Discussion

Answer: D

Correct answer: Undefined Behaviour

The array ary holds two pointers: ary[0] points at i and ary[1] points at j. Now read the expression ary[0][1]. Subscripting is defined as x[n] meaning *(x + n), so this is *(ary[0] + 1) - take the pointer to i, step one int past it, and read what is there. But i is a single int, not the first element of an array, so there is nothing legitimately one step past it. The standard permits forming a pointer one past a single object but not dereferencing it, and that makes this undefined behaviour. In practice the program will usually print whatever happens to sit next on the stack - often 10, the value of k, since many compilers lay these locals out adjacently - but that is an accident of the compiler and the optimisation level, not a result the language guarantees. It will not print 20, and it is neither a compile error nor a diagnosed runtime error, because nothing in the code is ill-formed and nothing traps. The listing in this record had lost the declaration the call in main depended on, and gave foo a parameter with the same name as one of its own locals; it is repaired above so that the program compiles and exhibits exactly the behaviour the question is about.