VC6中没有vsnprintf函数

VC6中没有vsnprintf函数

像下面的好用工具函数就无法使用：
string format_string(const char *fmt, ...)
{
 /* Guess we need no more than 512 bytes. */
 int n, size = 512;
 char *p;
 va_list ap;
 if ((p = (char*)malloc (size)) == NULL)
  return "";
 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)
  {
   string str(p);
   if(p)
    free(p);
   return str;
  }
  /* 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 ((p = (char*)realloc (p, size)) == NULL)
  {
   if(p)
    free(p);
   return "";
  }
 }
}