Why vsnwprintf missing

I using vsnprintf for ansi string and it works well, returns -1 only on error and returns count of needen size if it does not fit into buffer.
I need same function for wide strings, vsnwprintf just not exists. I also tried _vsnwprintf, but it acts really different, it returns -1 even if buffer too small, while I need to know how much to expand a buffer, but only when it's necessary, I don't want to make dummy calls just to determine needen size and only then expand a buffer.

vsnprintf

-1

vsnwprintf

_vsnwprintf

If look at vsnprintf code:

vsnprintf

int const _Result = __stdio_common_vsprintf(
_CRT_INTERNAL_LOCAL_PRINTF_OPTIONS | _CRT_INTERNAL_PRINTF_STANDARD_SNPRINTF_BEHAVIOR,
_Buffer, _BufferCount, _Format, NULL, _ArgList);
return _Result < 0 ? -1 : _Result;


While _vsnwprintf approximately has:

int const _Result = __stdio_common_vswprintf(
_CRT_INTERNAL_LOCAL_PRINTF_OPTIONS | _CRT_INTERNAL_PRINTF_LEGACY_VSPRINTF_NULL_TERMINATION,
_Buffer, _BufferCount, _Format, NULL, _ArgList);
return _Result < 0 ? -1 : _Result;


Looks like difference only in flags, I tried to call __stdio_common_vswprintf manually with _CRT_INTERNAL_PRINTF_STANDARD_SNPRINTF_BEHAVIOR flag, and it works exactly as I need, but also causing random crashes/freezes when application exiting. So I need vsnwprintf function, maybe someone know how to do it?

__stdio_common_vswprintf

_CRT_INTERNAL_PRINTF_STANDARD_SNPRINTF_BEHAVIOR

vsnwprintf

P.S. I using Microsoft Visual Studio 2017 on Windows 7.

2 Answers
2

The wide character version of vsnprintf is called vswprintf (note: no n). According to the documentation, it is compliant with ISO C so you can check the behaviour in the C Standard or here.

vsnprintf

vswprintf

n

It is correct that it returns -1 when the output would exceed the buffer size. There is no option to truncate output; you are supposed to make the "dummy call" to check buffer size.

-1

You could always wrap a vswprintf call in another function that has the behaviour you desire (make the dummy call and then either truncate to fit, or return a malloc'd buffer). Be ware of truncating in the middle of a UTF-16 character.

vswprintf

Note that MSVC's vsnprintf changed its behaviour at some point (2015 maybe?) to be ISO C compliant; the original MSVC behaviour can be retrieved by calling _vsnprintf.

vsnprintf

_vsnprintf

You can use _vsnwprintf(). Per the documentation for _vsnwprintf() (note the leading underscore):

_vsnwprintf()

_vsnwprintf()

...

int _vsnwprintf(
wchar_t *buffer,
size_t count,
const wchar_t *format,
va_list argptr
);


...

The value returned by all these functions does not include the
terminating null, whether one is written or not. When count is zero,
the value returned is the number of characters the functions would
write, not including any terminating null. You can use this result to
allocate sufficient buffer space for the string and its terminating
null, and then call the function again to fill the buffer.

count

count

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