The va_count macro places the number of quadwords (Alpha only) in
    the argument list into count. The value returned in count is the
    number of quadwords (Alpha only) in the function argument block
    not counting the count field itself.

    If the argument list contains items whose storage requirements
    are a quadword (Alpha only) of memory or less, the number in
    the count argument is also the number of arguments. However, if
    the argument list contains items that are longer than a quadword
    (Alpha only), count must be interpreted to obtain the number of
    arguments. Because a double is 8 bytes, it occupies one argument-
    list position on OpenVMS Alpha and Integrity server systems.

    The va_count macro is specific to Compaq C for OpenVMS Systems
    and is not portable.