Question

I heard a rumor that, in C, arrays that are contained inside structs may have padding added in between elements of the array. Now obviously, the amount of padding could not vary between any pair of elements or calculating the next element in an array is not possible with simple pointer arithmetic.

This rumor also stated that arrays which are not contained in structures are guaranteed to contain no padding. I know at least that part is true.

So, in code, the rumor is:

{
    // Given this:
    struct { int values[20]; } foo;
    int values[20];

    // This may be true:
    sizeof(values) != sizeof(foo.values);
}

I’m pretty certain that sizeof(values) will always equal sizeof(foo.values). However, I have not been able to find anything in the C standard (specifically C99) that explicitly confirms or denies this.

Does anyone know if this rumor is addressed in any C standard?

edit: I understand that there may be padding between the end of the array foo.values and the end of the struct foo and that the standard states that there will be no padding between the start of foo and the start of foo.values. However, does anyone have a quote from or reference to the standard where it says there is no padding between the elements of foo.values?

Answer 1

No, there will never be padding in between elements of an array. That is specifically not allowed. The C99 standard calls array types “An array type describes a contiguously allocated nonempty set of objects…”. For contrast, a structure is “sequentially”, not “contiguously” allocated.

There might be padding before or after an array within a structure; that is another animal entirely. The compiler might do that to aid alignment of the structure, but the C standard doesn’t say anything about that.