snprintf family of functions marked deprecated mustfix without a viable replacement

[mcicciotti-mwb]

The snprintf family of functions has no appropriate replacements and should not be marked deprecated mustfix. In particular, snprintf can be used in the absence of a buffer (represented by a null buffer of zero length) to determine the required buffer size. NTStrSafe replacements such as RtlStringCbPrintf simply are not designed to return the required buffer size in any way, while the snprintf_s family consider a null buffer of zero length a programming error that results in the invocation of the invalid argument handler.

The fix would be trivial: implement the _scprintf family of functions in the WDK.

[mcicciotti-mwb]

I'll note that vsnprintf_s is documented to implement this behavior, but the documentation is wrong:

Condition: count == 0

Behavior: Doesn't write any data and returns the number of characters that would have been written, not including the terminating NULL.

Return value: The number of characters that would have been written not including the terminating NULL.

(No such misinformation is present in the documentation for the variadic variant, snprintf_s.)

The published source code for snprintf_s unequivocally shows that a null buffer of length zero is an illegal argument, unless the maximum output length is also zero (which, however, results in a no-op and is of no use to compute the required buffer size):

template <typename Character>
_Success_(return >= 0)
static int __cdecl common_vsnprintf_s(
                                    unsigned __int64   const options,
    _Out_writes_z_(buffer_count)    Character*         const buffer,
                                    size_t             const buffer_count,
                                    size_t             const max_count,
                                    Character const*   const format,
                                    __crt_cached_ptd_host&   ptd,
                                    va_list            const arglist
    ) throw()
{
    _UCRT_VALIDATE_RETURN(ptd, format != nullptr, EINVAL, -1);

    if (max_count == 0 && buffer == nullptr && buffer_count == 0)
        return 0; // No work to do

    _UCRT_VALIDATE_RETURN(ptd, buffer != nullptr && buffer_count > 0, EINVAL, -1);