior in these respects. Such documentation shall define the instance of the abstract machine that corresponds to that implementation (referred to as the “corresponding instance” below).

Emphasis mine. In other words: A compiler-writer has to document exactly how the machine-code behaves, when the source code uses implementation-defined features.

Writing to a random non-null invalid pointer is one of the most unpredictable things you can do in a program, so this would require performance-reducing runtime-checks too.
Before we had MMUs, you could destroy hardware by writing to the wrong address, which comes very close to nasal demons ;-)

Answer 2

Undefined behavior is simply the result of a situation coming up that the writers of the specification did not foresee.

Take the idea of a traffic light. Red means stop, yellow means prepare for red, and green means go. In this example people driving cars are the implementation of the spec.

What happens if both green and red are on? Do you stop, then go? Do you wait until red turns off and it's just green? This is a case that the spec did not describe, and as a result, anything the drivers do is undefined behavior. Some people will do one thing, some another. Since there is no guarantee about what will happen you want to avoid this situation. The same applies to code.

Answer 3

Nitpicking: You have not quoted a standard.

These are the sources used to generate drafts of the C++ standard. These sources should not be considered an ISO publication, nor should documents generated from them unless officially adopted by the C++ working group (ISO/IEC JTC1/SC22/WG21).

Interpretation: Notes are not normative according to the ISO/IEC Directives Part 2.

Notes and examples integrated in the text of a document shall only be used for giving additional information intended to assist the understanding or use of the document. They shall not contain requirements ("shall"; see 3.3.1 and Table H.1) or any information considered indispensable for the use of the document e.g. instructions (imperative; see Table H.1), recommendations ("should"; see 3.3.2 and Table H.2) or permission ("may"; see Table H.3). Notes may be written as a statement of fact.

Emphasis mine. This alone rules out "comprehensive list of options". Giving examples however does count as "additional information intended to assist the understanding .. of the document".

Do keep in mind that the "nasal demon" meme is not meant to be taken literally, just as using a balloon to explain how universe expansion works holds no truth in physical reality. It's to illustrate that it's foolhardy to discuss what "undefined behavior" should do when it's permissible to do anything. Yes, this means that there isn't an actual rubber band in outer space.