On Sun, 20 Sep 2020, Warner Losh wrote:

> 0 is fine if sizeof(int) == sizeof(void *). Otherwise varadic function
> calls break.

I've never written anything that uses varargs, so I've never run into
that.  But I've actually done quite a bit of work with an environment
where this isn't true: MS-DOS using the large or huge model.  In this
environment, sizeof(int)=2, and sizeof(void*) is 4.