> Special-casing buff=NULL size=0 sounds like MS's attempt to add a bit
> more C99 compliance to the (v)snprintf functions, in addition to adding
> more C99 functions in general, which they've been talking about the last
> couple of years. This behavior may be different from older pre-VS2013
> versions.

I ran the same tests earlier today using VS2008 and VS2010, and I got the same results. Both vsnprintf and _vsnwprintf behave the same way.

I added two tests to dlls/msvcrt/tests/printf.c (not sure if that was the correct place) and submitted the diff to the testbot.

[1] https://testbot.winehq.org/JobDetails.pl?Key=10842
[2] https://testbot.winehq.org/JobDetails.pl?Key=10843

The tests pass on all VMs, except for W2000. That OS fails when calling vsnprintf(NULL, 0, fmt, args).

    ret = _vsnwprintf_wrapper( NULL, 0, format, one, two, three );
    ok( ret == 11, "got %d, expected 11\n", ret );

What's the best way to handle the different return value in Wine tests?

    ok( ret == 11 || ret == -1, "got %d, expected 11\n", ret );