What is the correct type for array indexes in C?

Question

What type for array index in C99 should be used? It have to work on LP32, ILP32, ILP64, LP64, LLP64 and more. It doesn't have to be a C89 type.

I have found 5 candidates:

• size_t
• ptrdiff_t
• intptr_t / uintptr_t
• int_fast*_t / uint_fast*_t
• int_least*_t / uint_least*_t

There is simple code to better illustrate problem. What is the best type for i and j in these two particular loops. If there is a good reason, two different types are fine too.

for (i=0; i<imax; i++) {
        do_something(a[i]);
}
/* jmin can be less than 0 */
for (j=jmin; j<jmax; j++) {
        do_something(a[j]);
}

P.S. In the first version of question I had forgotten about negative indexes.

P.P.S. I am not going to write a C99 compiler. However any answer from a compiler programmer would be very valuable for me.

Similar question:

• size_t vs. uintptr_t
  The context of this question if different though.

Answer 1

If you know the maximum length of your array in advance you can use

• int_fast*_t / uint_fast*_t
• int_least*_t / uint_least*_t

In all other cases i would recommend using

• size_t

or

• ptrdiff_t

depending on weather you want to allow negative indexes.

Using

• intptr_t / uintptr_t

would be also safe, but have a bit different semantics.

Answer 2

I think you should use ptrdiff_t for the following reasons

• Indices can be negative (thus all unsigned types, including size_t, are out of question)
• The type of p2 - p1 is ptrdiff_t. The type of i in the reverse thing, *(p1 + i), should be that type too (notice that *(p + i) is equivalent to p[i])