问题
I examine that C array maybe have some extra bytes at tail.
There are my code
int a = 5;
int test[] = {1,2,3,4};
int b = 5;
test[-1] = 11;
test[4] = 11;
cout << b << endl; // 11
cout << a << endl; // 5
You can see the running result there
the value of b is changed through changing test[-1]'s value. But when I change test[4]'s value, the value of a doesn't change;
I use gdb to check their addresses, found that
In g++ 6.4.0, the address of a substract address of test[4] is 8 bytes
In clang++ 3.8.1, the address of a substract address of test[4] is 4 bytes
So, I am curious that why the array has some bytes at tail?
Thanks @Peter A.Schneider to explaining the question. It is surely a UB , But it is just a experimental code. This isn't a discuss for practical code.
generally，variables at the runtime stack are close together. b is close to test， but why 'a' is not close to 'test+3'. That's the key of the problem.
回答1:
test[-1] = 11;
test[4] = 11;
This is undefined behavior.(Meaning anything could have happened). In your case you changed the value of b because they are adjacent in the memory where they are allocated. But you shouldn't rely on it. Because this may blow up your program or results in erroneous code behavior most of the time.
The UB you have is because `Accessing an array index out of bound in undefined behavior."
来源：https://stackoverflow.com/questions/47737913/why-c-array-has-extra-bytes-at-tail