Are negative array indexes allowed in C?

c arrays
bodacydo picture bodacydo · Aug 13, 2010 · Viewed 86.5k times · Source

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?

Answer

Matthew Flaschen picture Matthew Flaschen · Aug 13, 2010

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.

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