Error C4146 in Windows

[yuppox]

I'm seeing 4 errors in core.h on Windows. All of them are:

Error C4146 unary minus operator applied to unsigned type, result still unsigned

Here is the first instance

    for (i = 0; i < blocksize; i++) {
        barrier_mask = (unsigned char) (((i ^ xpadlen) - 1U) >> ((sizeof(size_t) - 1U) * CHAR_BIT));
        tail[-i]     = (tail[-i] & mask) | (0x80 & barrier_mask);
        mask |= barrier_mask;
    }

The second instance is here:

    for (i = 0U; i < blocksize; i++) {
        c          = tail[-i];
        is_barrier = (((acc - 1U) & (pad_len - 1U) & ((c ^ 0x80) - 1U)) >> 8) & 1U;
        acc |= c;
        pad_len |= (i & -is_barrier);
        valid |= (unsigned char) is_barrier;
    }

The problems are tail[-i] and -is_barrier. They are all size_t which is unsigned.

Why are those negative? I especially don't understand tail[-i]. It's counting from 0 to some size. Why would you want that to be negative when accessing an array?

[jedisct1]

p[x] is equivalent to *(p + x). So a negative value for x is not supposed to be an error.

Anyway, the notation was changed to *(p + x) in 63260875a1b66fd87d210d126406e88fde483706 which also makes the consistent with libsodium.

[yuppox]

Thanks for the update and explanation!