Is program termination of a C++ program observable behavior?

I could have also phrased this as What constitutes observable behavior?开发者_StackOverflow

The C++ Standard talks a lot about observable behavior, but I am not really sure if program termination is part of observable behavior.

That is, given a program such as:

int main() {
  for(;;) {}
  return 0;
}

is a conforming implementation allowed to ever terminate this program?

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Yes it is legal for the compile to generate an empty main body for the above code (thus terminating almost immediately).

The C++0x FCD says 6.5 says (pay special attention to the note):

A loop that, outside of the for-init-statement in the case of a for statement,
* makes no calls to library I/O functions, and
* does not access or modify volatile objects, and
* performs no synchronization operations (1.10) or atomic operations (Clause 29)

may be assumed by the implementation to terminate. [ Note: This is intended to allow compiler transformations, such as removal of empty loops, even when termination cannot be proven. — end note ]

So the compiler can assume that your for loop will terminate and since the body is empty it can optimize it away altogether.

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The quotation from the draft was copied from this question and verified against my copy.

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Is program termination of a C++ program observable behavior?

Yes, finishing the exectution of a program is an observable behavior since the program return an error code.

• 0 when no error
• not 0 value when not successful

Normal terminal does not show this states which is why you do not "see" it.

Is a conforming implementation allowed to ever terminate this program?

The program can not exit, but depending on the different OS, it can be terminated by for example signal, interruption ...

--EDIT--

The answer of the second question is a little bit erroneous

As Martin and Motti explained in a previous answer :

The discussion is based on allowed optimizations that the compiler can safely perform. Since the removal of a dead loop is explicitly allowed the compiler is free to junk the above loop thus allowing the program to terminate