Unix to Windows: Alternative to vsnprintf to determine length?

Answer provided in comments by Hans Passant:

The documented _vscprintf provides this functionality on Windows, so relying on "unspecified behaviour" is not necessary.

Take this example from the manual page of snprintf:

Here is how to allocate a buffer in order to fit your string.

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

char *
make_message(const char *fmt, ...)
{
    int n;
    int size = 100;     /* Guess we need no more than 100 bytes. */
    char *p, *np;
    va_list ap;

   if ((p = malloc(size)) == NULL)
        return NULL;

   while (1) {

       /* Try to print in the allocated space. */

       va_start(ap, fmt);
        n = vsnprintf(p, size, fmt, ap);
        va_end(ap);

       /* If that worked, return the string. */

       if (n > -1 && n < size)
            return p;

       /* Else try again with more space. */

       if (n > -1)    /* glibc 2.1 */
            size = n+1; /* precisely what is needed */
        else           /* glibc 2.0 */
            size *= 2;  /* twice the old size */

       if ((np = realloc (p, size)) == NULL) {
            free(p);
            return NULL;
        } else {
            p = np;
        }
    }
}