Are negative array indexes allowed in C?

I was just reading some code and found that the person was using arr[-2] to access the 2nd element before the arr, like so:

|a|b|c|d|e|f|g|
       ^------------ arr[0]
         ^---------- arr[1]
   ^---------------- arr[-2]

Is that allowed?

I know that arr[x] is the same as *(arr + x). So arr[-2] is *(arr - 2), which seems OK. What do you think?

That is correct. From C99 §6.5.2.1/2:

The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))).

There's no magic. It's a 1-1 equivalence. As always when dereferencing a pointer (*), you need to be sure it's pointing to a valid address.

This is only valid if arr is a pointer that points to the second element in an array or a later element. Otherwise, it is not valid, because you would be accessing memory outside the bounds of the array. So, for example, this would be wrong:

int arr[10];

int x = arr[-2]; // invalid; out of range

But this would be okay:

int arr[10];
int* p = &arr[2];

int x = p[-2]; // valid:  accesses arr[0]

It is, however, unusual to use a negative subscript.