What is _vsnwprintf_s in C?

_vsnwprintf_s in C writes formatted wide strings to a destination buffer and returns the number of wide characters written. _vsnwprintf_s is defined in the wchar.h header file and the function prototype is as follows:

int _vsnwprintf_s(wchar_t *dest, size_t destsz, size_t count, const wchar_t *format, va_list argptr);

Parameters

dest: Destination buffer of formatted wide string

destsz: Maximum size of destination buffer in bytes

count: Number of wide characters to write; does not include terminating NULL character

format: Format specification

argptr: Pointer to list of arguments

Return value

Upon success: Number of wide characters written

Upon failure: -1

Error handling

If dest or format is NULL, then -1 is returned and errno is set to EINVAL
If count is less than zero, then -1 returned and errno is set to EINVAL
If destsz is smaller than the characters to be written, then 1 returned and errno is set to ERANGE
If count is not equal to _TRUNCATE, then 1 returned and errno is set to ERANGE

NOTE: When count is equal to _TRUNCATE, _vsnwprintf writes as much of the wide string that fits in the destination buffer, leaving room for the NULL terminating wide character.

Example

#include <stdio.h>
#include <wchar.h>
#include <wtypes.h>
void formatted_widestring(wchar_t* format_widestring, ...)
{
   int chars_written = 0;
   wchar_t dest[30];
   va_list args;
   va_start(args, format_widestring);
   chars_written = _vsnwprintf_s(dest, sizeof(dest), _TRUNCATE, format_widestring, args);
   wprintf(L"Number of wide characters written: %d, Destination buffer: %ls\n", chars_written, dest);
   va_end(args);
}
int main() 
{
   formatted_widestring(L"%ls %ls %ls", L"Writing", L"wide", L"string");
   formatted_widestring(L"%ls %ls %ls %ls", L"Writing", L"formatted", L"wide", L"string");
   
   return 0;
}

Output

Number of wide characters written: 19, Destination buffer: Writing wide string

Number of wide characters written: 29, Destination buffer: Writing formatted wide string

_vsnwprintf_s is not supported by the GCC compiler, so you will get an implicit declaration of function… error. Use the following variant to get the job done: _vsnwprintf

Explanation

First, we import the necessary header files such as wchar.h for wide strings, and wtypes.h for variable arguments. The function formatted_widestring takes in a variable number of wide string arguments and prints them in formatted form using _vsnwprintf_s.

In lines 9-11, we initialize our destination buffer, start a variable argument list, and pass them to _vsnwprintf_s, where count is set to _TRUNCATE. Finally, we print the number of wide characters written and the destination buffer.

In lines 11, 18, and 19, the L identifier before the string indicates that the characters to follow are from the Unicode character set.

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