static void voutf(struct GlobalConfig *config,
                  const char *prefix,
                  const char *fmt,
                  va_list ap)
{
  size_t width = (79 - strlen(prefix));
  if(!config->mute) {
    size_t len;
    char *ptr;
    char *print_buffer;

    print_buffer = curlx_mvaprintf(fmt, ap);
    if(!print_buffer)
      return;
    len = strlen(print_buffer);

    ptr = print_buffer;
    while(len > 0) {
      fputs(prefix, config->errors);

      if(len > width) {
        size_t cut = width-1;

        while(!ISSPACE(ptr[cut]) && cut) {
          cut--;
        }
        if(0 == cut)
          /* not a single cutting position was found, just cut it at the
             max text width then! */
          cut = width-1;

         (void)fwrite(ptr, cut + 1, 1, config->errors);
         fputs("\n", config->errors);
         ptr += cut + 1; /* skip the space too */
        len -= cut + 1;
       }
       else {
         fputs(ptr, config->errors);
        len = 0;
      }
    }
    curl_free(print_buffer);
  }
}

The product reads data past the end, or before the beginning, of the intended buffer.